Mechanisms of Evolution
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Evolution is a change in allele frequencies; natural selection needs variation, heritability, overproduction and differential reproductive success.
- Selection can be measured in the wild, as in Galápagos finch beaks, and its direction depends on the environment.
- Genetic drift, bottlenecks, gene flow and mutation also change populations; isolation can lead to speciation.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Natural selection in populations
Evolution is a change in the inherited characteristics of a population over generations. Natural selection, its main driving force, depends on four conditions. Individuals vary. Much of that variation is heritable. More offspring are produced than can survive. And individuals with advantageous traits leave more offspring. Researchers saw this happen on the Galápagos Islands. During a severe drought in 1977, small seeds ran out, leaving only large, hard ones. Finches with deeper beaks could crack them and were more likely to survive. Their offspring inherited deeper beaks. So the average beak size increased in a single generation.
Other ways populations change
Natural selection is not the only force that changes populations. Genetic drift is change caused by chance. In a small population, random events such as storms or accidents can remove individuals regardless of their traits. So some alleles may be lost and others become common by luck. A bottleneck occurs when a population shrinks drastically, leaving little genetic variety. Elephant seals went through one after heavy hunting. Gene flow happens when individuals move between populations, carrying alleles with them. Mutation introduces new alleles. When populations are separated for long enough, these processes can make them so different that they become separate species.
Curriculum links: NGSS HS-LS4-2 · NGSS HS-LS4-3 · NGSS HS-LS4-4 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 12