Speciation and isolating mechanisms: WACE Year 12 Human Biology Unit 4
“Explain the process of speciation, including allopatric and sympatric speciation and the reproductive isolating mechanisms that maintain separate species”
Speciation is the formation of a new species, which happens when populations stop interbreeding and their gene pools diverge until they can no longer produce fertile offspring together. In allopatric speciation a physical barrier separates populations; in sympatric speciation they diverge without a geographic barrier. Reproductive isolating mechanisms, either pre-zygotic (preventing mating or fertilisation) or post-zygotic (preventing fertile offspring), maintain the separation.
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.
Speciation needs two things: isolation that blocks gene flow, and enough time for the separated gene pools to diverge through selection, drift and mutation. Geographic separation alone is not speciation; it is the lack of gene flow allowing the populations to become genetically incompatible that creates new species. State both the isolation and the divergence to gain full marks.
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).
A reproductive isolating mechanism is any biological feature that prevents members of different populations or species from interbreeding to produce fertile offspring. Pre-zygotic mechanisms stop mating or fertilisation from happening, while post-zygotic mechanisms prevent the resulting offspring from being viable or fertile.
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.
A common error is treating separation as speciation. Geographic isolation only blocks gene flow; speciation occurs only when the separated gene pools have diverged enough, through selection, drift and mutation, that the populations can no longer interbreed to produce fertile offspring. Always include the divergence over time, not just the barrier. Also remember that producing offspring is not enough; the offspring must be fertile for the groups to count as one species.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 marksDescribe the process of allopatric speciation, explaining the role of geographic isolation, selection and reproductive isolation in the formation of a new species.Show worked answer →
A 6 mark response needs the ordered process with each role explained.
- Geographic isolation
- A physical barrier (such as a mountain range, river or sea) divides one population into two, preventing gene flow between them.
- Independent change
- Each population experiences different environments and selection pressures, and may also undergo different mutations and genetic drift, so their gene pools change in different directions over many generations.
- Reproductive isolation
- The genetic differences accumulate until the two populations can no longer interbreed to produce fertile offspring, even if the barrier is removed.
- New species
- At this point the two populations are separate species under the biological species concept.
Markers reward the barrier stopping gene flow, divergent change under different selection, and accumulated differences causing reproductive isolation.
WACE 2023-style4 marksDistinguish between pre-zygotic and post-zygotic reproductive isolating mechanisms, giving one example of each.Show worked answer →
A 4 mark distinguish answer needs the timing distinction plus an example each.
Pre-zygotic mechanisms. Prevent mating or fertilisation, so no zygote forms. Examples include temporal isolation (breeding at different times) or behavioural isolation (different courtship displays).
Post-zygotic mechanisms. Act after fertilisation, so a zygote forms but the hybrid offspring are inviable or sterile. Example: hybrid sterility, such as a mule (from a horse and donkey) being unable to reproduce.
Markers reward the before-versus-after-fertilisation distinction and a correct example for each.