What evolution and natural selection are
Evolution is the change in the heritable traits of a population across generations. The key word is population. A single organism does not evolve during its life. Instead, the mix of genes carried by a whole group of organisms shifts from one generation to the next. Natural selection is the main mechanism that drives adaptive evolution. It is the process in which individuals with traits better suited to their environment tend to survive and reproduce more than individuals without those traits, so the helpful traits become more common over time. Charles Darwin described this mechanism in his 1859 book On the Origin of Species. The naturalist Alfred Russel Wallace reached the same idea independently, and the two first presented it together in 1858.
Individuals do not evolve, populations do
A rabbit born with thicker fur does not become more evolved during its life. It simply survives a cold winter better and leaves more offspring. Over many generations the thick-fur trait becomes more common in the population. Evolution is a change in the population, not a change inside one animal.
Darwin's logic: the four conditions
Natural selection follows from four observations about living things. When all four are true, the makeup of a population must change over generations. Nothing extra is needed.
| Condition | What it means | Example |
|---|---|---|
| Variation | Individuals in a population differ in their traits | Some beetles are green, some are brown |
| Heritability | Some of that variation is passed from parents to offspring through genes | Brown parents tend to have brown offspring |
| Overproduction | Populations produce more offspring than the environment can support, so there is competition | A frog lays thousands of eggs, but food and space are limited |
| Differential reproductive success | Individuals with certain traits survive and reproduce more than others | Brown beetles are eaten less by birds, so they leave more offspring |
The result is that helpful heritable traits become more common in the next generation. Repeat this over many generations and the population becomes better matched, or adapted, to its environment. A trait shaped this way is called an adaptation.
Mutation is random, selection is not
The variation that selection acts on comes from random events like mutation and the reshuffling of genes during reproduction. Those changes are not aimed at any goal. Natural selection is the part that is not random. The environment consistently favors whichever existing variants survive and reproduce best, so the outcome looks directed even though the raw variation is not.
Fitness and the types of selection
In biology, fitness does not mean strength or health in the everyday sense. Fitness means an individual's success at surviving and, above all, reproducing in its particular environment. An organism that lives a long time but leaves no offspring has a fitness of zero. The phrase survival of the fittest really means survival and reproduction of the best-matched, not survival of the strongest.
Fittest does not mean strongest or fastest
Fitness is always relative to a specific environment. A trait that boosts fitness in one place can lower it in another. Pale fur is an advantage in snow and a disadvantage in a dark forest. There is no single best organism, only organisms that fit their current surroundings well or poorly.
| Type of selection | What it favors | Effect on the population | Example |
|---|---|---|---|
| Directional | One extreme of a trait | Shifts the average toward that extreme | Bacteria surviving antibiotics, so resistance rises |
| Stabilizing | The middle, average trait | Reduces variation, narrows the range | Human birth weight, where very small and very large babies fare worse |
| Disruptive | Both extremes at once | Increases variation and can split a population | Two beak sizes favored while intermediate beaks lose out |
A related process is sexual selection, in which a trait spreads because it improves mating success rather than survival. The peacock's large tail makes it easier to spot by predators, yet it persists because peahens prefer it, so it raises reproductive success overall.
Other ways populations change
Natural selection is the mechanism behind adaptation, but it is not the only force that changes the genetic makeup of a population. A change in the frequency of alleles (versions of a gene) within a population over time is called microevolution. Four main mechanisms drive it.
| Mechanism | What it does | Notes |
|---|---|---|
| Mutation | A random change in DNA that creates new alleles | The original source of all new genetic variation |
| Gene flow | Movement of alleles between populations as individuals migrate and breed | Tends to make separate populations more alike |
| Genetic drift | Random change in allele frequencies from chance alone | Has a much larger effect in small populations |
| Natural selection | Non-random spread of alleles that improve fitness | The only mechanism that reliably produces adaptation |
Two special cases of genetic drift
The bottleneck effect happens when a disaster sharply shrinks a population, so the survivors carry only a chance sample of the original variation. The founder effect happens when a few individuals start a new, separate population and carry only part of the original gene pool. Both leave the new group with less genetic variation than the population it came from.
Evidence that populations change over time
Evolution is supported by several independent lines of evidence that all point to the same conclusion, that living things share common ancestors and have changed over long spans of time.
| Line of evidence | What it shows | Example |
|---|---|---|
| Fossil record | Species have changed over time, and transitional forms link older groups to newer ones | Tiktaalik shows features between fish and four-limbed animals |
| Homologous structures | Shared body plans point to a common ancestor even when the function differs | The same arm bones appear in a human arm, a whale flipper, and a bat wing |
| Vestigial structures | Reduced, largely non-functional parts are leftovers from ancestors | The tiny hip bones inside a whale and the human tailbone |
| Comparative embryology | Related species look alike in early development | Vertebrate embryos share features such as a tail and pharyngeal pouches early on |
| Molecular biology | All life shares DNA and the same genetic code, and closer relatives have more similar sequences | Human and chimpanzee DNA are extremely similar |
| Biogeography | The geographic spread of species matches shared ancestry and isolation | Island species most resemble mainland species from the nearest continent |
Homologous versus analogous
Homologous structures share the same underlying build from a common ancestor even if they do different jobs, like the forelimb bones of a human and a bat. Analogous structures do the same job but evolved separately and do not share that build, like the wing of an insect and the wing of a bird. Only homologous structures are evidence of common ancestry.
Evolution you can watch
Some evolution happens fast enough to observe directly. Bacteria evolving resistance to antibiotics, insects evolving resistance to pesticides, and the year-to-year shift in beak size in Galapagos finches during droughts are all measured cases of natural selection in action.
Speciation: how new species form
Speciation is the formation of a new species. In the biological species concept, a species is a group whose members can breed with one another and produce fertile offspring. A new species forms when one population becomes reproductively isolated from another, meaning they can no longer interbreed, and the two groups then accumulate their own genetic differences.
| Type | How isolation starts | Example |
|---|---|---|
| Allopatric speciation | A physical barrier separates two populations | A river or mountain range splits a population in two |
| Sympatric speciation | Groups split without a physical barrier, through differences such as timing, habitat, or mate choice | Insects that shift to feeding and mating on a different host plant |
Small changes within a population are called microevolution. The larger-scale patterns that appear over long time spans, such as the origin of entirely new species and groups, are called macroevolution. Macroevolution is the accumulated result of the same everyday mechanisms working over many generations.
Common misconceptions to avoid
Organisms do not develop traits because they need them
A giraffe's neck did not grow longer because giraffes stretched for high leaves and passed that stretching on. That idea, associated with Lamarck, is not how it works. Natural selection acts on variation that already exists. Giraffes with genes for longer necks reached more food, survived, and reproduced more, so those genes became common. Traits an animal acquires during its life, like bigger muscles from exercise, are not passed to offspring.
Evolution has no goal and no ladder
Evolution is not a march toward being more advanced, more complex, or more perfect. There is no final target. Bacteria are not less evolved than humans, just adapted to different conditions. Traits change to fit the current environment, and an environment that changes can favor simpler forms just as easily as complex ones.
Humans did not evolve from modern apes, and theory does not mean guess
Humans and chimpanzees share a common ancestor that lived millions of years ago. We did not descend from any living species of ape. Also, when scientists call evolution a theory, they do not mean a hunch. A scientific theory is a well-tested explanation supported by a large body of evidence, which is the strongest status an idea can have in science.
Try it yourself
beetle-color-generations.csvDownload beetle-color-generations.csv: https://restedteacher.com/practice-files/biology/beetle-color-generations.csv
Brown and green beetle counts from a sample of 200 beetles in a fictional population, taken at each of 5 generations.
Open it in a spreadsheet
Open it in Google Sheets or Excel. Add a column that divides brownbeetle_count by totalsampled to turn each generation's count into a frequency you can graph across generations.
1. A population of mice lives on dark soil. Most mice are brown, but a few are light tan. Hawks hunt them by sight. Predict how the coloring of the population will change over many generations, and name the four conditions of natural selection that make your prediction work.
2. A bird wing and an insect wing both allow flight, but their internal structure and origin are completely different. Are they homologous or analogous, and does that make them evidence of common ancestry?
3. A small group of lizards is blown onto a new island and starts the only population there. It happens to carry only a fraction of the genetic variation of the mainland population. Which mechanism of evolution does this describe?
4. Explain in one sentence why a very strong animal that never reproduces has a biological fitness of zero.
5. Using beetle-color-generations.csv, calculate the frequency (proportion) of brown beetles in Generation 1 and in Generation 5, using frequency = brownbeetle_count ÷ totalsampled.
6. What is the change in frequency of the brown phenotype from Generation 1 to Generation 5?
7. Based on the trend across all 5 generations and the four conditions of natural selection described earlier on this sheet, which coloring is being favored by selection? Is this an example of directional, stabilizing, or disruptive selection, and why?
8. In Generation 3, is green or brown the majority phenotype? Give the frequency of each to support your answer.