New species arise when these genetic differences lead to an inability of…

The idea

Speciation is complete when accumulated genetic differences mean members of two populations can no longer interbreed to produce fertile offspring. Both aspects matter: populations may be physically unable to mate, or their gametes may be incompatible. Even when mating succeeds, offspring can be infertile. For example, a horse and a donkey interbreed to produce a mule, but the mule's chromosomes cannot pair during meiosis, preventing gamete formation. A sterile hybrid is an evolutionary dead end, ensuring no alleles pass between the two gene pools, keeping them separate. Responsible genetic differences include changes in chromosome structure, courtship behaviour, breeding timing, or gamete surface proteins.

What earns the marks

• State that members of the two populations are no longer able to interbreed to produce fertile offspring.
• Explain that any hybrid offspring produced would be sterile due to incompatible chromosomes failing to pair during meiosis.
• Link the inability to interbreed successfully to the accumulation of genetic differences and mutations over time.
• Conclude that the two gene pools remain entirely separate because no alleles can be exchanged between them.

Common mistakes and how to put them right

• Stating only that populations 'cannot interbreed' without adding 'to produce fertile offspring'.

  Put it right: Include the requirement for fertile offspring, as some distinct species can mate but produce sterile hybrids.

• Claiming that physical separation alone makes two populations separate species.

  Put it right: Physical separation causes allopatric isolation, but speciation is only complete when genetic differences prevent successful interbreeding.

• Confusing the cause and effect of genetic differences.

  Put it right: The inability to interbreed is the consequence of accumulated genetic differences, not the cause of them.

Exam tips

• When describing the sequence of speciation, always conclude your explanation by stating that the populations can no longer interbreed to produce fertile offspring.
• Use the mule as a concrete example to illustrate that hybrid sterility, rather than just the physical inability to mate, confirms two populations are separate species.