Light pollution is the correct answer! Most of the world's population lives under light-polluted skies, and over 80% cannot see the Milky Way. Light pollution can be reduced by using shielded fixtures, dimming lights, and using motion sensors. Air pollution (from factories, cars, power plants) causes respiratory diseases, acid rain, and smog. Water pollution (from sewage, agricultural runoff, oil spills) harms aquatic life and contaminates drinking water. Soil pollution (from pesticides, heavy metals, landfills) affects plant growth and can contaminate groundwater. Noise pollution (from traffic, construction, aircraft) causes stress, hearing loss, and sleep disturbance. Plastic pollution affects oceans and wildlife (microplastics are now found everywhere, including in human blood). The World Health Organization (WHO) estimates that 7 million people die prematurely each year from air pollution. Polluters often pay the "external costs" – the cost of pollution is borne by society, not the polluter (market failure). Environmental regulations aim to internalize these costs. The polluter pays principle is a foundation of environmental law. The most polluted cities are often in low- and middle-income countries (Beijing, Delhi, Lahore, Dhaka).

Ground-level ozone is a harmful pollutant! It can trigger asthma, reduce lung function, and damage crops. Particulate matter (PM2.5) is especially dangerous because it can penetrate deep into the lungs and enter the bloodstream. The WHO has classified air pollution as a Group 1 carcinogen (causes cancer). The Great Smog of London (1952) killed an estimated 12,000 people (up to 4,000 directly, plus subsequent deaths). This disaster led to the Clean Air Act of 1956 (UK). In the United States, the Clean Air Act (1970, amended 1990) has significantly reduced air pollution (lead, SO₂, NOx, CO). However, ozone and particulate matter remain problems. The Air Quality Index (AQI) measures air pollution levels (0-500; higher is worse). PurpleAir and other sensors provide real-time air quality data. Air pollution disproportionately affects low-income communities and communities of color (environmental justice). Indoor air pollution (from cooking with solid fuels) kills about 3.2 million people per year, mostly in low-income countries. Masks (N95, KN95) can filter particulate matter. Air purifiers with HEPA filters remove particles. Smog is a combination of "smoke" and "fog." The Los Angeles basin is prone to smog due to its geography (mountains trap pollutants). Electric vehicles (EVs) reduce tailpipe emissions but not tire and brake dust (which is also particulate matter).

Nutrients (nitrogen and phosphorus) cause eutrophication! The Gulf of Mexico dead zone covers about 5,000-8,000 square miles annually. Other dead zones include the Baltic Sea, Chesapeake Bay, and the Black Sea. Eutrophication can be reduced by using less fertilizer, planting buffer strips, and treating wastewater. Sewage contains pathogens (bacteria, viruses) that cause waterborne diseases (cholera, typhoid, dysentery). According to the WHO, 2 billion people use drinking water contaminated with feces. Oil spills (Deepwater Horizon, 2010; Exxon Valdez, 1989) cause long-term damage to marine ecosystems. The Deepwater Horizon spill released about 4.9 million barrels of oil into the Gulf of Mexico. Plastic pollution kills marine animals (ingestion, entanglement). The Great Pacific Garbage Patch is a gyre of plastic debris (mostly microplastics) in the North Pacific Ocean, estimated to be 1.6 million square kilometers (twice the size of Texas). Microplastics (less than 5 mm) are found in seafood, salt, beer, and even human blood and lungs. The Clean Water Act (1972) in the U.S. regulates water pollution and has improved water quality, but many waterways remain impaired. Combined sewer overflows (CSOs) release untreated sewage during heavy rain. Green infrastructure (rain gardens, permeable pavement) reduces runoff. Desalination plants produce freshwater but also discharge concentrated brine, which can harm marine life.

The Great Pacific Garbage Patch is the largest! It is not a floating island of trash; it is a diffuse area of microplastics and larger debris. The plastic comes from land-based sources (rivers carry plastic to the ocean) and from fishing gear (ghost nets). The Ocean Cleanup project is developing technologies to remove plastic from the ocean. Single-use plastics (bags, straws, bottles, utensils) are a major problem because they are used for minutes but persist for centuries. Many countries have banned or restricted single-use plastics. Bioplastics (made from plants) are not always biodegradable; they require industrial composting. Biodegradable plastics may not degrade in the ocean (they need specific conditions). Plastic waste in the ocean kills over 1 million marine animals annually (including seabirds, sea turtles, whales, and dolphins). Microplastics have been found in Arctic ice, Antarctic snow, and the Mariana Trench (deepest ocean trench). Microplastics also contaminate drinking water and food. The average person may ingest 5 grams of plastic per week (about the weight of a credit card). Alternatives to plastic include glass, metal, paper, bamboo, and compostable materials. The 3 Rs: Reduce, Reuse, Recycle (in that order). Recycling rates vary by material: cardboard (89%), paper (66%), glass (27%), aluminum (50%), plastic (9%). The plastic industry has promoted recycling as a solution, but most plastic is not recyclable because of contamination or economic factors. Extended producer responsibility (EPR) laws require manufacturers to manage plastic waste. Plastic waste is often exported from rich countries to poorer countries (where it may be dumped or burned). The Basel Convention regulates the transboundary movement of plastic waste (amended in 2019).

Carbon dioxide is the primary greenhouse gas! CO₂ has a long lifetime in the atmosphere (hundreds to thousands of years). Methane (CH₄) is more potent (28 times more warming than CO₂ over 100 years) but has a shorter lifetime (about 12 years). Nitrous oxide (N₂O) is about 265 times more potent than CO₂. Fluorinated gases (HFCs, PFCs, SF₆) are extremely potent but less common. Global average temperature has increased by about 1.1°C (2°F) since pre-industrial times (1880). The Paris Agreement (2015) aims to limit warming to well below 2°C, preferably 1.5°C. The 2015-2022 period was the warmest on record. The Earth's carbon cycle has been disrupted: the concentration of CO₂ has increased from about 280 ppm (pre-industrial) to over 420 ppm (2023). This is the highest level in at least 800,000 years (and likely 3 million years). Consequences include sea level rise (thermal expansion, melting ice), more frequent and intense extreme weather events (heatwaves, floods, droughts, hurricanes), ocean acidification (harmful to shellfish and coral), and ecosystem disruption. Climate change is a global problem requiring international cooperation. Mitigation (reducing emissions) and adaptation (preparing for impacts) are both necessary. Renewable energy (solar, wind, hydro, geothermal) is rapidly replacing fossil fuels. Electrification (electric vehicles, heat pumps) reduces emissions if the electricity is clean. Carbon capture and storage (CCS) removes CO₂ from industrial processes or the air. Nature-based solutions (reforestation, wetland restoration) sequester carbon. The social cost of carbon (SCC) estimates the economic damages per ton of CO₂ emitted (estimated at $50-$200 per ton). Carbon pricing (carbon tax or cap-and-trade) internalizes this cost. The fossil fuel industry has known about climate change since the 1970s but misled the public (climate denial). The youth climate movement (Fridays for Future) has raised awareness. Individual actions (reducing energy use, driving less, eating less meat, reducing waste) help, but systemic change is essential.

Lead is a toxic heavy metal! Lead poisoning has affected millions of children worldwide. The Flint water crisis (Michigan, 2014) occurred when lead leached from old pipes into drinking water. Lead-based paint was banned in the US in 1978. Soil contamination can be remediated by excavation (removing contaminated soil), bioremediation (using bacteria to break down pollutants), or phytoremediation (using plants like sunflowers to absorb heavy metals). The Love Canal disaster (Niagara Falls, NY) was a neighborhood built on a toxic waste dump, leading to birth defects and cancer. Asbestos (used in insulation) causes lung cancer and mesothelioma. Arsenic (naturally occurring in some groundwater) causes skin lesions, cancer, and cardiovascular disease. Mercury (from coal burning, gold mining) bioaccumulates in fish (causing Minamata disease). Superfund (US) is a program to clean up hazardous waste sites. There are over 1,300 Superfund sites in the United States. Brownfields are abandoned industrial sites that may be contaminated but can be cleaned up and redeveloped. The polluter pays principle holds companies responsible for cleanup. Soil conservation practices (no-till farming, cover crops) prevent erosion and reduce contamination. Pesticides and herbicides can persist in soil for decades (DDT, banned in the US in 1972, is still found in soil). Organic farming reduces pesticide use. Composting recycles organic waste into fertilizer, reducing landfill use.

The decibel is the unit! Prolonged exposure to sounds above 85 dB can cause hearing loss. Noise-induced hearing loss is permanent. The EPA estimates that 100 million Americans are exposed to noise levels that can damage hearing. Noise pollution affects wildlife: sonar can harm whales (causing strandings); birds sing louder in noisy areas; bats may avoid noisy areas. Electric vehicles (EVs) are quieter than gas cars, but they are required to produce warning sounds at low speeds to alert pedestrians (especially blind people). Quiet pavements (porous asphalt) reduce tire noise. Noise barriers along highways reduce noise in neighborhoods. Soundproofing materials (double-pane windows, insulation) can reduce indoor noise. Airplane noise near airports can be reduced by quieter engines and flight path changes. The Noise Control Act of 1972 (US) regulates noise emissions. The European Union has a Environmental Noise Directive that requires mapping of noise levels. The Quiet Communities Act supports noise reduction efforts. There is no safe level of lead (as discussed) – a transition sentence? Actually, not needed. The concept of "noise pollution" was less recognized until the 1970s. White noise machines can mask disturbing sounds. The loudest sound ever recorded was the Krakatoa volcanic eruption (1883), heard 3,000 miles away (estimated 180 dB at 100 miles). Noise pollution is also called "sound pollution." London Heathrow Airport (and others) have night flight restrictions to reduce sleep disturbance. The WHO's "Night Noise Guidelines for Europe" recommend indoor noise levels below 40 dB (10:00 PM to 7:00 AM).

Reducing is the most effective! Recycling takes energy and resources; reducing eliminates the need entirely. The waste hierarchy: prevention (reduce) → preparing for reuse → recycling → recovery (energy) → disposal (landfill). Many countries have adopted circular economy principles (keeping materials in use, designing out waste and pollution, regenerating natural systems). Extended producer responsibility (EPR) requires manufacturers to take back products at end of life. Plastic bag bans (or fees) have reduced plastic bag use. Reusable bags should be used many times to offset their environmental impact (cotton bags need to be used over 100 times to beat plastic bags). Composting organic waste (food scraps, yard waste) reduces methane emissions from landfills. Zero waste lifestyle aims to send nothing to landfill. Upcycling means turning waste into a product of higher value (e.g., making bags from old sails). The United Nations Sustainable Development Goals (SDG 12) is "Responsible Consumption and Production." Individual actions include: using reusable water bottles and coffee cups, bringing reusable bags to stores, avoiding single-use plastics, repairing items instead of throwing them away, donating used items, buying second-hand, and composting. Community actions include: curbside recycling, composting programs, repair cafes, and tool libraries. Industrial ecology applies circular economy principles at the industrial scale. The "take-make-waste" linear economy is unsustainable. The circular economy is gaining traction. The 5 Rs (Refuse, Reduce, Reuse, Recycle, Rot) are an extension of the 3 Rs (refuse disposable items, rot = compost). The "buy nothing" movement promotes sharing and gifting used items. Carbon pricing and pollution taxes encourage waste reduction. Landfill bans on organic waste divert food scraps to composting. The European Union's Circular Economy Action Plan includes targets for recycling and waste reduction.

Asthma is linked to air pollution! Children are especially vulnerable because their lungs are still developing. Other health effects include: heart attacks (fine particles enter the bloodstream, causing inflammation), stroke, lung cancer (caused by PM2.5 and diesel exhaust), chronic bronchitis, and emphysema. The WHO classifies outdoor air pollution as a Group 1 carcinogen. The Global Burden of Disease study estimates that air pollution causes about 7 million premature deaths per year. Lead poisoning causes irreversible neurological damage, especially in children (lower IQ, behavioral problems). Mercury poisoning (from contaminated fish) causes Minamata disease (neurological symptoms). Arsenic poisoning (from contaminated groundwater) causes skin lesions, cancer, and cardiovascular disease. The Flint water crisis exposed over 100,000 residents to lead; many children have lifelong effects. Endocrine disruptors (BPA, phthalates, PFAS) in plastic and chemicals can affect hormones (reproduction, development, metabolism). PFAS (per- and polyfluoroalkyl substances) are "forever chemicals" linked to cancer, liver damage, and immune suppression. Noise pollution causes hearing loss, stress, and cardiovascular disease (elevated blood pressure, heart attacks). The health burden of pollution is highest in low- and middle-income countries, but pollution is a global problem. The Lancet Commission on pollution and health reported that pollution caused 9 million premature deaths in 2019 (three times more than AIDS, TB, and malaria combined; 15 times more than war). The health costs of pollution are estimated at trillions of dollars per year. Pollution prevention and control save lives and money. Universal access to clean drinking water and sanitation would prevent many waterborne diseases. Air quality alerts help vulnerable populations protect themselves (staying indoors, wearing masks).

Public transit, biking, and walking are great ways to reduce air pollution! Transportation accounts for about 29% of U.S. greenhouse gas emissions (the largest sector). Electric vehicles (EVs) produce no tailpipe emissions, but they still produce tire and brake dust (and the electricity must be clean). Biking and walking have zero emissions and provide health benefits. Telecommuting (working from home) also reduces transportation emissions. Other actions: combining errands (trip chaining), avoiding idling, maintaining proper tire pressure, and driving fuel-efficient vehicles. At home: switch to renewable energy (solar, wind) or buy green power from your utility; use energy-efficient appliances (Energy Star); insulate your home; use a programmable thermostat; air dry clothes; line dry laundry; repair leaks. In the kitchen: reduce food waste (which produces methane in landfills); compost food scraps; avoid single-use plastic; bring reusable bags; buy in bulk; avoid over-packaged products. In the yard: plant trees (they absorb CO₂ and filter air); use a push mower or electric mower (not gas); compost yard waste; avoid chemical fertilizers and pesticides. At the store: bring reusable bags; avoid plastic wrap; choose products with minimal packaging; buy second-hand; repair instead of replace. On vacation: stay at eco-friendly hotels; offset flights; travel by train instead of plane when possible. Community action: join a cleanup event; support local recycling and composting programs; participate in Earth Day events; write to elected officials; join a community solar garden; support bike lanes and public transit. Global action: support the Paris Agreement; urge your government to phase out fossil fuels; divest from fossil fuels; support climate justice. Every action counts. Pollution is a problem we can solve together. You have learned about different types of pollution, their causes and effects, and solutions. Now go out and make a difference!

Ecology studies interactions between organisms and their environment! It is not the same as environmentalism (advocacy). Ecologists use the scientific method to understand ecosystems. The word "ecology" comes from the Greek "oikos" (house) and "logos" (study). So the answer is interactions between organisms and their environment.

Ecosystem includes both living and non-living components! A population is only the same species. A community is only living organisms. The biosphere is all ecosystems combined. So the answer is ecosystem.

Producers are at the base! Primary consumers eat producers; secondary consumers eat primary consumers; tertiary consumers eat secondary consumers. Without producers, there would be no energy input into the food chain. So producers are the answer.

Energy loss explains why top predators are rare! If grass has 10,000 kcal, primary consumers get about 1,000 kcal, secondary consumers get about 100 kcal, and tertiary consumers get about 10 kcal. That is why a single top predator (like a lion) needs a large territory. So the answer is energy loss (10% rule).

Tropical rainforests have the highest biodiversity! They are warm and wet year-round. The Amazon rainforest is the largest. Deserts have low biodiversity; tundra has low biodiversity. So the answer is tropical rainforest.

Mutualism benefits both! Commensalism: barnacles on a whale (barnacles get transport, whale unaffected). Parasitism: tapeworms in an animal (tapeworm gets nutrients, host is harmed). So the answer is mutualism.

Carrying capacity is the maximum population size that can be sustained! When a population exceeds K, it leads to resource depletion and population decline. Examples: deer overpopulation on an island leads to overgrazing. So carrying capacity is the maximum population.

Photosynthesis removes CO₂! Respiration (by plants, animals, decomposers) releases CO₂. Deforestation and burning fossil fuels increase atmospheric CO₂. So the answer is photosynthesis.

Zebra mussels are invasive! Other examples: brown tree snake in Guam (caused extinction of native birds), kudzu vine in the southeastern US (grows over trees), cane toads in Australia (poisonous to native predators). So the answer is zebra mussels.

Habitat destruction is the biggest threat! Other threats include climate change, invasive species, overexploitation (hunting, fishing), and pollution. The current extinction rate is 100-1,000 times higher than the natural background rate. So the answer is habitat destruction.

The Cell Theory has three main tenets: (1) All living things are made of one or more cells; (2) The cell is the basic structural and functional unit of life; (3) All cells arise from pre-existing cells (Omnis cellula e cellula). Cells vary enormously in size (most are 1-100 micrometers), shape, and function. Two major cell types exist: prokaryotic (bacteria and archaea, lacking a nucleus) and eukaryotic (plants, animals, fungi, protists, containing membrane-bound organelles including a nucleus). Key cell structures include the cell membrane (controls what enters/exits), cytoplasm (jelly-like substance), ribosomes (protein synthesis), and genetic material (DNA). Eukaryotic cells contain specialized organelles like mitochondria (energy production), endoplasmic reticulum (protein/lipid synthesis), Golgi apparatus (modification and packaging), and lysosomes (waste breakdown). Understanding cells is essential for medicine, genetics, and biotechnology.

Photosynthesis occurs in chloroplasts, organelles containing chlorophyll (the green pigment that captures light energy). The process has two stages: light-dependent reactions (thylakoid membranes) and the Calvin cycle (stroma). Light-dependent reactions convert light energy to ATP and NADPH, splitting water molecules and releasing oxygen. The Calvin cycle uses ATP and NADPH to fix carbon dioxide into organic molecules (glucose). Photosynthesis is essential for nearly all life on Earth – it produces oxygen for respiration, removes carbon dioxide from the atmosphere, and forms the base of most food chains. C3, C4, and CAM plants have different photosynthetic adaptations to various environments. Factors affecting photosynthesis include light intensity, carbon dioxide concentration, temperature, and water availability. Without photosynthesis, Earth's atmosphere would lack oxygen and complex life could not exist.

Cellular respiration consists of four main stages: glycolysis (cytoplasm, breaks glucose into pyruvate, produces 2 ATP), pyruvate oxidation (mitochondrial matrix, converts pyruvate to acetyl-CoA), the Krebs cycle (mitochondrial matrix, produces NADH, FADH₂, and 2 ATP), and the electron transport chain (inner mitochondrial membrane, produces about 34 ATP). The total theoretical yield is about 36-38 ATP per glucose molecule, though actual yield is around 30-32 ATP. Aerobic respiration requires oxygen as the final electron acceptor, producing water. Without oxygen, cells perform anaerobic respiration (fermentation), producing lactate (animals) or ethanol and CO₂ (yeast) and only 2 ATP per glucose. Cellular respiration is why we breathe – oxygen is consumed and carbon dioxide is produced. Metabolism includes both catabolism (breaking down molecules, releasing energy) and anabolism (building molecules, consuming energy).

The DNA double helix resembles a twisted ladder: sugar-phosphate backbones form the sides, and base pairs form the rungs. Base pairing is specific: A pairs with T (two hydrogen bonds), and C pairs with G (three hydrogen bonds). The sequence of bases along a DNA strand encodes genetic information. DNA is organized into chromosomes (humans have 46 chromosomes, 23 pairs). Genes are specific DNA segments that code for proteins or functional RNA molecules. DNA replication is semi-conservative – each new DNA molecule contains one original strand and one newly synthesized strand. Mutations are changes in DNA sequence that can be caused by errors during replication, radiation, or chemicals. The Human Genome Project (completed in 2003) sequenced all 3 billion base pairs of human DNA. Genetic engineering, CRISPR-Cas9 gene editing, and DNA fingerprinting are practical applications of DNA knowledge.

Darwin's theory of evolution by natural selection was published in "On the Origin of Species" (1859). Key principles include: (1) Variation exists within populations; (2) More offspring are produced than can survive; (3) Competition for limited resources occurs; (4) Individuals with advantageous traits are more likely to survive and reproduce; (5) Over time, favorable traits become more common in the population. Evidence for evolution includes the fossil record (transitional forms like Tiktaalik), comparative anatomy (homologous structures like mammal forelimbs), embryology (similar early development across species), molecular biology (DNA sequence similarities), and direct observation (antibiotic resistance in bacteria, Darwin's finches beak changes). Evolution explains the unity (common ancestry) and diversity of life. Misconceptions include believing evolution is "just a theory" (scientific theories are well-substantiated explanations) or that individuals evolve (populations evolve). Modern synthesis combines Darwinian evolution with Mendelian genetics.

Ecological organization levels: individual → population (same species, same area) → community (all populations in an area) → ecosystem (community + abiotic factors) → biome (large region with similar climate) → biosphere (global sum of all ecosystems). Producers (autotrophs) – plants, algae, cyanobacteria – capture energy through photosynthesis. Consumers (heterotrophs) include herbivores (primary consumers), carnivores (secondary/tertiary consumers), omnivores, and decomposers (bacteria, fungi) which break down dead matter. Food chains show linear energy transfer; food webs show interconnected relationships. Energy pyramids demonstrate that only about 10% of energy transfers between trophic levels – the rest is lost as heat. Key biogeochemical cycles: carbon (photosynthesis, respiration, combustion, fossil fuels), nitrogen (fixation, nitrification, denitrification), water (evaporation, condensation, precipitation, runoff). Biodiversity increases ecosystem stability and resilience. Human activities like deforestation, pollution, and climate change disrupt ecosystems.

Genetics is the correct answer! The field of genetics has many branches: molecular genetics (study of DNA and genes), population genetics (study of genetic variation in populations), quantitative genetics (study of traits controlled by many genes), and genomics (study of entire genomes). Genetics explains why children resemble their parents, why some diseases run in families, and how evolution works. The Human Genome Project (1990-2003) sequenced the entire human genome (about 3 billion base pairs). Genetics has applications in medicine (gene therapy, genetic testing), agriculture (GMOs, crop breeding), and forensics (DNA fingerprinting). The central dogma of molecular biology: DNA → RNA → protein. DNA (deoxyribonucleic acid) is the hereditary material. RNA (ribonucleic acid) carries instructions from DNA to make proteins. Proteins do most of the work in cells. Genetic variation arises from mutations (changes in DNA sequence) and recombination (shuffling of genes during reproduction). Genetics is a rapidly advancing field. CRISPR-Cas9 is a revolutionary gene-editing tool (Nobel Prize 2020). Ethical issues include genetic engineering of humans, GMOs, and genetic privacy. The study of genetics began with Mendel's laws. He published his work in 1866, but it was ignored until 1900. The word "gene" was coined by Wilhelm Johannsen in 1909. The structure of DNA was discovered by Watson and Crick in 1953 (using data from Rosalind Franklin).

Mendel used pea plants! He chose seven traits: seed shape (round/wrinkled), seed color (yellow/green), pod shape (inflated/constricted), pod color (green/yellow), flower color (purple/white), flower position (axial/terminal), and stem height (tall/short). He crossed plants with different traits (e.g., purple flowers with white flowers) and observed the offspring. He discovered the law of segregation (each trait is determined by two factors (alleles) that separate during gamete formation) and the law of independent assortment (genes for different traits are inherited independently). Mendel's work was published in 1866 but was largely ignored. In 1900, three scientists (Hugo de Vries, Carl Correns, Erich von Tschermak) independently rediscovered Mendel's laws. Mendel's work became the foundation of modern genetics. The "Mendelian ratio" (3:1) is the ratio of dominant to recessive traits in the F2 generation of a monohybrid cross. Mendel became an abbot later in life and stopped his experiments due to administrative duties. He died in 1884, unaware that his work would revolutionize biology. The term "Mendelian inheritance" is used for traits controlled by a single gene with dominant and recessive alleles. Not all traits follow Mendelian inheritance (incomplete dominance, codominance, polygenic traits).

DNA is a double helix! The two strands are antiparallel (run in opposite directions). DNA is found in the nucleus of eukaryotic cells (and in mitochondria and chloroplasts). The human genome contains about 3 billion base pairs of DNA. If you stretched out the DNA from one human cell, it would be about 6 feet (2 meters) long! All the DNA in your body (about 37 trillion cells) would stretch from Earth to the Sun and back hundreds of times. DNA replication (copying) occurs during cell division. It is semi-conservative: each new DNA molecule has one old strand and one new strand. The genetic code is read in groups of three bases (codons). Each codon specifies an amino acid (or start/stop). There are 20 amino acids that make up proteins. The DNA sequence is transcribed into RNA, which is translated into protein (central dogma). DNA can be damaged by radiation, chemicals, and replication errors. Cells have DNA repair mechanisms. Mutations (changes in DNA sequence) can be harmful, neutral, or beneficial. DNA fingerprinting (DNA profiling) is used in forensics (identifying suspects), paternity testing, and genealogy. The structure of DNA is a double helix. The discovery of DNA's structure was one of the most important scientific breakthroughs of the 20th century. Rosalind Franklin's X-ray diffraction image (Photo 51) was critical to the discovery. She died in 1958; Nobel Prizes are not awarded posthumously, so she was not honored. Watson and Crick won the Nobel Prize in 1962.

A gene is a segment of DNA! Genes are the instructions for making proteins (which do most of the work in cells). Some genes code for RNA molecules (rRNA, tRNA) that are not translated into protein. Alleles are different versions of the same gene (e.g., gene for eye color has alleles for blue, brown, green). Humans have about 20,000-25,000 genes, but only about 1.5% of DNA codes for proteins; the rest is non-coding DNA (some regulates gene expression, some is junk DNA). Chromosomes are visible under a light microscope during cell division. Humans have 46 chromosomes (23 pairs). The 23rd pair determines sex: XX = female, XY = male. The X chromosome carries about 1,000 genes; the Y chromosome carries about 70 genes. The Y chromosome contains the SRY gene, which triggers male development. Chromosomal abnormalities (Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter syndrome (XXY)) can cause genetic disorders. The Human Genome Project (1990-2003) mapped all human genes. Genetic testing can identify mutations (e.g., BRCA1/BRCA2 for breast cancer risk). Gene therapy aims to treat genetic diseases by replacing defective genes. Epigenetics (changes in gene expression without changing DNA sequence) is an emerging field. The study of chromosomes is cytogenetics. The word "chromosome" means "colored body" (they stain with dyes). The number of chromosomes varies among species: fruit flies have 8, dogs have 78, potatoes have 48, some ferns have over 1,000. Humans have 46. The first human chromosome to be sequenced was chromosome 22 in 1999.

The dominant allele is expressed! Dominant alleles are denoted by capital letters (e.g., B for brown eyes); recessive alleles by lowercase letters (e.g., b for blue eyes). Genotype refers to the genetic makeup (e.g., BB, Bb, bb). Phenotype refers to the observable traits (e.g., brown eyes, blue eyes). Homozygous means having two identical alleles (BB or bb). Heterozygous means having two different alleles (Bb). The Punnett square is a tool to predict the probability of offspring genotypes. In a monohybrid cross (Bb × Bb), the genotypic ratio is 1:2:1 (BB:Bb:bb) and the phenotypic ratio is 3:1 (brown:blue). Not all traits follow simple dominant-recessive inheritance. Incomplete dominance (e.g., snapdragons: red × white = pink), codominance (e.g., AB blood type), polygenic traits (e.g., height, skin color) are controlled by multiple genes. Sex-linked traits (e.g., hemophilia, red-green color blindness) are carried on the X chromosome. They are more common in males (who have only one X). The human genome contains about 20,000-25,000 genes. Many human traits are polygenic (influenced by environment as well). Dominant disorders include Huntington's disease (caused by dominant allele; onset in middle age). Recessive disorders include cystic fibrosis and sickle cell anemia. Carriers are heterozygous individuals who do not express the recessive trait but can pass it to offspring. Genetic testing can identify carriers. The Hardy-Weinberg principle describes allele frequencies in populations (in the absence of evolution).

Punnett squares are useful for predicting inheritance! In a monohybrid cross (Bb × Bb), the Punnett square has four boxes. The possible genotypes: BB (1), Bb (2), bb (1). Probability of bb = 1/4 = 25%. In a test cross (Bb × bb), the probability of bb is 50%. In a cross between two homozygous parents (BB × bb), all offspring are Bb (100% heterozygous). Punnett squares can also be used for dihybrid crosses (two traits). Mendel's law of independent assortment states that genes for different traits are inherited independently. In a dihybrid cross (RrYy × RrYy), the phenotypic ratio is 9:3:3:1 (9 round-yellow, 3 round-green, 3 wrinkled-yellow, 1 wrinkled-green). Punnett squares can be used to predict the probability of genetic disorders. For example, if both parents are carriers of a recessive disorder (Bb × Bb), the probability that a child will have the disorder is 25%. The probability that a child will be a carrier is 50%. Punnett squares assume random fertilization and independent assortment. They are a simplified model (real inheritance is more complex). Punnett squares are a valuable teaching tool. They are named after Reginald Punnett, who was a geneticist and co-discoverer of the Hardy-Weinberg principle. The first Punnett square was published in 1905. Punnett squares can be extended to three or more traits (though they become large). For a trihybrid cross (AaBbCc × AaBbCc), the Punnett square would have 64 boxes. Probability rules can be used instead: multiply probabilities (product rule). For example, probability of aabbcc = (1/4) × (1/4) × (1/4) = 1/64. The sum rule is used for mutually exclusive outcomes.

Mutations are changes in DNA! Point mutations: substitution (one base replaced), insertion (extra base added), deletion (base removed). Frameshift mutations (insertion or deletion) change the reading frame and usually cause non-functional proteins. Chromosomal mutations: deletion, duplication, inversion, translocation (segments moved). Most mutations are neutral (occur in non-coding DNA). Some are harmful (cause genetic disorders). Some are beneficial (provide selective advantage). Sickle cell anemia is caused by a point mutation in the hemoglobin gene (substitution). The mutation also provides resistance to malaria (beneficial in regions where malaria is common). This is an example of balancing selection. The BRCA1 and BRCA2 gene mutations increase the risk of breast and ovarian cancer. Mutations can be somatic (occur in body cells, not passed to offspring) or germline (occur in egg or sperm, heritable). Somatic mutations can cause cancer (when they occur in tumor suppressor genes or oncogenes). The mutation rate in humans is about 1 in 100 million base pairs per generation (about 30-70 new mutations per individual). Most new mutations are neutral or harmful. Beneficial mutations are rare but are the raw material for evolution. The Ames test is used to test chemicals for mutagenicity (ability to cause mutations). Mutagens include UV radiation (causes thymine dimers), ionizing radiation (X-rays, gamma rays), and chemicals (benzene, formaldehyde, tobacco smoke). The BRCA test identifies mutations in cancer susceptibility genes. Genetic testing can determine if a person carries harmful mutations. Genetic counseling helps individuals understand their risks. Mutation is the ultimate source of all genetic variation. Without mutation, evolution would not be possible. However, most mutations are harmful, which is why cells have DNA repair mechanisms. The human genome has over 100 DNA repair genes. Xeroderma pigmentosum is a disorder where DNA repair is defective; individuals are highly susceptible to skin cancer. The study of mutations is important for understanding evolution, cancer, and genetic diseases.

Charpentier and Doudna won the Nobel Prize for CRISPR-Cas9! CRISPR-Cas9 is faster, cheaper, and more precise than previous gene-editing methods. It uses an RNA guide to direct the Cas9 enzyme to a specific DNA sequence, where it cuts the DNA. The cell's repair machinery then repairs the cut, allowing scientists to insert, delete, or replace genes. Applications: creating genetically modified organisms (GMOs), correcting genetic diseases (e.g., sickle cell anemia, cystic fibrosis), engineering crops (drought resistance, pest resistance), and creating gene drives (spreading genes through populations). Ethical concerns: human germline editing (editing embryos) could affect future generations. The first gene-edited babies (twin girls) were born in China in 2018 (He Jiankui) – widely condemned as unethical. Many countries have banned human germline editing. Somatic gene therapy (editing only the patient's cells) is more widely accepted. Gene therapy for sickle cell disease and other disorders is in clinical trials. CRISPR can also be used to create animal models of human diseases. The CRISPR patent dispute (between UC Berkeley and the Broad Institute) involves millions of dollars. CRISPR can also be used for diagnostics (e.g., SHERLOCK, DETECTR to detect viruses like COVID-19). The future of genetic engineering is bright but must be regulated. Genetic modification of crops has been controversial. The first GMO (Flavr Savr tomato) was introduced in 1994. Most GMOs are engineered for herbicide resistance or insect resistance. The term "GMO" is often used in debates about food safety (most scientific organizations say GMOs are safe). Golden rice is engineered to produce beta-carotene (vitamin A) to prevent blindness in developing countries. Ethical guidelines for genetic engineering are evolving. The Asilomar Conference (1975) established safety guidelines for recombinant DNA research. The National Academies of Science, Engineering, and Medicine have issued reports on gene editing. Public engagement is important for responsible innovation.

Males are more affected by X-linked disorders! For example, red-green color blindness affects about 8% of males but only 0.5% of females. Hemophilia (the "royal disease") affected many descendants of Queen Victoria (who was a carrier). Her son Leopold had hemophilia; her daughters Alice and Beatrice were carriers. Tsarevich Alexei (son of Tsar Nicholas II) had hemophilia. Y-linked disorders are rare and only affect males (e.g., male infertility). X-linked dominant disorders (like Rett syndrome) are more common in females. Sex-limited traits are expressed only in one sex (e.g., male pattern baldness, milk production). Sex-influenced traits are expressed differently in males and females (e.g., pattern baldness is dominant in males but recessive in females). Genetic counseling can help families understand the risk of passing on genetic disorders. Prenatal testing (amniocentesis, chorionic villus sampling) can detect some genetic disorders. Newborn screening tests for disorders like phenylketonuria (PKU) can prevent disability. Gene therapy for X-linked disorders is being researched. Hemophilia is treated with clotting factor replacement therapy. Duchenne muscular dystrophy (caused by mutation in the dystrophin gene) is one of the most common genetic disorders. The Muscular Dystrophy Association funds research. The history of hemophilia in European royalty (the "royal disease") is a famous example of X-linked inheritance. Queen Victoria was likely a spontaneous mutant (neither of her parents had hemophilia).

Genetic variation is essential for evolution! Without variation, natural selection would have nothing to act on. The Hardy-Weinberg principle describes conditions under which allele frequencies remain constant (no evolution). Evolution occurs when allele frequencies change over time. Mechanisms of evolution: natural selection, genetic drift (random changes in small populations), gene flow (migration), and mutation. Sexual reproduction (meiosis) generates variation through independent assortment and crossing over. Crossing over (exchange of genetic material between homologous chromosomes) creates new combinations of alleles. Independent assortment (random alignment of homologous chromosomes at metaphase I) produces gametes with different combinations of chromosomes. Fertilization (random union of gametes) adds variation. Genetic variation is measured by heterozygosity (proportion of heterozygous loci). Loss of genetic variation reduces a population's ability to adapt to environmental changes (e.g., inbreeding depression). Conservation genetics studies genetic variation in endangered species. Cheetahs have very low genetic variation (they are nearly identical genetically). This makes them vulnerable to disease. The human genome is about 99.9% identical between any two individuals (the 0.1% difference accounts for all our variation). Most human genetic variation is found within populations, not between populations. This is why race is a social construct, not a biological category. The study of human genetic variation has medical applications (e.g., pharmacogenomics: tailoring drugs to an individual's genotype). Genetic variation is also important in agriculture (crop and livestock breeding). The Green Revolution used genetic variation to develop high-yield crops. The future of genetics includes personalized medicine, gene therapy, and synthetic biology. Understanding genetics helps us understand ourselves and the world around us. You have learned about DNA, genes, inheritance, mutations, and genetic engineering. You are now a genetics expert!

Rain is a type of weather! Weather includes rain, snow, sunshine, clouds, wind, fog, thunderstorms, and hail. Weather happens in the lowest part of our atmosphere, called the troposphere. Weather affects what clothes you wear, what activities you can do, and how you feel. A weather scientist is called a meteorologist – they study weather and make forecasts!

You should wear sunscreen on sunny days! The sun gives off ultraviolet (UV) rays that can hurt your skin and cause sunburn. Sunscreen blocks these rays. You should also wear a hat and sunglasses to protect your eyes. Even on cloudy days, UV rays can still reach your skin, so many doctors recommend wearing sunscreen every day when you go outside.

Precipitation is the name for water that falls from clouds! Rain is liquid precipitation. Snow is frozen precipitation that falls as ice crystals. Hail is frozen rain that forms in big thunderstorms. Sleet is rain that freezes before it hits the ground. All of these are types of precipitation. The water cycle never stops – water is always moving between the ground, the air, and back again!

Cumulus clouds are the fluffy white clouds! They usually mean good weather. When you see them, it is probably a nice day. But if they grow very tall and dark, they can become cumulonimbus clouds – thunderstorm clouds! Cloud watching is a fun way to predict the weather. Low, dark clouds often mean rain is coming. High, wispy clouds can mean a storm is on the way in a day or two.

When the temperature rises above freezing, snow melts and turns into water! That is why you see puddles on the ground after a snowstorm warms up. Snow is important because it provides water for rivers and lakes when it melts in the spring. Some animals, like polar bears and arctic foxes, have white fur to blend in with the snow – that is called camouflage.

An anemometer measures wind speed! The cups catch the wind and spin around. The faster they spin, the stronger the wind. Wind can be fun – it helps you fly a kite or sail a boat. But wind can also be dangerous – very strong winds from hurricanes and tornadoes can knock down trees and damage buildings. Wind is also a source of clean energy – wind turbines use the wind to make electricity!

You should go inside a safe building during a thunderstorm! Stay away from windows, and do not use phones that are plugged into the wall, computers, or other electronics. Do not take a bath or shower during a thunderstorm – water can conduct electricity. If you are outside and cannot get indoors, never stand under a tall tree. Crouch down low with your feet together. Lightning is beautiful but very dangerous – always respect it!

Spring is the season of new growth! After cold winter, the weather warms up. Snow melts, rain falls, and plants start growing again. Trees grow new leaves, flowers like tulips and daffodils bloom, and animals come out of hibernation. Birds build nests and lay eggs. Spring is a time of renewal and life. Summer is hot, autumn has falling leaves, and winter is cold with snow in many places.

A thermometer measures temperature! Most thermometers have a liquid inside that expands (gets bigger) when it is hot and contracts (gets smaller) when it is cold. You read the number next to the top of the liquid. Temperature is measured in degrees Fahrenheit (°F) in the United States and degrees Celsius (°C) in most other countries. Water freezes at 32°F (0°C) and boils at 212°F (100°C). Normal body temperature is about 98.6°F (37°C).

You should go to a basement or a small interior room (like a closet or bathroom) on the lowest floor! Stay away from windows. Cover your head with a blanket, pillow, or your arms. If you are in a mobile home, get out and go to a sturdy building. For hurricanes, you may need to evacuate (leave the area) if officials tell you to. Always listen to weather warnings and follow what your parents and teachers say. Being prepared keeps you safe!

A cat is a living thing! Living things (also called organisms) share five characteristics: (1) they need food, water, and air, (2) they grow and change, (3) they can reproduce (make babies), (4) they respond to their environment, and (5) they have cells. Rocks, toys, and water are non-living – they do not need food or water and they do not grow!

A teddy bear is non-living! Even though it looks like a bear, it is just a stuffed toy. It does not need food, water, or air. It does not grow or make baby teddy bears. Understanding the difference between living and non-living helps scientists study our world. Living things are called "biotic" and non-living things are called "abiotic."

A squirrel needs air to live! All animals need oxygen from the air to turn food into energy. Fish get oxygen from water using gills, but they still need oxygen. Plants need carbon dioxide from the air to make food through photosynthesis. Even tiny bacteria need air! Air is essential for all life on Earth.

A sunflower needs water to live! Plants use water to carry nutrients from the soil to their leaves and to make food through photosynthesis. Animals need water for digestion, to remove waste, and to keep their bodies at the right temperature. All living things – from the smallest bug to the largest whale – need water. That is why scientists look for water on other planets – water means life might be possible!

A bean sprout is growing! Growth is one of the most important signs of life. Living things get bigger and more complex as they get older. Plants grow from seeds to seedlings to full plants. Animals grow from babies to adults. This growth takes energy from food and water. Non-living things never grow on their own – they only change if someone changes them.

A chicken can reproduce! Reproduction is the process of creating new living things of the same kind. Some animals give birth to live babies (like humans, dogs, and cats). Some animals lay eggs (like chickens, fish, and frogs). Plants make seeds that grow into new plants. This is how life continues from generation to generation. A toy cannot make new toys – that is one reason it is non-living!

You pull your hand away from a hot stove! That is a reflex – your body responds to danger without you even thinking about it. Living things respond to light, temperature, sound, touch, and other changes. Plants grow toward sunlight. Worms dig away from light. Flowers close at night. These responses help living things survive. A rock does not move when you touch it – it has no response!

A cotton t-shirt was once alive! Cotton comes from cotton plants. The plant grew, needed water and sunlight, and was alive. After it was harvested and made into fabric, the shirt is no longer alive. Other examples: paper comes from trees, wool sweaters come from sheep, and wooden furniture comes from trees. Even gasoline comes from ancient plants and animals that died millions of years ago!

Fire is NOT alive! Even though fire needs oxygen and can spread, it is just a chemical reaction called combustion. Fire does not have cells – all living things are made of cells. Fire cannot reproduce (make baby fires) – it can only spread if there is more fuel. Fire does not grow from the inside like a living thing; it just burns more material. So fire is non-living. That is an important science distinction!

There are 3 living things: the butterfly (an insect), the mushroom (a fungus – mushrooms are alive even though they are not plants or animals), and the oak tree (a plant). The bicycle, cloud, and television are non-living. You are getting very good at telling the difference! Remember: if it needs food/water/air, grows, responds, reproduces, and has cells – it is alive. Great job, scientist!

Evolution is the correct term! Evolution explains how life has changed from common ancestors over billions of years. Evidence for evolution includes the fossil record (transitional fossils like Tiktaalik, Archaeopteryx), comparative anatomy (homologous structures like the forelimbs of mammals, birds, and reptiles), comparative embryology (similar embryos in related species), molecular biology (DNA similarities), and direct observation (antibiotic resistance in bacteria, beak size changes in finches). Evolution is both a fact (the pattern – life has changed over time) and a theory (the mechanism – how it happens). The word "theory" in science means a well-substantiated explanation supported by evidence. The theory of evolution is one of the most robust theories in science. Misconceptions: evolution is not "just a theory" (meaning guess), humans did not evolve from monkeys (we share a common ancestor), and evolution does not have a goal or direction (it is not progressive).

Darwin's finches are the correct answer! They became a classic example of adaptive radiation (evolution of multiple species from a common ancestor). The ground finch has a thick beak for cracking seeds; the tree finch has a slender beak for insects; the cactus finch has a long beak for cactus flowers. During droughts, finches with larger beaks survive better because they can eat harder seeds (observed by Peter and Rosemary Grant in the 1970s-2000s). This is direct observation of natural selection in action. Darwin also studied tortoises (different shell shapes on different islands), mockingbirds, and marine iguanas. He was not the first to propose evolution (Lamarck did earlier), but he was the first to propose a plausible mechanism (natural selection). Darwin was a naturalist, not a biologist by training (he studied medicine and theology). He was a close friend of botanist Joseph Dalton Hooker and geologist Charles Lyell, whose "Principles of Geology" influenced him.

Natural selection is the mechanism of evolution! The process requires three conditions: (1) variation (individuals in a population differ in their traits), (2) heritability (traits are passed from parents to offspring), and (3) differential reproductive success (individuals with certain traits leave more offspring). For example, peppered moths in England: before the Industrial Revolution, light-colored moths were more common because they were camouflaged on light-colored lichen. After pollution darkened the trees, dark-colored moths became more common because they were better hidden from predators. This is a classic example of natural selection in action. Natural selection does not create new traits; it selects from existing variation. Mutation (random changes in DNA) is the ultimate source of new variation. Natural selection can act on any heritable trait: morphology (body shape), physiology (ability to digest certain foods), behavior (mating calls, migration patterns). Not all evolution is due to natural selection; genetic drift (random changes) can also cause evolution, especially in small populations.

Homologous structures are the correct answer! They provide strong evidence for common ancestry. Vestigial structures (appendix in humans, pelvis in whales, wings in flightless birds) are remnants of organs that had a function in ancestors but are reduced or unused now. The human appendix was once part of a larger cecum for digesting cellulose; it is now vestigial but may have some immune function. Whale pelvises are tiny bones embedded in muscle – they are remnants of land-dwelling ancestors with hind legs. DNA evidence is even more powerful: humans share about 98-99% of DNA with chimpanzees, about 85% with mice, about 60% with fruit flies, and about 50% with bananas. The differences accumulate over time; the more closely related two species are, the more similar their DNA. Pseudogenes (non-functional copies of genes) also show common ancestry. For example, humans and other primates share a pseudogene for the enzyme L-gulonolactone oxidase, which is needed to make vitamin C; it is broken (non-functional) in both. This could only happen if they inherited the broken gene from a common ancestor.

Speciation is the correct term! The Galápagos finches are an example of adaptive radiation (rapid speciation from a common ancestor). The Hawaiian honeycreepers (birds) and cichlid fish in African lakes are other classic examples. Reproductive isolation mechanisms include prezygotic barriers (habitat isolation, temporal isolation, behavioral isolation, mechanical isolation, gametic isolation) and postzygotic barriers (reduced hybrid viability, reduced hybrid fertility, hybrid breakdown). The concept of a species is complex; there are over 20 different species concepts (morphological, biological, phylogenetic, ecological). The biological species concept does not apply to asexual organisms (bacteria, many plants) or fossils. Speciation can take thousands to millions of years; however, some rapid speciation has been observed in plants (polyploidy can create a new species in one generation). Polyploidy (having more than two sets of chromosomes) is common in plants (wheat, cotton, strawberries are polyploids). Polyploidy is rare in animals because it often disrupts sex determination.

Antibiotic resistance is evolution in action! Alexander Fleming (who discovered penicillin) warned about this in 1945. Bacteria can also exchange resistance genes through plasmids (horizontal gene transfer), spreading resistance between species. This is why finishing the full course of antibiotics is important: stopping early may leave the most resistant bacteria alive. Overuse of antibiotics in agriculture (to promote growth in livestock) is a major contributor. New antibiotics are being developed, but bacteria evolve faster than we can invent new drugs. Other examples of observed evolution: pesticide resistance in insects (e.g., DDT resistance in mosquitoes), herbicide resistance in weeds (e.g., glyphosate-resistant pigweed), and heavy metal resistance in plants. The evolution of resistance is a classic demonstration of natural selection in real time. Some bacteria have evolved resistance to multiple antibiotics (superbugs). Methicillin-resistant Staphylococcus aureus (MRSA) is a common superbug in hospitals. The World Health Organization (WHO) has declared antibiotic resistance one of the top ten global public health threats.

Tiktaalik is a famous transitional fossil! It lived about 375 million years ago. Its fins had wrist-like bones that could flex and support weight, suggesting it could prop itself up in shallow water. This was a crucial step in the evolution of vertebrates from water to land. Archaeopteryx (about 150 million years ago) is another famous transitional fossil; it had teeth, a long bony tail, and claws on its wings (like dinosaurs) but also feathers and wings for flight (like birds). The fossil record is incomplete but growing; many transitional fossils have been discovered (Ambulocetus – "walking whale" – transitional between land mammals and whales; Pakicetus – early whale with legs). Critics of evolution often point to "missing links," but every fossil discovery fills a gap and creates new gaps. The theory predicts that transitional forms should exist, and they have been found. The fossil record also shows patterns like mass extinctions (e.g., the asteroid that killed the non-avian dinosaurs 66 million years ago) and adaptive radiation (rapid diversification after extinctions).

Homo habilis was the first tool-maker! They made simple stone flakes for cutting meat and scraping hides. Australopithecus (including Lucy, lived about 3.2 million years ago) walked upright but had a small brain (about 400-500 cm³). Homo habilis had a larger brain (about 600 cm³). Homo erectus had an even larger brain (about 900-1,000 cm³) and was the first to control fire, leave Africa, and use more advanced tools (Acheulean hand axes). Homo erectus may have been the ancestor of Homo heidelbergensis, which led to Homo sapiens (modern humans) and Neanderthals. The earliest Homo sapiens fossils (300,000 years old) were found in Morocco (Jebel Irhoud). Neanderthals (Homo neanderthalensis) lived in Europe and Asia until about 40,000 years ago. They had larger brains than modern humans, made tools, buried their dead, and may have had language. Modern humans outside Africa have about 1-4% Neanderthal DNA. Denisovans are known only from a finger bone and teeth found in Denisova Cave, Siberia; their DNA is present in some modern humans (especially in Melanesia and Tibet). The Out of Africa theory proposes that modern humans evolved in Africa and spread to the rest of the world, replacing other hominids (with some interbreeding).

Genetic drift is the correct term! The bottleneck effect occurs when a population is drastically reduced (by a natural disaster, disease, or hunting), and the surviving population has different allele frequencies (by chance). The Northern elephant seal population was hunted to about 20 individuals in the 1890s; the current population of over 100,000 has very low genetic diversity. The founder effect occurs when a small group leaves to start a new population; the new population's gene pool reflects only that small group. The Amish population in Pennsylvania has a high frequency of Ellis-van Creveld syndrome (a form of dwarfism) because one of the founders carried the allele. Genetic drift can cause harmful alleles to become common or beneficial alleles to be lost by chance. In large populations, the effects of genetic drift are minimal. The neutral theory of molecular evolution (Motoo Kimura) proposes that most evolutionary change at the molecular level is due to genetic drift, not natural selection. Genetic drift is a major evolutionary mechanism, especially in small populations like endangered species.

Humans share a common ancestor with chimpanzees! Evolution does not claim that humans evolved from modern apes; it claims that humans and modern apes share a common ancestor. That common ancestor was neither a human nor a chimpanzee but an earlier ape species. Misconceptions often arise from the phrase "humans evolved from monkeys" – which is incorrect. Evolution is not progressive; it does not have a goal (it is not "trying" to make humans). Bacteria are just as "evolved" as humans; they have been evolving for the same amount of time (3.5 billion years). Evolution does not explain how life began (abiogenesis); it explains how life changed after it began. The theory of evolution is one of the most supported theories in science; the "theory" in "theory of evolution" means a well-substantiated explanation, not a guess (that would be a hypothesis). Individuals do not evolve; populations evolve over generations. Your children may have different traits than you because of evolution, but you yourself do not evolve during your lifetime (except through epigenetic changes, which are not heritable in the same way). Acceptance of evolution is nearly universal among scientists (about 99% of scientists accept evolution). Religious leaders (including the Catholic Church) accept evolution as compatible with faith (theistic evolution). The Catholic Church teaches that evolution is not incompatible with belief in God; the Church accepts the scientific consensus on evolution.

Mitochondria are the correct answer! The Krebs cycle and electron transport chain occur in the mitochondria. So the answer is mitochondria.

Werner Heisenberg is the answer! He won the Nobel Prize in Physics in 1932. So the answer is Werner Heisenberg.

Noble gases are unreactive! So the answer is they are very unreactive.

Chloroplasts are the site of photosynthesis! So the answer is chloroplasts.

Inertia is the answer! So the answer is inertia.

Thymine pairs with adenine! So the answer is thymine.

Volume decreases! So the answer is volume decreases.

Ribosomes are found in the cytoplasm and on the rough ER! So the answer is on the rough endoplasmic reticulum.

About 8 minutes! So the answer is about 8 minutes.

Hans Krebs is the answer! So the answer is Hans Krebs.

Engineering is the correct answer! The term STEM was coined by the National Science Foundation (NSF) in the 1990s. STEAM adds Arts (creativity and design). STEM careers include software developer, data scientist, engineer, biochemist, statistician, and many others. Women and minorities are underrepresented in many STEM fields, leading to efforts to increase diversity (STEM equity). The U.S. government has designated STEM fields as critical for economic competitiveness. Many countries offer special visas for STEM workers (e.g., H-1B visa in the US). STEM careers often require a bachelor's degree or higher, but some technicians require an associate degree or certificate. The median salary for STEM workers is about double that of non-STEM workers. STEM fields are projected to grow faster than non-STEM fields (about 10% growth for STEM vs. 5% for non-STEM from 2020-2030). The "T" in STEM (Technology) includes computer science, information technology, and cybersecurity. The "M" (Mathematics) includes statistics, data science, and applied mathematics. STEM education in K-12 emphasizes hands-on learning, problem-solving, and critical thinking. Project-based learning (PBL) is common in STEM classes. STEM schools and magnet schools focus on STEM subjects. After-school programs (robotics clubs, coding clubs) encourage STEM engagement. Competitions like FIRST Robotics, Science Olympiad, and MathCounts promote STEM skills. STEM careers are often team-oriented (interdisciplinary). Many STEM professionals work in research and development (R&D). The National Academy of Sciences, Engineering, and Medicine provides guidance on STEM policy.

Software developers write code! They turn ideas into functional software. Specializations: front-end (user interface), back-end (server-side), full-stack (both), mobile app developer (iOS, Android), game developer (video games), embedded systems (software for devices). The median annual salary for software developers in the US is about $110,000 (2022). Many software developers work remotely. The field has a low unemployment rate and high job satisfaction. Programming languages: Python (data science, AI, web), JavaScript (web, apps), Java (enterprise, Android), C++ (games, systems), C# (Windows, games). The software development life cycle (SDLC) includes planning, analysis, design, implementation, testing, deployment, and maintenance. Agile methodologies (Scrum, Kanban) are common. Version control systems (Git, GitHub) track changes. Open source software is developed collaboratively. Computer science degrees are common, but many developers are self-taught or have bootcamp certificates. Coding bootcamps (12-24 weeks) teach practical skills. The demand for developers is driven by digital transformation, AI, and cloud computing. The Bureau of Labor Statistics projects 22% growth for software developers from 2020-2030 (much faster than average). Women in tech (only about 20% of developers are women). Diversity initiatives aim to increase representation. Famous software developers: Bill Gates (Microsoft), Mark Zuckerberg (Facebook), Linus Torvalds (Linux), Ada Lovelace (first programmer). Software developers often work in teams, use issue-tracking software (Jira, Trello), and practice code reviews. Continuous integration/continuous deployment (CI/CD) automates testing and deployment. DevSecOps integrates security. The field is constantly evolving, requiring lifelong learning. Certification programs (AWS, Microsoft, Google) demonstrate expertise.

Mechanical engineers work on a wide range of products! Automotive engineering (cars, trucks), aerospace (aircraft, spacecraft), energy (wind turbines, solar panels, power plants), manufacturing (assembly lines, robotics), biomedical (prosthetics, implants), and consumer products (appliances, sports equipment). They use computer-aided design (CAD) software (SolidWorks, AutoCAD, CATIA). They analyze stress, heat transfer, fluid dynamics, and vibrations. The median annual salary for mechanical engineers is about $90,000 (US). A bachelor's degree in mechanical engineering is required; many go on to earn Professional Engineer (PE) license. Mechanical engineering is accredited by ABET. Co-ops and internships are important for gaining experience. Mechanical engineers often work in teams with electrical engineers, software engineers, and industrial designers. The field is diverse: HVAC (heating/ventilation/air conditioning), automotive, aerospace, robotics, manufacturing, nanotechnology, and renewable energy. Famous mechanical engineers: Nikola Tesla (AC motor, turbines), James Watt (steam engine), Henry Ford (automotive manufacturing). Mechanical engineers also work in sales, management, and consulting. The job outlook is steady (about 7% growth). Emerging fields: additive manufacturing (3D printing), robotics, biomechanics, and micro-electro-mechanical systems (MEMS). Mechanical engineers need strong math, physics, and problem-solving skills. Creativity and attention to detail are important. Many mechanical engineers work in research and development (R&D). The American Society of Mechanical Engineers (ASME) sets professional standards. Sustainability is increasingly important (energy efficiency, green design). The field is evolving with AI and automation. Mechanical engineers are often called "generalists" because they have a broad skill set. The median age of mechanical engineers is about 45 years (an aging workforce). Women make up about 10% of mechanical engineers (underrepresented). Diversity programs aim to increase representation. Professional societies (ASME, SAE) offer networking and continuing education.

Data scientists analyze data! They clean and process raw data, perform exploratory data analysis (EDA), build machine learning models, and communicate results to stakeholders. The field emerged in the 2000s with the explosion of big data. Data science requires skills in statistics (regression, hypothesis testing), programming (Python, R, SQL), data visualization (Tableau, Matplotlib), machine learning (scikit-learn, TensorFlow, PyTorch), and database management (SQL, NoSQL). A bachelor's degree in a quantitative field (CS, statistics, math, physics, engineering) is common; many data scientists have master's or PhD degrees. The median annual salary for data scientists in the US is about $100,000-$120,000. The job outlook is excellent (projected growth of 36% from 2021-2031). Data scientists are sometimes called "unicorns" because they need both technical and business skills. The data science pipeline: data collection → cleaning → exploration → modeling → evaluation → deployment → monitoring. Machine learning algorithms: linear regression, decision trees, random forests, neural networks, k-means clustering. Deep learning (neural networks with many layers) is used for image recognition, natural language processing, and speech recognition. Big data technologies (Hadoop, Spark) handle massive datasets. Cloud platforms (AWS, Google Cloud, Azure) provide data science tools. Ethics in data science: privacy, bias, fairness, transparency. The GDPR (General Data Protection Regulation) in Europe regulates data privacy. Data scientists must avoid p-hacking (manipulating data to get significant results). The field overlaps with business intelligence (BI), data analytics, and machine learning engineering. Many data scientists work in e-commerce (recommendation systems), finance (fraud detection), healthcare (disease prediction), and autonomous vehicles (computer vision). The Data Science profession has several subfields: business analytics, operations research, bioinformatics, and geospatial analysis. Data scientists often present findings using dashboards (Tableau, Power BI). The Open Source community provides many free tools (Python, R, Jupyter Notebooks).

Biomedical engineers design medical devices! Examples: prosthetic limbs, artificial heart valves, stents, insulin pumps, dialysis machines, and surgical robots. They also develop diagnostic equipment (MRI, CT, ultrasound, X-ray). Tissue engineering aims to grow replacement organs (3D bioprinting). Biomaterials (ceramics, polymers, metals) must be biocompatible. Rehabilitation engineering designs assistive devices for people with disabilities. Clinical engineers maintain hospital equipment. Regulatory affairs ensures devices meet FDA or CE standards. A bachelor's degree in biomedical engineering or a related field is required; many go on to graduate school. The median annual salary is about $92,000 (US). The field is growing faster than average (about 10% growth). Biomedical engineers often work in hospitals, research labs, medical device companies, or government agencies (FDA, NIH). Famous biomedical inventions: pacemaker (Wilson Greatbatch), artificial heart (Robert Jarvik), cochlear implant (Graeme Clark). Ethical issues: animal testing, clinical trials, access to medical devices. 3D printing is revolutionizing prosthetics (custom-fit, low-cost). Wearable devices (Fitbit, Apple Watch) monitor health metrics. Neuroengineering interfaces brain to computer (control prosthetic limbs with thoughts). Point-of-care diagnostics (rapid tests, portable devices) improve access in developing countries. Regenerative medicine (stem cells, 3D printed organs) is an emerging field. The Biomedical Engineering Society (BMES) is the professional organization. Biomedical engineers need strong math, physics, biology, and problem-solving skills. They often collaborate with physicians, scientists, and regulatory experts. The field is highly interdisciplinary. Graduate degrees (MS or PhD) are common for research positions. Licensing (PE) is not always required. Certifications (Certified Clinical Engineer) are available. Diversity in biomedical engineering is improving, but women remain underrepresented in some subfields. The aging population increases demand for medical devices and implants. Personalized medicine tailors treatments to individual patients (requires diagnostic devices). Telemedicine devices (remote monitoring) have grown during the COVID-19 pandemic. Startups are active in biomedical engineering (venture capital funding).

Chemists study matter! Subfields: organic chemistry (carbon compounds, including pharmaceuticals and plastics), inorganic chemistry (metals, minerals, catalysts), analytical chemistry (identifying and quantifying substances), physical chemistry (thermodynamics, kinetics), biochemistry (chemical processes in living organisms), and materials chemistry (polymers, nanomaterials). Chemists work in labs wearing safety goggles and gloves. A bachelor's degree in chemistry is required for entry-level positions; research positions often require a PhD. The median annual salary is about $80,000 (US). Job growth is average (about 6% growth). Chemists work in pharmaceutical companies, chemical manufacturers, environmental consulting firms, government labs (EPA, FDA), and academia. The American Chemical Society (ACS) is the professional organization. Famous chemists: Marie Curie (radioactivity), Linus Pauling (chemical bonding), Rosalind Franklin (DNA structure), Dorothy Hodgkin (X-ray crystallography). Chemistry is often called the "central science" because it connects physics, biology, and environmental science. Green chemistry aims to reduce hazardous substances (design safer chemicals, reduce waste). Analytical chemistry uses instruments like gas chromatography-mass spectrometry (GC-MS), high-performance liquid chromatography (HPLC), and NMR spectroscopy. Computational chemistry uses computer simulations. Chemists must maintain safety protocols (fume hoods, protective equipment). Some chemists work in forensics (crime labs) analyzing evidence. Quality control (QC) chemists test products for purity and consistency. Environmental chemists test water, soil, and air for pollutants. Medicinal chemists design and synthesize new drugs. Materials chemists develop polymers, nanomaterials, and composites. Chemistry is essential for energy storage (batteries, fuel cells) and renewable energy (solar cells, hydrogen production). The Chemistry field has many specializations. Careers in chemistry often require continuing education (to keep up with new techniques). Internships and co-ops are valuable for gaining experience. Women in chemistry have made significant contributions but remain underrepresented in leadership roles. The Nobel Prize in Chemistry is awarded annually. Chemistry outreach (Chemistry Olympiad, ChemClubs) encourages students. The chemical industry is a major sector of the economy (about $5 trillion worldwide).

Civil engineers design bridges! Other projects: highways, tunnels, airports, dams, levees, water treatment plants, wastewater treatment plants, pipelines, and buildings. Subfields: structural engineering (buildings, bridges), transportation engineering (roads, traffic systems), geotechnical engineering (soil, foundations), water resources engineering (dams, flood control), environmental engineering (pollution control), construction engineering (project management). Civil engineers use CAD software (AutoCAD, Civil 3D), structural analysis software (SAP2000, ETABS), and project management tools. A bachelor's degree in civil engineering is required; many earn Professional Engineer (PE) license. The median annual salary is about $88,000 (US). Job growth is average (about 8%). Civil engineers work for government agencies (DOT, EPA), consulting firms, construction companies, and utilities. Famous civil engineers: Gustave Eiffel (Eiffel Tower), John Roebling (Brooklyn Bridge), Joseph Bazalgette (London sewers). The American Society of Civil Engineers (ASCE) gives the U.S. infrastructure a grade (e.g., C- in 2021). Sustainability is increasingly important (green building, LEED certification). Civil engineers must consider environmental impacts (stormwater management, erosion control). Natural hazards (earthquakes, hurricanes, floods) require resilient design. Building codes and standards (ACI, AISC, ASCE 7) ensure safety. Civil engineers often work on large-scale projects requiring permits and environmental impact statements. Public-private partnerships (P3s) finance infrastructure. The Infrastructure Investment and Jobs Act (2021) provides funding for U.S. infrastructure. Civil engineers collaborate with architects, urban planners, and construction managers. Construction management involves scheduling, budgeting, and safety oversight. Geotechnical engineers investigate soil conditions (drilling, testing). Structural engineers design steel frames, concrete foundations, and earthquake-resistant structures. Water resources engineers design dams, reservoirs, and flood control systems. Environmental engineers design air and water pollution control systems. Civil engineering is a stable career with good demand. Professional licensing (PE) requires passing the FE (Fundamentals of Engineering) exam, gaining work experience (4 years), then passing the PE exam. Continuing education is required to maintain license. Women and minorities are underrepresented; diversity initiatives aim to increase representation. The civil engineering field is adapting to climate change (sea level rise, extreme weather).

Environmental scientists protect the environment! They work on issues like air and water quality, hazardous waste cleanup, environmental impact assessments, conservation, and renewable energy. Subfields: ecology, toxicology, climatology, hydrology, environmental chemistry, and environmental policy. Environmental scientists often work for government agencies (EPA, NOAA, state DEP), consulting firms, non-profits (Nature Conservancy, WWF), or industry. A bachelor's degree in environmental science or a related field is required; many have master's degrees. The median annual salary is about $73,000 (US). Job growth is average (about 8%). Environmental scientists use GIS (geographic information systems), remote sensing, and statistical software. They collect samples (water, soil, air) and analyze them in labs. Environmental impact statements (EIS) are required for major projects (NEPA). The Clean Air Act, Clean Water Act, and CERCLA (Superfund) are key U.S. laws. Climate change is a major focus: studying impacts (sea level rise, extreme weather) and mitigation (carbon capture, renewable energy). Environmental scientists also work on restoration ecology (rehabilitating damaged ecosystems). Brownfield remediation cleans up contaminated sites for redevelopment. Environmental scientists communicate findings to the public and policymakers. The field is interdisciplinary (biology, chemistry, geology, sociology). Environmental justice addresses disproportionate pollution burdens on low-income and minority communities. Sustainability science balances environmental, economic, and social goals. The United Nations Sustainable Development Goals (SDGs) include environmental targets. Environmental scientists often work outdoors (field sampling) and in labs. Fieldwork can be physically demanding. Some environmental scientists work in emergency response (oil spills, chemical releases). The Environmental Protection Agency (EPA) was established in 1970. Environmental science careers include: environmental consultant, conservation scientist, environmental health and safety officer, and climate change analyst. The job outlook is favorable as environmental regulations continue. Environmental scientists may earn certifications (Certified Environmental Scientist, LEED Green Associate). The American Academy of Environmental Engineers and Scientists is a professional organization. Many environmental scientists are involved in public outreach and environmental education. The field is growing due to climate change and sustainability concerns. The Green New Deal proposal would create many environmental jobs. Environmental scientists often work in interdisciplinary teams. The work can be rewarding (protecting the planet).

Statisticians analyze data! They use probability theory, regression analysis, hypothesis testing, Bayesian inference, and machine learning. They work in clinical trials (drug testing), public health (disease tracking), quality control (manufacturing), finance (risk analysis), and sports analytics (predicting performance). Statisticians often hold a master's or PhD in statistics or biostatistics. The median annual salary is about $92,000 (US). Job growth is faster than average (about 33% growth). Statisticians work for government agencies (Census Bureau, CDC, FDA), pharmaceutical companies, research institutes, and tech companies. Statistical software: R, Python, SAS, SPSS, Stata, MATLAB. The American Statistical Association (ASA) is the professional organization. Famous statisticians: Florence Nightingale (used statistics to improve military hygiene), Ronald Fisher (founder of modern statistics), William Gosset (Student's t-test), John Tukey (coined "bit" and "software", developed FFT). The Census Bureau employs statisticians to conduct the decennial census. The Bureau of Labor Statistics (BLS) publishes economic data. Statisticians ensure data quality and validity. They avoid bias and p-hacking. The field of "data science" overlaps with statistics. Actuaries (statisticians who assess risk) work in insurance. Biostatisticians work in medical research. Statistical literacy is important for understanding news reports (polls, studies). The reproducibility crisis in science (many published results cannot be replicated) has led to reforms (preregistration, open data). Statisticians also work in election polling (estimating voter preferences). Quality control statisticians use control charts to monitor manufacturing processes. The Six Sigma methodology uses statistical tools to reduce defects. Bayesian statistics incorporates prior knowledge. Statisticians must be careful with causality (correlation does not imply causation). Many statisticians work in academia (teaching and research). The demand for statisticians is driven by big data and evidence-based decision-making. Statisticians often collaborate with domain experts (doctors, economists, engineers). The field of computational statistics uses simulations and resampling methods (bootstrap, Monte Carlo). Statistical computing is an essential skill. Statisticians also work in social science research. The American Statistical Association offers certifications (Graduate Statistician, Accredited Professional Statistician). Diversity in statistics is improving, but women and minorities remain underrepresented. The field is expected to grow rapidly due to increased data availability.

All of these skills are important! Math: algebra, calculus, statistics, geometry. Science: biology, chemistry, physics, earth science. Problem-solving: breaking down complex problems, systematic reasoning. Communication: writing reports, giving presentations, explaining technical concepts to non-experts. Other important skills: teamwork (working in interdisciplinary teams), critical thinking (evaluating evidence, avoiding bias), creativity (generating new ideas), adaptability (learning new technologies), and attention to detail (accuracy). STEM professionals often need to manage projects, budget resources, and meet deadlines. Ethical reasoning is important (safety, privacy, environmental impact). Leadership skills are needed for advancement. Many STEM jobs require collaboration (pair programming, design reviews, team meetings). Remote work has become common in many STEM fields. International collaboration is common (scientific conferences, shared data). Lifelong learning: taking courses, attending conferences, reading journals. Online learning platforms (Coursera, edX, Udacity) offer STEM courses. Professional certifications demonstrate expertise. Networking through professional societies (IEEE, ACM, ASME, ACS) is valuable. Mentoring is important for career development. STEM careers offer intellectual challenge, good pay, and opportunities to make a difference. The STEM pipeline includes K-12 education, undergraduate and graduate degrees, and continuing education. Underrepresented groups (women, minorities) may face barriers; diversity programs aim to increase participation. Hands-on experience (internships, co-ops, research) is highly valued. The demand for STEM professionals is high and growing. Some STEM careers do not require a four-year degree (e.g., computer programmer, technician, machinist). Vocational training and associate degrees can lead to STEM jobs (e.g., medical laboratory technician, wind turbine technician). The shortage of STEM workers is a concern in many countries. Immigration policies (H-1B visas) allow foreign STEM workers to fill gaps. Early exposure to STEM (science fairs, robotics clubs) encourages interest. STEM camps and afterschool programs support learning. Role models (diverse scientists and engineers) inspire students. You can prepare for STEM careers by taking challenging math and science courses, participating in extracurriculars, and seeking internships. STEM careers are rewarding and impactful. You can be a scientist, engineer, technologist, or mathematician and make a difference in the world!

Natural disasters include many events! Earthquakes occur when stress builds up along fault lines and is released as seismic waves. The Richter scale measures magnitude (energy released). Hurricanes (typhoons, cyclones) are large rotating storms that form over warm ocean water. Wildfires (forest fires) can be caused by lightning, human activity, or volcanic activity. Floods occur when water overflows onto normally dry land. Tornadoes are violently rotating columns of air. Volcanic eruptions release magma, ash, and gases. Tsunamis are giant waves caused by underwater earthquakes or landslides. Droughts are extended periods of abnormally low rainfall. Landslides are downslope movements of rock and soil. Heatwaves are extended periods of abnormally hot weather. Natural disasters are classified by their origin (geological, hydrological, meteorological, climatological, biological). Climate change is increasing the frequency and intensity of many natural disasters (heatwaves, droughts, floods, hurricanes). Understanding natural disasters can help us prepare and reduce risk. Early warning systems (seismic networks, weather satellites, tsunami buoys) save lives. Building codes, land-use planning, and evacuation plans reduce vulnerability. The United Nations Office for Disaster Risk Reduction (UNDRR) promotes disaster risk reduction. The Sendai Framework (2015-2030) sets global targets for reducing disaster risk.

The Richter scale measures magnitude! The moment magnitude scale (Mw) is now more commonly used for large earthquakes (it is more accurate). Earthquakes are caused by tectonic plate movement (convergent, divergent, transform boundaries). The San Andreas Fault in California is a transform boundary. Earthquakes can also be caused by volcanic activity. Aftershocks are smaller earthquakes that follow the main shock. Foreshocks may precede a main shock (not always). Seismographs record ground motion. P-waves (primary waves) are fastest; S-waves (secondary waves) are slower; surface waves cause the most damage. The 1906 San Francisco earthquake (magnitude 7.9) caused extensive fire damage. The 2010 Haiti earthquake (magnitude 7.0) killed over 200,000 people (poor building construction). The 2011 Tohoku earthquake (magnitude 9.0) triggered a tsunami and the Fukushima nuclear disaster. Earthquake early warning systems (e.g., ShakeAlert in California) can provide seconds to minutes of warning before shaking arrives (using faster P-waves). Building codes (seismic retrofitting, base isolation) reduce earthquake damage. Drop, cover, and hold on is the recommended safety procedure during an earthquake. Do not stand in doorways (old advice; modern doorways are not stronger). Earthquakes cannot be predicted, but hazard maps show risk. Seismic hazard maps are used to set building codes. The "Ring of Fire" (Pacific Ocean) has the most earthquakes and volcanoes. The USGS (United States Geological Survey) monitors earthquakes worldwide. The "Big One" refers to a hypothetical major earthquake on the San Andreas Fault.

The Saffir-Simpson scale measures hurricane intensity! Hurricane Katrina (2005) was Category 5 at peak intensity but Category 3 at landfall; it caused catastrophic storm surge and levee failures in New Orleans, killing over 1,800 people. Hurricane Maria (2017) devastated Puerto Rico. Hurricane Dorian (2019) stalled over the Bahamas with 185 mph winds. Hurricane Patricia (2015) had the strongest winds ever recorded (215 mph) in the Eastern Pacific. Hurricanes are called "typhoons" in the Northwest Pacific and "cyclones" in the Indian Ocean and South Pacific. The Atlantic hurricane season runs from June 1 to November 30 (peaks in September). Hurricanes form from tropical disturbances that organize into tropical depressions, then tropical storms (named), then hurricanes. The National Hurricane Center (NHC) issues forecasts. Storm surge (rising sea level) is the deadliest part of a hurricane (caused by low pressure and strong winds pushing water). Hurricane Harvey (2017) dumped over 60 inches (1.5 m) of rain in Texas (record). Climate change is increasing hurricane intensity (more Category 4 and 5 storms) and rainfall rates. Evacuation is the best protection. Hurricane hunters (NOAA) fly into hurricanes to collect data. The "cone of uncertainty" shows the probable path of the hurricane. Residents in hurricane-prone areas should have a plan, supplies, and know their evacuation zone. The deadliest Atlantic hurricane is the Great Hurricane of 1780 (20,000-27,500 deaths). The costliest is Katrina ($125 billion). The strongest (by pressure) is Wilma (2005) at 882 mb. Retired hurricane names (Katrina, Sandy, Harvey, Irma, Maria) are not reused. The World Meteorological Organization (WMO) maintains lists of names.