Speciation

What counts as a species?

The most widely used definition is the biological species concept: a species is a group of populations whose members can interbreed in nature and produce viable, fertile offspring, and who can't do that with other groups. The key idea is reproductive isolation. Once two populations can't exchange genes, they evolve separately, and each is a species.

The definition has limits. It can't be applied to organisms that reproduce asexually, like many bacteria, or to species known only from fossils. For these, biologists group organisms by their body form (the morphological species concept) or by their position on a phylogenetic tree built from DNA.

Barriers that keep species apart

Anything that stops two species producing fertile hybrids is a reproductive barrier. Barriers are sorted by when they act. Prezygotic barriers act before a zygote (a fertilised egg) forms:

• Habitat isolation: the species live in different places and rarely meet. Two cricket species in North America prefer different kinds of soil.
• Temporal isolation: they breed at different times. Two closely related frog species breed in different months of the year.
• Behavioural isolation: they don't recognise each other's courtship. Each firefly species flashes its own light pattern.
• Mechanical isolation: their reproductive parts don't fit together, as in many damselflies, or a flower's shape suits only one pollinator.
• Gametic isolation: sperm and egg meet but can't fuse, because the proteins that let them bind don't match. This matters a lot for animals that release gametes into the sea.

Postzygotic barriers act after a hybrid zygote has formed:

• Hybrid inviability: the hybrid embryo fails to develop or dies young.
• Hybrid sterility: the hybrid survives but can't produce working gametes, like the mule.
• Hybrid breakdown: first-generation hybrids are fertile, but their offspring are weak or sterile.

Why are mules sterile? A horse has 64 chromosomes and a donkey has 62, so a mule gets 32 from one parent and 31 from the other: 63 in total. In meiosis, chromosomes must pair with a matching partner. The horse and donkey chromosomes differ in number and structure, so many can't pair properly, and most gametes end up with the wrong set of chromosomes.

Allopatric speciation: separated by geography

Allopatric speciation (from Greek for "other homeland") happens when a population is split by a physical barrier. It's generally thought to be the most common way new animal species form.

1. 1A population is split. This can happen by vicariance, when a barrier such as a river, mountain range or rising sea divides it, or by dispersal, when a few individuals move to a new place, such as an island.
2. 2Gene flow between the two populations stops.
3. 3Each population changes independently: different mutations arise, genetic drift shifts allele frequencies, and natural selection adapts each one to its own environment.
4. 4Over time, the differences build up. Some of them happen to affect mating or hybrid fertility, as a side effect.
5. 5If the populations meet again and can no longer produce fertile offspring, they have become separate species.

When one ancestral species spreads into many new habitats and splits into many species, each adapted to its own way of life, it's called adaptive radiation. The Hawaiian honeycreepers are a classic case: from a single ancestor that reached the islands, species evolved with very different beaks for seeds, nectar or insects.

Sympatric speciation: same place, new species

Sympatric speciation ("same homeland") happens without any geographic separation. For this to work, something must cut gene flow within a single area. The best-understood way is polyploidy: having more than two complete sets of chromosomes.

In autopolyploidy, a mistake in meiosis gives a plant diploid (2n) gametes instead of haploid (n) ones. If two such gametes fuse, the offspring is tetraploid (4n). It can breed with other tetraploids, but if it crosses with its diploid parents, their n gametes plus its 2n gametes make triploid (3n) offspring. Triploids can't pair their chromosomes evenly in meiosis, so they are mostly sterile. The tetraploid is reproductively isolated from its parents in a single generation.

In allopolyploidy, two different species hybridise. The hybrid is usually sterile, because its two sets of chromosomes come from different species and can't pair. But if its chromosome number doubles, each chromosome suddenly has an identical partner, meiosis can work, and the plant becomes fertile. That's exactly what happened to Karpechenko's radish–cabbage hybrids. Bread wheat, cotton and tobacco are all allopolyploids.

Sympatric speciation can also happen without polyploidy, when part of a population starts using a different habitat, food or mating signal. In some African lakes, cichlid fish in the same lake have split into groups with different body shapes and feeding styles, which may be the early stages of new species.

When separated populations meet again

Sometimes two diverging populations come back into contact before isolation is complete. The area where they meet and interbreed is a hybrid zone. What happens next depends on the hybrids.

• If hybrids are less fit than either parent, individuals that avoid mating with the other population leave more offspring. Selection strengthens prezygotic barriers. This is called reinforcement.
• If hybrids are just as fit, gene flow can merge the two populations back into one. This is called fusion.
• If hybrids keep being produced at a steady rate, the hybrid zone may stay stable for a long time.

How fast does all this happen? Some fossil lineages seem to change slowly and steadily (gradualism). Others seem to stay the same for long periods and then change quickly around the time new species appear (punctuated equilibrium). Both patterns occur in the record.

Key terms

Biological species concept

A species is a group of populations whose members interbreed in nature and produce viable, fertile offspring.

Reproductive isolation

The inability of two populations to exchange genes by producing fertile offspring.

Prezygotic / postzygotic barrier

A barrier that acts before fertilisation / one that acts on the hybrid after fertilisation.

Allopatric speciation

Speciation after a population is split by a geographic barrier.

Sympatric speciation

Speciation within the same geographic area, for example by polyploidy.

Polyploidy (auto- / allo-)

Having more than two chromosome sets, from one species / from a hybrid of two species.

Adaptive radiation

One ancestral species rapidly splitting into many species adapted to different ways of life.

Reinforcement

Selection against poorly fit hybrids that strengthens prezygotic barriers.

Check yourself

Try answering in your head before you open each answer.

• 1.Classify each barrier as prezygotic or postzygotic: (a) two orchid species are pollinated by different bee species; (b) hybrid embryos of two frogs stop developing after a few days; (c) two sea urchin species release gametes at the same time, but the sperm of one can't bind to the eggs of the other.Show answerHide

  (a) Prezygotic, a mechanical (or behavioural, via the pollinator) barrier. (b) Postzygotic: hybrid inviability. (c) Prezygotic: gametic isolation. The test is whether a zygote has formed yet.

• 2.A population of beetles is split by a new river. After 5,000 years the river dries up and the two groups meet. How could you find out whether they are now separate species?Show answerHide

  See whether they interbreed in nature and whether their offspring survive and are fertile. If they mate freely and the hybrids are fertile, they're still one species, and gene flow will mix them again. If they won't mate, or their hybrids die or are sterile, they are separate species. If hybrids are only less fit, reinforcement may strengthen the barriers over time.

• 3.A gardener finds a tetraploid (4n) plant among a population of its diploid (2n) species. Why would crossing it with a normal plant rarely give fertile offspring?Show answerHide

  The tetraploid makes 2n gametes and the diploid makes n gametes, so their offspring are triploid (3n). In meiosis, three copies of each chromosome can't be split evenly into two cells, so most gametes are unbalanced and the triploid is largely sterile. The tetraploid is reproductively isolated from its parents.