If two populations of one species become so different in phenotype that…

The idea

Speciation happens when populations of one species become reproductively isolated, so gene flow between them stops. Different selection pressures and random mutations then make their phenotypes diverge. Over many generations the accumulated differences become so great that members of the two populations can no longer interbreed successfully to produce fertile offspring. At that point they are classified as two separate species. For example, a river may split a population of beetles; each group experiences different food and climate, so different alleles are favoured. Eventually the two groups may mate but produce infertile young, or not mate at all, showing that new species have formed.

What earns the marks

• Reproductive isolation occurs when populations are separated so that they no longer interbreed and gene flow between them stops.
• Different selection pressures in the two environments favour different alleles, so phenotypes diverge over many generations.
• Mutation introduces new alleles, and natural selection acting on these variants increases the differences between the populations.
• Speciation is complete when the two populations can no longer interbreed to produce fertile offspring, so they are recognised as separate species.
• A named example, such as two beetle populations separated by a river, can illustrate how isolation and selection lead to new species.

Common mistakes and how to put them right

• Thinking that a new species forms as soon as two populations look different.

  Put it right: Phenotype differences alone are not enough, because reproductive isolation and infertile or no offspring are the defining test.

• Believing that individual organisms change their phenotype to suit the environment.

  Put it right: Mutations create new alleles, and natural selection favours those already present, so populations change over generations.

• Confusing speciation with extinction.

  Put it right: Speciation produces two new species from one ancestral species, whereas extinction is the loss of a species.

Exam tips

• Use the phrase 'no longer interbreed to produce fertile offspring' when defining the point at which two new species have formed.
• Link each stage of your answer in a logical chain: isolation, different selection pressures, mutation, natural selection, reproductive isolation.
• Include a specific example, such as beetles separated by a river, to show how the process works in practice.