Evolution and Species Diversity

Analyse asks you to identify the components of a process and show the relationships between them. It is a step above explain: you must break speciation into its parts and show how each part depends on the one before.

6 marks. Set out the sequence as distinct stages, show what each stage contributes, and finish by showing how one round of the sequence can repeat to give many species rather than two.

Speciation usually begins with geographic isolation, in which a single population is physically divided so that its parts can no longer interbreed. A river changes course, a sea level rise cuts an island from the mainland, a mountain range rises, or a band of unsuitable habitat separates two areas. Gene flow between the two groups stops, which is the essential consequence: the two gene pools are now independent.

The two environments are rarely identical, so different selective pressures act on each group: different climate, food sources, predators or competitors. Each population accumulates different allele frequencies as natural selection favours different traits, and random processes add to the divergence, particularly in a small isolated population where chance alone can fix alleles. Because no gene flow connects them, nothing averages the two gene pools back together.

As the gene pools diverge they may accumulate differences that prevent successful interbreeding even if the two groups meet again: differences in breeding season or courtship behaviour, incompatible reproductive structures, or genetic incompatibilities that make hybrids inviable or sterile. At that point the two populations are separate species, because they can no longer produce fertile offspring together.

Species diversity, rather than a single pair of species, arises because this sequence can repeat and can run many times in parallel. Where one ancestral population reaches an area offering many unexploited ways of living, populations may become isolated in different habitats and be selected for different resources, producing a group of related species each adapted to a distinct niche. This is adaptive radiation, and it is why a single colonising lineage can give rise to dozens of descendant species.

The eucalypts illustrate the outcome at scale: from Australian ancestors the group has diversified into several hundred species occupying habitats from alpine country to arid inland and coastal forest, differing in bark, leaf and fruit structure and in tolerance of drought, frost and fire. The diversity is the accumulated product of many rounds of isolation, divergent selection and reproductive isolation across a long period on a large and environmentally varied continent.

Explain here means giving the reason one criterion counts and the other does not. The answer turns entirely on the definition of a species, so state that definition early.

4 marks. Give the species definition, show why geographic isolation fails it, show why reproductive isolation satisfies it, then give a case that separates the two.

A species is generally defined as a group of organisms that can interbreed under natural conditions to produce fertile offspring. The test for a new species is therefore about reproductive compatibility, not about where the organisms live.

Two populations separated by a barrier are not interbreeding, but that says nothing about whether they could. Remove the barrier and they may resume breeding and producing fertile offspring immediately, which would show they never stopped being one species. Geographic isolation is therefore a circumstance that may allow speciation, and it is reversible.

Reproductive isolation is a property of the organisms themselves rather than of their geography. Once two populations have diverged enough that they breed at different times, fail to recognise each other's courtship signals, cannot mate successfully, or produce hybrids that are inviable or sterile, they will not exchange genes even if they are brought back into contact. The two gene pools are then permanently independent, which is what makes them two species.

The horse and the donkey make the distinction concrete. They are not geographically isolated and will readily mate, producing a mule, so neither separation nor an inability to mate divides them. They are nevertheless two species, because the mule is almost always sterile, so no genes flow between the two populations across the generations. Reproductive isolation is present without geographic isolation, and it is the reproductive isolation that settles the classification.