Human Speciation

Species refers to a group of individuals that can reproduce and produce fertile offspring.  As populations of the same species become isolated from each other, their gene pools become different enough that their offspring would no longer be fertile.  I usually teach this by talking about several common examples:

Horse + Donkey = Mule

Lion + Tiger = Liger

Horse + Zebra = Zorse

We know that the organisms on the left are distinct species because the animals on the right are infertile.  Mules cannot make more mules.

Calamities of Nature have given me a new example of speciation in human populations: