Populations (A-level only): Species exist as one or more populations.

The idea

A species is a group of organisms able to interbreed and produce fertile offspring, but its members are rarely spread evenly across the landscape. They usually occupy separate patches of suitable habitat, and each patch holds a population. Lemurs of one species living on opposite sides of a river, or monkeyflowers growing at different altitudes on the same mountain, are one species divided into several populations. What keeps them a single species is gene flow: if individuals move between the populations, or gametes do, as when a bee carries pollen from one hillside to another, alleles are shared and the two gene pools stay similar.

What earns the marks

• Defining a species by the ability to interbreed and produce fertile offspring.
• A population as the members of one species living in the same place at the same time.
• Gene flow between populations keeping the gene pools of one species similar.
• Recognising that separated populations of one species can later become separate species.

Common mistakes and how to put them right

• Using species and population as if they meant the same thing in a genetics or ecology answer.

• Saying two populations are different species simply because they live apart or look different.

• Writing that a population contains several species, which describes a community.

• Assuming geographical separation instantly creates a new species, with no reference to accumulating genetic differences.

Exam tips

• Check which word the question uses; allele frequency and gene pool questions always concern a population, not a whole species.
• If a map shows two groups divided by a barrier, call them separate populations of one species until the data show they cannot interbreed.
• Name gene flow explicitly, because it is the idea that links separate populations back into one species.