Natural selection

Biology · D Continuity and change. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All Biology topics

The idea

Natural selection is the mechanism of evolution. Individuals in a population vary. Mutation produces new alleles, and sexual reproduction (meiosis and fertilization) produces new combinations of alleles.

More offspring are produced than the environment can support, so there is competition for resources. Individuals with traits that suit the environment are more likely to survive and reproduce. If these traits are heritable, their alleles are passed on and become more frequent over generations. Traits acquired during life are not inherited, so they cause no evolution.

In sexual selection, a trait spreads because it helps an individual to obtain mates, even if it lowers the chance of survival.

Rules to remember

Sequence: variation → overproduction → competition → differential survival and reproduction → heritable traits passed on → allele frequencies change.
Mutation gives new alleles. Sexual reproduction gives new combinations of alleles.
Fitness means reproductive success: the number of offspring that survive to breed, not how long an individual lives.
Selection pressures can be biotic (predators, competition, disease) or abiotic (temperature, drought). Abiotic factors are usually density-independent.
Resistance alleles exist before the antibiotic or insecticide is used. The chemical selects them. It does not create them.
Guppies (Endler): female choice favours bright males, predators favour dull males. Colour depends on the balance between the two.

Common mistake

Students write that organisms change because they need to, or that a chemical causes the resistance mutation. The variation is already present by chance. The environment only selects which individuals survive and breed.

Worked example

Guppies from a pool with few predators are moved to a stream with many predators. Predict and explain the change in male colour over several generations.

Male colour varies and is heritable.
Bright males are seen more easily, so with many predators more of them are eaten before they breed.
Duller males survive and reproduce more, even though females prefer bright males.
Alleles for dull colour become more frequent in each generation.

Answer: The males become less brightly coloured, because natural selection by predators now outweighs sexual selection by females.

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Read the full notes

Keep going

← Homeostasis Stability and change →Take a Biology test