Explain how reproductive isolation leads to speciation, including allopatric and sympatric modes.

What this dot point is asking

What is a species

Under the biological species concept, a species is a group of organisms that can interbreed in nature to produce fertile, viable offspring. Members of one species share a gene pool, the total of all alleles in the population. Speciation occurs when one gene pool splits into two that no longer mix.

The general process of speciation

Speciation usually follows a sequence:

1. A single population shares one gene pool.
2. The population is split or divided so gene flow between the groups is reduced or stopped.
3. The separated groups experience different selection pressures, mutations, and genetic drift, so their allele frequencies diverge over many generations.
4. The groups become so genetically different that, even if reunited, they can no longer interbreed to produce fertile offspring. They are now separate species.

Allopatric speciation

Allopatric speciation happens when a physical (geographic) barrier separates populations. The barrier might be a mountain range, a river, an ocean, or new land formed by geological change. With gene flow blocked, the two populations evolve independently. This is thought to be the most common form of speciation. The Galapagos finches are a classic example: ancestral finches reached different islands, and isolation plus different food sources led to distinct beak shapes and eventually distinct species.

Sympatric speciation

Sympatric speciation happens without geographic separation, while populations still share the same area. Gene flow is interrupted by biological factors instead. Examples of how this can occur include:

• Polyploidy: a sudden change in chromosome number, common in plants, that immediately prevents successful breeding with the parent population.
• Behavioural shifts: groups using different food sources or breeding at different times stop interbreeding.

Sympatric speciation is generally rarer than allopatric speciation in animals but is important in plants.

Rates of speciation: gradualism and punctuated equilibrium

Speciation does not always happen at a steady pace. Two models describe how fast it can occur. Gradualism proposes that species change slowly and steadily over very long periods, with new species emerging through the gradual accumulation of small changes. Punctuated equilibrium proposes that species stay much the same for long periods (stasis) and then change rapidly over a relatively short time, often when the environment changes quickly or a small population becomes isolated. The fossil record shows examples that fit both patterns, so they are best seen as two ends of a spectrum rather than rival theories. Recognising that speciation can be slow or rapid helps explain why some transitional fossils are common while others are rare, and why isolated island groups such as the Galapagos finches can diversify comparatively quickly once they reach a new, empty habitat with many vacant niches.

Isolating mechanisms

Once divergence begins, isolating mechanisms keep gene pools separate. They are grouped by whether they act before or after mating.

• Prezygotic mechanisms prevent mating or fertilisation. These include temporal isolation (breeding at different times), behavioural isolation (different courtship signals), and ecological or habitat isolation (living in different microhabitats).
• Postzygotic mechanisms act after fertilisation. These include hybrid inviability (offspring do not survive) and hybrid sterility (offspring survive but cannot reproduce, like the mule).