How does natural selection lead to adaptation in populations?
Explain how natural selection acts on variation to produce adaptation over generations.
How natural selection acts on heritable variation, the role of selection pressures, and the structural, physiological, and behavioural adaptations that result, for TCE Biology Unit 4.
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The mechanism of natural selection
Natural selection, first explained by Charles Darwin and Alfred Russel Wallace, depends on a small number of conditions all being met:
- Variation: individuals in a population differ in their characteristics.
- Heritability: some of that variation is genetic and can be passed to offspring.
- Overproduction: organisms produce more offspring than the environment can support.
- Selection pressure and competition: limited resources, predators, disease, and other factors mean not all individuals survive and reproduce.
- Differential reproductive success: individuals whose traits suit the conditions leave more offspring, so the alleles for those traits become more common over time.
The key measure is fitness, which in biology means reproductive success, not physical strength. An individual is fit if it passes on more of its alleles to the next generation.
Selection pressures
A selection pressure is any environmental factor that affects an organism's chance of survival and reproduction, and therefore drives natural selection. Examples include predation, competition for food or mates, climate, availability of water, and disease. When a selection pressure changes, the alleles that are favoured can change too, which is why populations can shift over time.
Types of adaptation
Adaptations are usually grouped into three categories:
- Structural: physical features of the body, such as the thick fur of an alpine mammal or the streamlined shape of a fast fish.
- Physiological: internal or biochemical processes, such as the production of venom, or kidneys that conserve water in a desert animal.
- Behavioural: actions or patterns of behaviour, such as migration, nocturnal activity to avoid heat, or huddling for warmth.
Selection pressure changing over time
If the environment is stable, selection tends to keep a population well adapted. If conditions change (a new predator arrives, the climate warms, a food source disappears), the traits that confer fitness change, and the population evolves in response, provided suitable variation already exists. Populations with more genetic variation are better able to adapt to change, which is one reason genetic diversity matters for survival.
Artificial selection
Humans can act as the selecting agent, choosing which individuals breed to favour particular traits. This is artificial selection, seen in farm crops, livestock, and dog breeds. It works by the same principle as natural selection (selecting heritable variation) but with a human, rather than the environment, deciding which traits are favoured. It demonstrates how quickly populations can change when selection is strong.