These are the words that come up most often across the site, in plain language. Each one has a short example and, where possible, a link to the article that explains it in full.
A
Abiogenesis
The process by which life first arose from non-living chemistry. It is a separate question from evolution, which explains how life changes once it exists.
Example: The Miller-Urey experiment tested whether simple building blocks of life could form from basic chemicals.
Read more: the origin of life
Adaptation
An inherited trait that helps an organism survive or reproduce in a particular environment.
Example: Fur color that makes an animal harder for predators to see can be an adaptation.
Read more: natural selection
Allele
One version of a gene. Different alleles of the same gene can lead to different traits.
Example: A population may have different alleles linked to different flower colors.
Read more: mutation
Allele frequency
How common one allele is in a population, often given as a percentage. Evolution can be described as a change in allele frequencies over generations.
Example: If a resistance allele goes from 1 percent to 40 percent of a bacterial population, its frequency has risen.
Read more: what is evolution?
Analogous structure
A feature that looks or works alike in two groups because they evolved it separately, not because they inherited it from a shared ancestor.
Example: Bird wings and insect wings both allow flight, but they are built in completely different ways.
Read more: convergent evolution and homology
Ancestor
An earlier organism or population from which later groups descended.
Example: Humans and chimpanzees share ancestors in the distant past.
Read more: did humans evolve from monkeys?
Antibiotic resistance
The ability of bacteria to survive a drug that used to kill them. It evolves when resistant bacteria survive treatment and multiply.
Example: Some staph infections no longer respond to common antibiotics such as methicillin.
Read more: antibiotic resistance
Antigenic drift
Small, gradual changes in the surface of a virus that make it harder for the immune system to recognize.
Example: Antigenic drift is the main reason the flu vaccine is updated every year.
Read more: virus evolution and vaccines
Artificial selection
Evolution guided by people, who choose which plants or animals get to breed.
Example: All dog breeds were shaped from wolf ancestors by people selecting the traits they wanted.
Read more: artificial selection
B
Bottleneck
A sharp drop in population size that leaves only a small, random sample of the original genetic variation.
Example: After a disaster wipes out most of a population, the survivors may carry only a few of the alleles that were once present.
Read more: genetic drift
C
Carbon-14 dating
A radiometric dating method that measures how much carbon-14 is left in once-living material. It works only for remains up to about 50,000 years old.
Example: Carbon-14 can date a mammoth bone from the last ice age, but not a dinosaur.
Read more: how we date fossils
Common ancestor
A single ancestral population from which two or more later groups descended.
Example: Humans and chimpanzees share a common ancestor that lived roughly 6 to 7 million years ago.
Read more: did humans evolve from monkeys?
Common ancestry
The idea that different species trace back to shared ancestors, so all life is related like one huge family tree.
Example: Similar DNA in related species supports common ancestry.
Read more: genetic evidence
Convergent evolution
When unrelated groups evolve similar traits because they face similar challenges.
Example: Sharks and dolphins both have streamlined bodies and fins, even though one is a fish and the other a mammal.
Read more: convergent evolution and homology
D
DNA
The molecule that carries inherited information in living things. It is passed from parents to offspring.
Example: Mutations are changes in DNA.
Read more: genetic evidence
Domestication
The long process of shaping wild plants or animals, through artificial selection, into forms that live and breed alongside people.
Example: Corn was domesticated from a wild Mexican grass called teosinte about 9,000 years ago.
Read more: artificial selection
E
Endogenous retrovirus
A leftover piece of viral DNA that became part of an animal’s genome and is now passed down through generations.
Example: Humans and chimpanzees carry many of the same viral leftovers in the same places in their DNA, which points to a shared ancestor.
Read more: genetic evidence
Evolution
Change in the inherited traits of a population over generations.
Example: A resistant form of bacteria becoming more common is evolution.
Read more: what is evolution?
Extinction
When every member of a species or lineage has died out.
Example: Most species that ever lived are now extinct and are known only from fossils.
Read more: fossil evidence
F
Fitness
How well an organism survives and reproduces in a particular environment, compared with others. In biology, it means leaving offspring, not being strong or athletic.
Example: A small, plain bird that raises many chicks has higher fitness than a large, bold one that raises none.
Read more: natural selection
Fossil
Preserved remains or traces of past life.
Example: Bones, footprints, and shells can all become fossils under the right conditions.
Read more: fossil evidence
Founder effect
What happens when a few individuals start a new population, so the new group carries only part of the original population’s variation.
Example: A handful of birds blown to a new island may start a population where one rare color becomes common.
Read more: genetic drift
G
Gene
A stretch of DNA that carries instructions affecting growth, structure, or function.
Example: Genes help influence traits such as blood proteins or eye development.
Read more: mutation
Gene flow
The movement of alleles from one population to another when individuals move and breed.
Example: When people from distant regions have children together, gene flow mixes variants that were once separated.
Read more: microevolution and macroevolution
Genetic drift
Change in a population caused by chance alone. It has the biggest effect in small populations.
Example: After a disaster, some genetic variants may become rare simply because the survivors were a random sample.
Read more: genetic drift
Genome
The complete set of DNA in an organism.
Example: Comparing the human and chimpanzee genomes shows how closely the two species are related.
Read more: genetic evidence
H
Half-life
The time it takes for half of a radioactive material to break down. It acts like a clock for dating rocks.
Example: Carbon-14 has a half-life of about 5,730 years.
Read more: how we date fossils
Hominin
Humans and the extinct species more closely related to us than to chimpanzees.
Example: Neanderthals, Homo erectus, and Australopithecus are all hominins.
Read more: human evolution timeline
Homologous structure
A feature shared by different groups because they inherited it from a common ancestor, even if it is now used in different ways.
Example: A human arm, a bat wing, and a whale flipper have the same basic set of bones.
Read more: convergent evolution and homology
Hybrid
The offspring of parents from two different populations or species.
Example: Some new plant species formed when two species produced a hybrid that then doubled its chromosomes.
Read more: microevolution and macroevolution
I
Index fossil
A fossil of a species that lived for a short time but over a wide area, so it helps match rock layers of the same age in different places.
Example: Finding the same index fossil in two distant cliffs tells geologists the layers formed at about the same time.
Read more: how we date fossils
Inherited trait
A trait that can be passed from parents to offspring through DNA.
Example: Coat color in many animals has an inherited component.
Read more: what is evolution?
L
Lineage
A line of descent from an ancestor through its descendants over time.
Example: The whale lineage can be traced back to land-dwelling mammals.
Read more: speciation
M
Macroevolution
Large-scale evolution seen over long periods, such as the origin of new species and larger groups.
Example: The change from land mammals to whales over millions of years is macroevolution.
Read more: microevolution and macroevolution
Microevolution
Small-scale evolution within a population, such as a change in how common an allele is over a few generations.
Example: Finch beaks becoming deeper after a drought is microevolution.
Read more: microevolution and macroevolution
Mutation
A change in DNA. Mutations are the original source of new inherited variation.
Example: A single DNA letter change is one kind of mutation.
Read more: mutation
N
Natural selection
The process by which inherited traits that help survival or reproduction become more common over generations.
Example: Better-camouflaged individuals may leave more offspring in a predator-rich environment.
Read more: natural selection
P
Phylogeny (evolutionary tree)
A branching diagram showing how groups of living things are related through common ancestors.
Example: A phylogeny shows that birds are more closely related to crocodiles than to lizards.
Read more: genetic evidence
Population
A group of the same species living in the same place at the same time. Populations, not individuals, are what evolve.
Example: All the frogs of one species in a wetland can form a population.
Read more: what is evolution?
R
Radiometric dating
Measuring the age of rocks using the steady breakdown of radioactive elements inside them.
Example: Volcanic ash layers above and below a fossil can be dated to tell how old the fossil is.
Read more: how we date fossils
Reproductive isolation
Anything that stops two populations from breeding together or producing fertile offspring.
Example: Two groups of flies that mate on different host plants may stop interbreeding over time.
Read more: speciation
S
Selection pressure
Any feature of the environment that affects which individuals survive and reproduce.
Example: An antibiotic is a strong selection pressure on bacteria.
Read more: natural selection
Sexual selection
A kind of natural selection where traits spread because they help individuals win mates, not because they help survival.
Example: Bright peacock tails attract mates even though they also make the birds easier for predators to spot.
Read more: natural selection
Speciation
The formation of new species when one lineage splits into two or more.
Example: Separated populations can become different enough over time to count as separate species.
Read more: speciation
Species
A group whose members can usually breed with one another and have fertile offspring.
Example: Different dog breeds are still part of one species.
Read more: speciation
T
Theory, scientific
A broad explanation of how nature works, supported by many observations and tests. It is much stronger than a guess.
Example: Evolution is a scientific theory, just like germ theory.
Read more: is evolution just a theory?
Transitional fossil
A fossil with a mix of older and newer traits that helps connect earlier and later groups.
Example: Tiktaalik had both fish-like fins and features linked to life on land.
Read more: are there no transitional fossils?
V
Variant
One version of a gene, a trait, or a virus that differs from others.
Example: The Omicron variant of SARS-CoV-2 spread quickly because of changes in its surface proteins.
Read more: virus evolution and vaccines
Variation
Differences among individuals in a population. Without variation, natural selection has nothing to act on.
Example: Members of a bird population may vary in beak size, color, or song.
Read more: natural selection
Vestigial structure
A body part that has lost most or all of its original function but is still inherited. Vestigial means reduced, not always useless.
Example: Whales still carry small hip bones left over from their four-legged ancestors.
Read more: vestigial structures
How to use this page
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