Mechanisms of speciation and the role of selection

1Two apes, one river

Chimpanzees live in the forests north and east of the great Congo River; bonobos live only to the south of it. Neither swims. The two are each other's closest relatives, yet they differ in body shape and, strikingly, in behaviour: bonobo groups are led by females and settle conflicts more peacefully. Many scientists think that the river split the range of their common ancestor about one to two million years ago, and that the populations on each side then evolved separately. Splitting one species into two is called speciation. This lesson asks what a species is, how populations become reproductively isolated, and what roles natural and artificial selection play.

2What is a species?

The biological species concept defines species by breeding: lions and tigers can produce hybrids in zoos, but in the wild they do not interbreed, so they are separate species. The concept has limits. It cannot be applied to organisms that reproduce only asexually, such as bacteria, or to fossils, whose breeding cannot be observed. In these cases biologists use similarities of structure (the morphological concept) or of DNA sequences (the phylogenetic concept).

3Isolating mechanisms

4Allopatric and sympatric speciation

Allopatric speciation is thought to be the commonest route. A barrier (a river, a mountain range, a stretch of sea, a desert, or the drying of a lake) divides a population. No genes flow between the parts. Each part experiences different selection pressures, gains different mutations and drifts differently, so their allele frequencies diverge. If the differences come to include isolating mechanisms, the two will not interbreed even if they meet again: they are separate species. Darwin's finches arose in this way, as birds colonised different islands of the Galápagos.

Sympatric speciation happens without a geographical barrier:

• Polyploidy, mainly in plants. An error in meiosis can produce diploid gametes; if two fuse, the offspring is tetraploid and cannot produce fertile offspring with its diploid parents (their cross gives sterile triploids). A new species can arise in one generation. When two different species hybridise, the hybrid is usually sterile because its chromosomes have no partners to pair with in meiosis; if its chromosome number then doubles, every chromosome has a partner, and the plant becomes a fertile new species (an allopolyploid). Common cord grass, Spartina anglica, arose this way on the coast of southern England in the late nineteenth century.
• Ecological and behavioural isolation. If some members of a population start to use a different food, habitat or breeding time, or prefer mates of a different colour, gene flow between the groups falls, and disruptive selection can drive them apart in the same place.

Adaptive radiation. Lake Victoria holds hundreds of species of cichlid fish, each specialised for a different food, from algae scraped off rocks to snails, insects or other fish, although many scientists think the lake almost dried out about 15 000 years ago. In some, females choose males by colour, and populations whose females prefer red or blue males have begun to separate even where they live together. Such rapid radiations happen when a group meets many empty niches with few competitors.

5The role of selection in speciation

Natural selection drives most speciation. When separated populations live in different environments, selection adapts each to its own conditions, and the differences that build up in body form, behaviour, timing or chromosomes can make interbreeding fail as a side effect. Where diverging populations meet again, hybrids that are poorly adapted to either environment leave few offspring. Selection then favours individuals that choose mates from their own population, strengthening prezygotic isolation; this is called reinforcement. Disruptive selection can split a population even without a barrier.

Artificial selection shows how much change selection can produce, but by itself it rarely produces new species. Dog breeds as different as a Chihuahua and a Great Dane came from wolves by artificial selection, yet all dogs belong to one species and can produce fertile offspring (sometimes only through intermediate-sized dogs). Breeders have, however, made new species of crop plants by deliberate hybridisation and chromosome doubling, the method of Worked example 1. Triticale, a cross of wheat and rye made in the twentieth century, combines the grain quality of wheat with the hardiness of rye and is grown as a crop. Human activity also causes speciation unintentionally, by moving organisms to new places, as with Spartina anglica, whose American parent arrived in English harbours on ships.