Evolution (biology)
Biological evolution is the change in heritable characteristics of biological populations over successive generations, driven by mechanisms such as natural selection, genetic drift, gene flow, and mutation. It unifies all of biology by explaining the diversity of life on Earth through descent with modification from common ancestors. Evolution operates at multiple levels, from changes in allele frequencies within populations (microevolution) to the origin of new species and higher taxa (macroevolution).
Major Mechanisms of Biological Evolution
| Mechanism | Definition | Direction | Population Size Effect |
|---|---|---|---|
| Natural Selection | Differential reproductive success | Directional — favours fit traits | Effective in large populations |
| Genetic Drift | Random allele frequency change | Random — no consistent direction | Strong in small populations |
| Gene Flow | Migration of alleles between populations | Homogenises populations | Reduces divergence |
| Mutation | New alleles arising spontaneously | Random — introduces novelty | Rate independent of size |
| Sexual Selection | Mate choice and competition | Favours attractive/competitive traits | Effective in large populations |
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Related Terms
Natural Selection
Natural selection is the process by which individuals with heritable traits that improve survival and reproduction in a given environment leave more offspring than those without such traits, causing those traits to become more common in the population over generations. It is the primary mechanism of adaptive evolution, first described by Charles Darwin and Alfred Russel Wallace in 1858. Natural selection acts on phenotypic variation and requires heritable differences in reproductive success to drive evolutionary change.
Speciation
Speciation is the evolutionary process by which populations evolve to become distinct species, typically through the accumulation of reproductive barriers that prevent gene flow between diverging lineages. It is the source of biodiversity and occurs through several modes, most commonly allopatric speciation (geographic isolation) and sympatric speciation (divergence without geographic barrier). The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring, making reproductive isolation the key criterion.
Gene Flow
Gene flow, also called gene migration, is the transfer of alleles or genes from one population to another through the movement and interbreeding of individuals. It tends to homogenise allele frequencies between populations, reducing genetic differentiation, and can introduce new alleles into a population or change the frequencies of existing ones. Gene flow counteracts the genetic divergence produced by natural selection, drift, and mutation, and is a critical factor in whether populations will diverge enough to speciate.
From Latin "evolutio" (unrolling, opening), derived from "evolvere" (to roll out, unfold). Applied to biological change by Charles Darwin in "On the Origin of Species" (1859), though the word appeared in biological contexts as early as 1762 with Charles Bonnet.