Explain the process of speciation, including allopatric and sympatric speciation and the reproductive isolating mechanisms that maintain separate species

A focused answer to the WACE Year 12 Human Biology Unit 4 dot point on speciation. The biological species concept, allopatric and sympatric speciation, and the pre-zygotic and post-zygotic reproductive isolating mechanisms that keep species separate.

Generated by Claude Opus 4.77 min answerUpdated 2026-05-29

Reviewed by: AI editorial process; not yet individually human-reviewed

Have a quick question? Jump to the Q&A page

What this dot point is asking

WACE wants you to explain how the gene-pool changes from the population genetics topic can accumulate until one species becomes two. This builds on isolation, drift, selection and mutation, and introduces the barriers that keep species distinct.

What a species is

A species is usually defined as a group of organisms that can interbreed in nature to produce fertile offspring. The key word is fertile: a horse and a donkey can mate to produce a mule, but the mule is sterile, so horses and donkeys are separate species. Speciation has occurred when two populations can no longer interbreed to produce fertile offspring.

Allopatric speciation

Allopatric speciation involves geographic isolation. A physical barrier, such as a mountain range, river, sea or distance, splits a population into two that can no longer interbreed. With gene flow now blocked, the two gene pools change independently: they experience different selection pressures, accumulate different mutations, and are subject to genetic drift. Over many generations they diverge so much that, even if reunited, they can no longer interbreed to produce fertile offspring. They are now separate species.

Sympatric speciation

Sympatric speciation occurs without geographic separation: populations living in the same area diverge into separate species. This can happen through behavioural differences, exploiting different niches or food sources, or chromosomal changes that prevent interbreeding with the original population. Disruptive selection, which favours both extremes of a trait, can contribute by pulling a population toward two distinct forms.

Reproductive isolating mechanisms

Once populations have diverged, isolating mechanisms keep them from interbreeding and so keep the species separate. They fall into two groups:

Pre-zygotic mechanisms prevent mating or fertilisation, so no zygote forms. Examples include different breeding seasons (temporal isolation), different courtship behaviours (behavioural isolation), incompatible reproductive structures (mechanical isolation), and living in different habitats (ecological isolation).
Post-zygotic mechanisms act after fertilisation, so a zygote forms but the offspring are unfit. Examples include the offspring dying early (hybrid inviability) or being sterile, like the mule (hybrid sterility).

Explaining how a barrier leads to two species

A river divides a population

Suppose a river changes course and splits one population of animals into two, with no individuals able to cross. Gene flow between the two groups stops. On each side, the environment differs slightly, so different selection pressures act, different mutations arise and accumulate, and genetic drift adds random change, especially if a group is small. Over many generations the two gene pools diverge. If the river later dries up and the groups meet again, they may no longer recognise each other as mates or may produce infertile offspring; they can no longer interbreed to produce fertile offspring and are now separate species. A marker wants the sequence: isolation blocks gene flow, the gene pools diverge through selection, drift and mutation, and reproductive isolation completes the split.

How this maps to the exam

Expect questions that ask you to define a species, describe allopatric speciation step by step, distinguish allopatric from sympatric speciation, or classify a given barrier as a pre-zygotic or post-zygotic isolating mechanism. Scenario questions usually give a barrier and ask you to explain how a new species could form.