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.
Understanding Biology: Speciation
A population contains variation from mutations and from reshuffling of genes during reproduction. Most new mutations have little effect, but some change a trait that affects survival or mating. In one place, a certain beak shape, fur colour, flowering time, or tolerance to salt may be useful.
In another place, a different version may help more. Natural selection makes helpful inherited traits more common. Chance matters too.
In a small population, genetic drift can make a gene version common simply because its carriers happened to leave more offspring. Over a long time, selection, drift, and mutation can pull separated populations in different directions.
Reproductive barriers do not need to appear all at once. A prezygotic barrier prevents fertilisation from happening. Birds may use different songs, insects may respond to different chemical signals, or plants may flower in different months.
Some organisms live in the same region but use different food sources or breed in different parts of a lake. A postzygotic barrier acts after fertilisation. An embryo may fail to develop, or a hybrid may survive but be infertile.
Mules are a familiar example. They are offspring of horses and donkeys, yet most cannot produce offspring themselves.
Polyploidy shows that new species can sometimes form quickly, especially in plants. A polyploid organism has extra complete sets of chromosomes. This can happen when cells make gametes incorrectly.
A plant with doubled chromosome sets may be unable to breed successfully with its parent population, while it can breed with another doubled plant. If it reproduces by self pollination or finds a similar mate, a separate lineage can begin in very few generations.
Many crop plants have polyploid ancestors, including wheat. This process is less common in animals because their development and sex determination are often more easily disrupted by extra chromosome sets.
Speciation is usually easier to recognise in hindsight than while it is happening. Researchers compare DNA, body features, behaviour, fossils, and the places where organisms live. DNA evidence can reveal close relatives that look very different because they adapted to different conditions.
It can also show organisms that look alike but have been separate for a long time. The biological species concept is useful for many animals, yet it has limits. It cannot be directly tested for extinct organisms.
It is difficult to apply to organisms that reproduce asexually. Some closely related species can even produce rare hybrids in the wild.
When learning this topic, keep the levels of explanation separate. Mutations create new genetic variation in individuals. Evolution changes how common inherited variants are in populations over generations.
Speciation requires populations to remain different long enough for barriers to reproduction to build up. A barrier alone does not guarantee a new species. If individuals later mix freely and produce fertile offspring, gene flow can reduce the differences.
Real populations are often messy, with partial barriers and occasional hybrids. That complexity is normal evidence of evolution in progress, not a failure of the idea.
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.