Whenever a lineage splits into more than one branch, it has undergone speciation. Yet, I should hate to be asked exactly when the time comes to call them ‘split’. That is like asking exactly when a child becomes an adult. I am content to know that initially there was one lineage and now there are more.

But I would like also to know what mechanism drives the speciation. Like the maturation process of a human being, that mechanism can be studied and described.

Three basic modes of speciation have given us life's diversity:

• Geographical speciation

• Polyploidy

• Competitive speciation

Geographical speciation is the most orthodox. Polyploidy is the best tested. Competitive speciation is the most controversial. You probably learned other words to describe speciation processes, but I believe they all describe one of these three modes.

Geographical speciation

Here is an outline of the process of geographical (also called allopatric) speciation.

• A geographical barrier restricts gene flow within a sexually reproducing population.

• The isolated subpopulations evolve separately for a time.

• They become unlike enough to be called different species.

• Often the barrier breaks down and the isolates overlap but do not interbreed (or they interbreed with reduced success).