Allopatric and sympatric speciation.

The idea

Allopatric and sympatric speciation differ in what causes reproductive isolation. Allopatric speciation begins with a geographical barrier, like a river or mountain range, physically dividing a population. The separated groups then diverge. Sympatric speciation occurs without a geographical barrier, in populations sharing the same area. Isolation here is behavioural (e.g., different courtship displays), temporal (e.g., breeding at different times), or mechanical. In plants, it also occurs via polyploidy, where chromosome doubling produces individuals unable to make fertile offspring with the parent population. Both routes lead to separate gene pools and different allele frequencies.

What earns the marks

• Naming geographical isolation, or allopatric speciation, where a physical barrier separates the populations.
• Quoting evidence of overlapping distributions before suggesting sympatric speciation.
• Naming the isolating mechanism in sympatric speciation, such as different pollinators, different breeding times or polyploidy.
• Linking either route to separate gene pools and different allele frequencies.

Common mistakes and how to put them right

• Describing a geographical barrier without naming geographical isolation or allopatric speciation, when the question specifically asks for the term.

• Suggesting sympatric speciation in a clearly allopatric context, which is rejected.

• Claiming sympatric speciation with no evidence that the two populations share an area.

• Treating different altitudes as a complete explanation, instead of stating the barrier or isolating mechanism it creates.

• Assuming sympatric speciation is too rare to mention, when polyploidy in plants is a standard example.

Exam tips

• Use the term and then the evidence, for example allopatric, because the two species are separated by the river shown in Figure 2.
• If the distributions overlap, say so first, because the sympatric marking point is often awarded only once you have quoted that overlap.
• Both routes end the same way, so finish either answer with separate gene pools and an inability to produce fertile offspring.