The cladistic species concept

The most important definition of a species over time is the cladistic species concept.

In the cladistic species concept, a species is a lineage of populations between two phylogenetic branch points (or speciation events).

The cladistic concept recognizes species by branch points, independently of how much change occurs between them.

Because of this in the figure, the two patterns are cladistically identical: in both there are three species and species 1 gives birth to two new species at the branch point. On all the non-temporal species concepts we have seen, there are two species in the figure, (a) and three in (b).

Figure: three patterns of evolution. Time goes up the page. The x-axis represents phenetic distance. In (a) the ancestral species does not change phenetically after a daughter species evolves. In (b) the ancestral species changes after the evolution of the new species: species 1 and 2 are both phenetically and cladistically different. In (c) there is a species that changes gradually through time.