Reproductive separation of two populations can result in the accumulation…

The idea

Reproductive separation occurs when a barrier prevents gene flow between populations. This initial separation can be allopatric (geographical barriers like rivers) or sympatric (ecological or behavioural barriers in the same area). Once separated, the two gene pools change independently. Different mutations arise in each population. They face different selection pressures—either from different environments or distinct ecological niches—favouring different alleles. Consequently, allele frequencies diverge. Genetic drift adds further divergence, especially in small populations. Over time, these genetic differences accumulate until the populations can no longer interbreed to produce fertile offspring, resulting in reproductive isolation.

What earns the marks

• Reproductive separation occurs, preventing gene flow between the two populations.
• The two populations now have separate gene pools.
• Different mutations arise independently in each population.
• Different selection pressures act on each population, favouring different alleles.
• Allele frequencies change differently, accumulating differences until reproductive isolation occurs.

Common mistakes and how to put them right

• Conflating reproductive separation (the initial barrier to gene flow) with reproductive isolation (the final evolutionary outcome).

• Writing 'no inbreeding' instead of 'no interbreeding' between the two populations.

  Put it right: The wrong term is rejected.

• Assuming genetic differences appear immediately, rather than accumulating over many generations.

Exam tips

• Use the phrases 'no gene flow' or 'separate gene pools', as both are directly creditworthy.
• Order matters: state separation first, then independent mutation and different selection pressures, leading to changed allele frequencies and finally reproductive isolation.