Concept preview

What you will be able to do

1. 01Define a species and describe pre-zygotic and post-zygotic reproductive isolating mechanisms.
2. 02Compare allopatric and sympatric speciation.
3. 03Explain gene pools and use the Hardy–Weinberg equations to calculate allele and genotype frequencies.
4. 04Describe genetic drift, gene flow, bottleneck and founder effects, and distinguish micro- from macroevolution.

To understand how new species form, we first need to know what a species actually is. The most widely used definition is the biological species concept: a species is a group of organisms that can interbreed in nature to produce fertile offspring. By this definition a horse and a donkey are different species, because although they can mate, their offspring — the mule — is almost always sterile. The key idea is reproductive isolation: the members of one species share a gene pool that is sealed off from that of other species. This concept works well for most animals, but it has limits — it cannot be applied to fossils, to asexual organisms such as bacteria, or to organisms that hybridise freely — so biologists also use other species concepts, based on appearance or DNA, when the biological definition cannot be tested.