(g) the role of isolating mechanisms in the evolution of new species

The idea

Speciation occurs when populations become reproductively isolated so that gene flow between them stops. Isolating mechanisms prevent interbreeding and so allow the separated gene pools to diverge. Geographical (allopatric) isolation, such as a river or mountain range separating populations, is a common starting point. Once separated, different mutations arise in each population, and natural selection acts differently where environments or selection pressures differ, changing allele frequencies. Over many generations the populations accumulate enough genetic and phenotypic difference that they can no longer interbreed to produce fertile offspring, so they are separate species.

What earns the marks

• States that speciation requires reproductive isolation so that gene flow between populations stops.
• Describes geographical (allopatric) isolation, where a physical barrier separates populations and prevents interbreeding.
• Explains that separated populations experience different selection pressures and accumulate different mutations, so allele frequencies diverge.
• Identifies reproductive isolating mechanisms such as temporal, behavioural or mechanical isolation, or hybrid infertility, that maintain separation.
• Concludes that accumulated differences eventually prevent the populations producing fertile offspring, so they become separate species.

Common mistakes and how to put them right

• Confusing geographical isolation with reproductive isolation.

  Put it right: A physical barrier starts separation but reproductive isolation is what maintains separate gene pools.

• Claiming that individual organisms evolve or choose to change.

  Put it right: Allele frequencies in populations change over generations through selection.

• Assuming isolation alone creates a new species immediately.

  Put it right: Divergence through mutation and selection over many generations is required.

• Treating all isolating mechanisms as geographical.

  Put it right: Recognise temporal, behavioural and mechanical reproductive barriers too.

Exam tips

• Use the sequence: isolation, no gene flow, different selection pressures, allele frequency change, reproductive isolation, new species.
• Name a specific isolating mechanism and explain how it prevents interbreeding rather than just naming it.
• Link divergence explicitly to mutation and natural selection acting on each population's gene pool.