Evolution by Natural Selection
How species change over generations, the evidence that proves it, and the four mechanisms that drive allele frequencies up or down.
Primary sources: UC Berkeley Understanding Evolution · NCBI Molecular Biology of the Cell
01 · What is evolution?
A change in allele frequencies over time.
The modern definition of evolution is a change in allele frequencies in a population across generations. It is a population-level phenomenon — individual organisms do not evolve; populations do.
Charles Darwin proposed the mechanism of natural selection in On the Origin of Species (1859). The modern synthesis of the 1930s–40s unified Darwin's ideas with Mendelian genetics, giving us the framework still used today. See our natural selection guide for the four conditions required for selection to act.
The genetic basis of inheritance — alleles, genotypes and heredity — is covered in our DNA, genes and heredity guide.
02 · Four mechanisms
Selection is not the only driver.
Allele frequencies change through four processes. Natural selection is the only one that produces adaptation.
Natural selection
Heritable traits that increase survival and reproduction become more common over generations. The primary mechanism of adaptive evolution.
Genetic drift
Random fluctuations in allele frequencies. Most important in small populations; can fix or eliminate alleles by chance alone (bottleneck, founder effects).
Mutation
The source of all new genetic variation. Mutations are random with respect to fitness — selection then acts on whatever variation exists.
Gene flow
Movement of alleles between populations through migration and interbreeding. Tends to reduce genetic differences between populations.
03 · Evidence
Multiple independent lines of evidence.
Evolution is one of the most thoroughly evidenced theories in science. Key lines of evidence include:
- Fossil record — preserved remains show gradual morphological change over geological time. Transitional fossils (e.g. Tiktaalik linking fish to tetrapods, Archaeopteryx linking dinosaurs to birds) document major evolutionary transitions.
- Molecular biology — DNA and protein sequences can be compared across species. More closely related species have more similar sequences. Pseudogenes and shared endogenous retroviruses are powerful evidence of common descent.
- Comparative anatomy — homologous structures (same developmental origin, different functions) reveal shared ancestry. Analogous structures (similar function, different origin) show convergent evolution.
- Direct observation — antibiotic resistance in bacteria, beak changes in Galápagos finches, coat colour in Italian wall lizards have all been measured in real time. See our natural selection guide for specific examples.
04 · Speciation
When populations diverge into species.
Speciation occurs when populations become reproductively isolated — they can no longer interbreed to produce fertile offspring. The most common route is allopatric speciation: a geographic barrier (mountain range, ocean, river) divides a population. Each subpopulation then evolves independently under different selection pressures until reproductive isolation is complete.
Sympatric speciation (without geographic isolation) occurs when disruptive selection or polyploidy creates divergent lineages within the same habitat. Many plant species originated this way through allopolyploidy (chromosome doubling after hybridisation).
Modern genomics allows us to reconstruct evolutionary history from DNA — the study of phylogenomics. CRISPR tools allow us to test functional hypotheses about specific genes' roles in evolution. See our CRISPR article.
Questions about evolution
- Evolution is defined as a change in allele frequencies in a population across generations. It is not the same as natural selection — selection is one mechanism. Others include genetic drift (random changes, important in small populations), mutation (the source of new alleles) and gene flow (movement of alleles between populations). See our natural selection guide for the mechanism in detail.
- Evidence comes from: fossils (dating evolutionary change over geological time); homologous structures (same bone arrangement in a human arm, whale flipper and bat wing); DNA sequence similarity (humans and chimps share ~98.8% of coding DNA); biogeography (island species resemble nearby mainland species); direct observation (antibiotic resistance, Darwin's finches). See UC Berkeley Understanding Evolution.
- Darwin's original theory lacked a mechanism for inheritance. The modern synthesis integrated natural selection with Mendel's genetics, population genetics (Hardy-Weinberg), and later molecular biology. Key figures: R.A. Fisher, J.B.S. Haldane, Sewall Wright and Theodosius Dobzhansky. See our DNA, genes and heredity guide for the genetic side.
- Evolution has no foresight or goal. Natural selection is a blind process: it acts on heritable variation that exists now, not variation that might be useful in future. Adaptations that were once advantageous can become neutral or even harmful when the environment changes. The same species can evolve in opposite directions if selection pressures reverse.
Read: natural selection in depth
The natural selection guide covers the four conditions, types of selection, and classic evidence including Darwin's finches.