5.24—Isolation, gene flow and speciation

Speciation is the formation of a new species when populations become isolated, gene flow falls, allele frequencies diverge and the populations can no longer produce fertile offspring together.

Geographical barriers produce allopatric speciation. Sympatric speciation occurs without a physical barrier when timing, behaviour or reproductive structures prevent interbreeding.

A mountain range separates two tree populations. Different selection pressures and genetic drift change allele frequencies until hybrids are no longer fertile; this is a mechanism, not a single sudden mutation.

Different appearance alone does not prove separate species. Identify the isolation mechanism and the reproductive outcome before claiming speciation.