What the exam asks
- Section I (Multiple Choice): Stimulus sets often provide data on mating calls, breeding seasons, hybrid survival, or geographic distributions, asking you to identify the specific isolating mechanism or evolutionary pattern.
- Section II (Free Response): Frequently appears in Q4 (Conceptual Analysis) or Q1/Q2 (Experimental Results), where you must explain how reproductive isolation blocks gene flow, predict the evolutionary effect of a habitat change (such as building a dam or removing a barrier), and justify your prediction using evidence.
Core ideas
The biological species concept and reproductive isolation
Under the biological species concept, a species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring, and who are reproductively isolated from other such groups.
Speciation requires the interruption of gene flow (the transfer of alleles between populations). Once gene flow stops, the isolated populations independently accumulate genetic differences through mutation, natural selection, and genetic drift until they can no longer interbreed.
Prezygotic vs. postzygotic barriers
Reproductive barriers prevent successful gene flow either before or after fertilization (zygote formation).
| Barrier Category | Mechanism | How it prevents gene flow |
|---|---|---|
| Prezygotic (prevents fertilization) | Habitat isolation | Populations live in different microhabitats within the same area and rarely encounter one another. |
| Prezygotic | Temporal isolation | Populations breed during different times of day, seasons, or years. |
| Prezygotic | Behavioral isolation | Differences in courtship rituals, pheromones, or mating calls prevent mate recognition. |
| Prezygotic | Mechanical isolation | Morphological differences in reproductive structures prevent successful mating or pollination. |
| Prezygotic | Gametic isolation | Sperm and egg molecular receptors are incompatible, so fusion cannot occur. |
| Postzygotic (prevents hybrids from passing on genes) | Reduced hybrid viability | Zygotes fail to develop, or hybrid offspring are frail and die before reaching reproductive maturity. |
| Postzygotic | Reduced hybrid fertility | Hybrids survive and are healthy, but are sterile (often because differing chromosome numbers prevent proper pairing in meiosis I). |
| Postzygotic | Hybrid breakdown | First-generation () hybrids are viable and fertile, but offspring are weak or sterile. |