Natural selection and evolution explained
Inheritance, variation and evolution • Variation and evolution
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Definition of evolution
Evolution is a change in the inherited characteristics of a population over time. The change occurs across generations when allele frequencies shift within the population, producing different phenotypes at the population level. Evidence for evolution includes fossil records, observed changes in modern populations, and genetic mapping linking traits across species. Limiting factors for defining evolution include timescale (changes require many generations for most traits), inheritance mechanism (heritable genetic change required), and measurable allele frequency shifts rather than single individual changes.
Mechanism: natural selection - stepwise cause→effect
Variation exists within populations because of genetic differences and occasional mutations. Variation causes some individuals to possess characteristics that affect survival and reproduction in a given environment. Competition for limited resources and environmental pressures create selection pressures that reduce survival or reproduction for less-suited individuals. Differential survival and reproductive success cause advantageous heritable traits to become more common in the population over successive generations. Repetition of these steps across many generations causes the population’s inherited characteristics to change. Limiting factors include the rate of beneficial mutations, gene flow between populations (which can dilute local adaptation), genetic drift in small populations (which can change allele frequencies randomly), and rapid environmental change that can outpace adaptation.
Speciation by accumulated change and isolation
Speciation occurs when accumulated inherited changes prevent previously interbreeding populations from producing fertile offspring. Geographic isolation separates populations into different environments so that natural selection, mutation and genetic drift act independently, magnifying differences. Over many generations, reproductive isolation arises and distinct species form. Famous textbook examples include island populations diverging after isolation and freshwater sticklebacks splitting during ice-age separations. Limiting factors for speciation include ongoing gene flow (which prevents divergence), insufficient time, and the requirement that genetic changes affect reproductive compatibility.
Examples that illustrate natural selection
Darwin’s finches show variation in beak shape linked to available food; selection for beak shapes suited to island habitats produces different populations over time. This demonstrates selection acting on variation to produce large-scale change. Antibiotic resistance in bacteria provides a rapid modern example: random mutations or gene transfer create resistant variants; antibiotic use kills susceptible bacteria, so resistant strains survive and reproduce, increasing resistance allele frequency in the population. This process provides direct evidence of evolution by natural selection.
Co-evolution and evolutionary arms races
Co-evolution occurs when interacting species exert mutual selective pressures that drive reciprocal adaptations. Plant–pollinator pairs and predator–prey systems illustrate this: changes in one species cause selection for complementary changes in the other. Repeated reciprocal selection can lead to specialised traits and rapid divergence in interacting species. Examples include flower spur length and pollinator proboscis length evolving together, and prey toxins and predator resistance escalating in an arms race. Limiting factors include dependency on the interaction persisting and the availability of genetic variation for adaptation.
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