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Speciation & Reproductive Isolation Mechanisms Cheat Sheet
A printable reference covering species concepts, allopatric and sympatric speciation, prezygotic barriers, postzygotic barriers, and reinforcement for grades 11-12.
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Speciation is the process by which one population becomes two or more separate species over evolutionary time. This cheat sheet helps students connect genetic change, natural selection, geographic separation, and reproductive isolation. It is useful for comparing mechanisms of speciation and for explaining how barriers prevent gene flow between populations. The most important idea is that new species form when populations stop exchanging genes and become genetically distinct. Reproductive barriers can act before fertilization, such as habitat, timing, behavior, mechanical fit, or gamete compatibility. They can also act after fertilization, when hybrids have low survival, low fertility, or reduced fitness across generations.
Key Facts
- Speciation occurs when gene flow between populations is reduced or stopped and genetic differences accumulate over many generations.
- Allopatric speciation happens when a physical barrier, such as a river, mountain range, or island distance, separates populations.
- Sympatric speciation happens without geographic separation, often through polyploidy, habitat shifts, sexual selection, or niche specialization.
- Prezygotic barriers prevent mating or fertilization before a zygote forms.
- Postzygotic barriers reduce the survival, fertility, or long-term success of hybrid offspring after a zygote forms.
- Reinforcement strengthens reproductive isolation when hybrids have lower fitness than offspring produced within each parent population.
- Reduced gene flow plus mutation, natural selection, genetic drift, and nonrandom mating can cause populations to diverge.
- A biological species is often defined as a group of organisms that can interbreed in nature and produce viable, fertile offspring.
Vocabulary
- Speciation
- Speciation is the evolutionary process in which one lineage splits into two or more distinct species.
- Reproductive Isolation
- Reproductive isolation is any barrier that prevents populations from successfully interbreeding and exchanging genes.
- Prezygotic Barrier
- A prezygotic barrier prevents mating or fertilization before a zygote is formed.
- Postzygotic Barrier
- A postzygotic barrier reduces the viability, fertility, or fitness of hybrid offspring after fertilization.
- Allopatric Speciation
- Allopatric speciation is the formation of new species after populations become geographically separated.
- Sympatric Speciation
- Sympatric speciation is the formation of new species in the same geographic area.
Common Mistakes to Avoid
- Confusing geographic isolation with reproductive isolation is wrong because a physical barrier can start divergence, but reproductive barriers must prevent gene flow for species to remain separate.
- Calling every difference between populations a new species is wrong because speciation requires enough reproductive isolation to limit successful interbreeding.
- Mixing up prezygotic and postzygotic barriers is wrong because prezygotic barriers act before fertilization, while postzygotic barriers act after fertilization.
- Assuming hybrids are always sterile is wrong because some hybrids survive and reproduce, while others have reduced fertility or lower fitness only in later generations.
- Ignoring gene flow is wrong because ongoing interbreeding can keep populations genetically similar and slow or prevent speciation.
Practice Questions
- 1 A canyon splits one lizard population into two groups. After 500,000 years, the groups can no longer produce fertile offspring. What type of speciation is most likely, and what reproductive barrier now exists?
- 2 Two frog species live in the same pond, but one breeds in March and the other breeds in June. Identify the reproductive isolation mechanism and state whether it is prezygotic or postzygotic.
- 3 A plant species with 2n = 14 chromosomes produces a polyploid offspring with 4n = 28 chromosomes that can self-fertilize but cannot successfully breed with the original 2n population. What type of speciation could this cause?
- 4 Explain why natural selection might favor stronger mating preferences when two related populations produce hybrids with low fitness.