A species is a group of individuals which are:

1. Morphologically alike

2. Capable of interbreeding to produce viable and fertile offspring.

3. Able to demonstrate a limited flow of genes between species.

There are two ways species can develop...

Anagenesis

This is where the evolution of a species is linear, and gradually changes from one species to another.

Cladogenesis

This is where a multiple of species develop from one. This process is sometimes referred to as divergent evolution or speciation.