1What a species is

The biological species concept defines a species by reproduction: members can breed together, in nature, and their offspring survive and are themselves fertile. Horses and donkeys can mate, but their offspring, mules, are almost always sterile, so horses and donkeys are separate species.

The concept works well for sexually reproducing organisms living at the same time. It cannot be applied to organisms that reproduce only asexually, such as bacteria, or to fossils, whose breeding cannot be tested. For those, biologists group organisms by shared structure or DNA sequence instead.

Two populations are separate species when genes no longer flow between them: no interbreeding, or no fertile offspring if they do.

2Allopatric and sympatric speciation

Allopatric speciation happens when populations are geographically isolated: a river changes course, a mountain range rises, sea level rises and cuts off a peninsula, or a few individuals reach an island (Figure 1a). With no gene flow between them, the two populations are exposed to different selective pressures, drift independently and gain different mutations. Over many generations they become so different that, if the barrier later disappears, they no longer interbreed successfully. This is the commonest route, because a physical barrier stops gene flow completely.

Sympatric speciation happens in populations that overlap geographically, living in the same area (Figure 1b). Something other than distance must reduce gene flow. The CED's example is the apple maggot fly (Rhagoletis) in North America. These flies mate on the fruit of the tree they feed on as larvae. Hawthorn was the original host; after apple trees were introduced, some flies began to use them. Apples ripen earlier than hawthorn fruit, so flies on apples mate earlier in the year. Flies that mate on different trees at different times rarely mate with each other, and the apple and hawthorn populations now differ genetically, even though they live in the same orchards and hedgerows. In plants, an error in cell division that doubles the chromosome number can also create a new species in a single generation, because the doubled offspring cannot produce fertile offspring with the parent species.

3The barriers that keep species apart

The CED's exam advice is blunt: name the mechanism precisely. Sort by timing: does the barrier act before a zygote forms or after?

All of these do the same job: they prevent gene flow between the populations. Once gene flow stops, the populations evolve independently.

4How fast? Gradualism and punctuated equilibrium

The fossil record shows two patterns, and both happen.

Gradualism: a lineage changes slowly and steadily, by small amounts, over hundreds of thousands or millions of years. In successive rock layers, the fossils shift a little each time, with many intermediates.

Punctuated equilibrium: a lineage stays almost unchanged (stasis) for a long time, then changes rapidly, often when a new species forms, then stays stable again. In the rock layers, one form persists through many layers and then is replaced over a short span by a different form, with few intermediates.

Which pattern a lineage shows depends on its environment. A stable environment with stabilising selection favours stasis. A sudden change, such as a new habitat, a new competitor or a climate shift, or the isolation of a small population, can produce rapid change.

5Divergent evolution and adaptive radiation

Divergent evolution happens when related populations adapt to different habitats or ways of life and become more and more different in phenotype. Homologous structures (topic 7.6) are its result: one ancestral forelimb has diverged into arms, wings and flippers.

Adaptive radiation is divergent evolution at high speed: one ancestral species gives rise to many new species in a short time, each adapted to a different way of life. It happens when many new habitats or resources become available at once with few competitors: on a newly formed island chain, in a new lake, or after a mass extinction empties many niches. The CED's examples:

• Hawaiian Drosophila: hundreds of species of fruit fly, found nowhere else, descended from very few colonists that reached the young, isolated islands and spread into habitats on different islands and at different altitudes.
• Caribbean Anolis lizards: on each large island, lizards have diversified into species specialised for different parts of the habitat, such as tree trunks near the ground, high canopy or thin twigs, with legs and toe pads to match.

6Convergent evolution

Convergent evolution happens when similar selective pressures produce similar phenotypes in populations or species that are not closely related. The resulting similar structures are analogous, not homologous.

• Sharks (fish) and dolphins (mammals) both have streamlined bodies and fins, because fast swimming in water selects for that shape.
• The Anolis lizards show convergence too: the same set of specialists (trunk, canopy, twig) evolved separately on different islands. The twig lizard on one island is more closely related to the other lizards on its own island than to the twig lizard on the next island, yet they look alike because they live alike.

Convergence is why morphology alone can mislead a phylogeny, and why molecular data are preferred (topic 7.9).

7Predicting the effects of a change

• A barrier forms between parts of a population: gene flow stops; the populations diverge; given enough time and different pressures, they may become separate species.
• A barrier disappears before isolation is complete: gene flow resumes and the populations may merge again; or, if hybrids are less fit, selection may favour individuals that mate only with their own kind, strengthening prezygotic barriers.
• A new host, food or habitat appears within a population's range: individuals that use it and mate there may become a separate breeding group, the start of sympatric speciation.
• Islands or empty niches become available: expect rapid divergence and possibly an adaptive radiation.