Speciation and species concepts

Last time

Before we start: last time, in Genetic drift, Hardy-Weinberg and molecular evidence, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

When does one species become two?

Biologists name over a million species, yet they argue about what a species actually is. There is more than one definition, and each has strengths and gaps. Then there is the process by which one species splits into two, which needs a barrier to gene flow. Exam questions combine both, usually with a named example.

The biological species concept

The biological species concept says that a species is a group of organisms that can interbreed to produce fertile offspring, and that is reproductively isolated from other groups. A horse and a donkey can mate, but the mule is sterile. The horse has 64 chromosomes and the donkey 62, so the mule has 63. An odd number cannot pair up in meiosis. The concept fails for organisms that reproduce asexually, for fossils, and for species that hybridise.

Other species concepts

The morphological species concept uses physical appearance and structure. It works for fossils and asexual organisms, but variation within a species can mislead. The ecological species concept groups organisms by their niche, and the phylogenetic concept uses the smallest group sharing a common ancestor, shown by D N A sequences. Each concept has limits, so in practice several lines of evidence are combined.