Okay, I'll buy that...but what causes the process to happen? Is it slight inaccuracies in gene transfer at meiosis?
Also...what does it take to get a population of animals that are cross-fertile with one another, but no longer cross-fertile with other descendants from their ancestral line?
"Okay, I'll buy that...but what causes the process to happen? Is it slight inaccuracies in gene transfer at meiosis?"
Several:
1. Primarily, mistakes in DNA replication (for animals and plants this has to be in the germ cell line). If you are not familiar read the history of hemophilia in the royal households of Europe, all began by a mutation in one of Victoria's eggs.
2. Natural mutagenic things - cosmic rays, chemicals like polyaromatics, mutagens in your food, radiation, UV light, etc. (I know these are mutagens, but they still are "natural").
Also...what does it take to get a population of animals that are cross-fertile with one another, but no longer cross-fertile with other descendants from their ancestral line?
Physical separation for one, but I will have to defer to others for other answers.