Speciation is the evolutionary process by which one species splits into two or more distinct species. It matters because it explains how Earth’s biodiversity grows over long periods of time. When populations stop sharing genes, they can follow different evolutionary paths.
Over many generations, those differences can become large enough that the groups no longer interbreed successfully.
The central mechanism behind speciation is reproductive isolation, which blocks gene flow between populations. Isolation can happen because of a physical barrier, different habitats, different mating behaviors, or genetic changes. Allopatric speciation occurs when populations are separated by geography, while sympatric speciation occurs without a geographic barrier.
Under the biological species concept, two groups are considered separate species when they cannot produce fertile offspring together in natural conditions.
Key Facts
- Speciation = the evolutionary process by which one species splits into two or more distinct species.
- Gene flow decreases divergence, while reproductive isolation allows divergence to increase.
- Allopatric speciation occurs when a physical barrier separates populations, such as a river, mountain, or island distance.
- Sympatric speciation occurs in the same geographic area through factors such as habitat choice, sexual selection, or polyploidy.
- Biological species concept: species are groups that can interbreed and produce fertile offspring, and are reproductively isolated from other groups.
- Hardy-Weinberg model for no evolution: p^2 + 2pq + q^2 = 1 and p + q = 1.
Vocabulary
- Speciation
- Speciation is the formation of two or more distinct species from one ancestral species.
- Reproductive isolation
- Reproductive isolation is any barrier that prevents different populations from interbreeding successfully.
- Gene flow
- Gene flow is the movement of alleles between populations through mating or migration.
- Allopatric speciation
- Allopatric speciation is speciation that occurs when populations are separated by a geographic barrier.
- Sympatric speciation
- Sympatric speciation is speciation that occurs without physical separation of populations.
Common Mistakes to Avoid
- Confusing adaptation with speciation is wrong because a population can evolve new traits without becoming a separate species.
- Assuming geographic separation always causes speciation is wrong because populations may reconnect and interbreed before reproductive isolation evolves.
- Thinking two organisms are different species only because they look different is wrong because the biological species concept focuses on successful reproduction, not appearance alone.
- Ignoring gene flow is wrong because even small amounts of interbreeding can slow or prevent divergence between populations.
Practice Questions
- 1 A population of 600 lizards is split by a new river into two groups of 250 and 350. After many generations, the two groups no longer produce fertile offspring when brought together. What process has occurred, and what type of speciation is most likely?
- 2 In a plant population, 16% of individuals show a recessive phenotype. Assuming Hardy-Weinberg equilibrium, find q, p, and the frequency of heterozygotes using q^2 = 0.16, p + q = 1, and 2pq.
- 3 Two insect populations live in the same forest, but one mates in early spring and the other mates in late summer. Explain how this could lead to speciation without a geographic barrier.