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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.
The two processes are represented here graphically for a hypothetical species. Time is a function of variability. The horizontal width of any area shows the number of individuals. A horizontal line shows rapid evolution, and a vertical one shows little.
It can be seen from this graph that after the initial episode of cladogenesis, the process was about to repeat again with the species on the right, but the process did not succeed.
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