17.3.3—Speciation through genetic isolation

Speciation is the formation of a new species from a pre-existing population. Genetic isolation is required: the separated populations no longer exchange genes sufficiently for their gene pools to remain the same.

Allopatric pathway — geographic separation:

1. A geographic barrier divides one population.
2. The groups cannot interbreed across the barrier, so gene flow stops or is greatly reduced.
3. Mutation, natural selection and genetic drift act independently in the two populations.
4. Their allele frequencies and phenotypes diverge over many generations.
5. Reproductive differences accumulate until the groups can no longer successfully interbreed; separate species have formed.

Sympatric pathway — separation in the same area:

1. There is no geographic barrier, but ecological or behavioural differences separate groups within the area.
2. The groups use different environments or behaviours and no longer exchange genes sufficiently.
3. Different selection pressures and genetic changes drive divergence.
4. Reproductive isolation develops, so the groups no longer successfully interbreed and can become separate species.

The starting separation differs, but the required logic is shared: isolation → reduced gene flow → independent gene-pool change → phenotypic divergence → reproductive isolation → speciation. A single visible difference or barrier is not, by itself, proof that two species have already formed; the change takes many generations and must include reproductive isolation.

In sympatric speciation, distinguish the factor that first separates the groups from the later differences that prevent interbreeding. Allopatric means geographic isolation; sympatric means no geographic barrier. This card owns the isolation-to-speciation mechanism, not the general evolution definition or DNA-sequence evidence.