One of the hallmarks of evolutionary theory is speciation, however most people can’t grasp this concept. You’ll often things like, “If humans came from monkeys , why are there still monkeys?”
The answer is for the same reason there are still wolves even though dogs descended from them. Namely you have two species who’ve adapted to their different habitats. This is speciation in action.
When two populations of the same species are separated by location and time their environments trigger mutations and when the genetic difference between them is great enough they can no longer produce viable offspring.
Mules are the quintessential example of this. Born from a male donkey and female horse, most of the time they are sterile and their characteristics vary wildly due to the uneven number of chromosomes they have.
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| image by dimitri c via sxc.hu |
As for humans, their closest genetic cousin is the chimpanzee. The difference between our DNA is two percent. Owing to their 48 chromosomes versus 46 for us. Thus we can’t produce viable offspring.
The other misconception about speciation deals with confusion over what a species. For example you’ll here people talk about “kinds” of animals. The problem with this is species can be genetic cousins like humans and chimps or they can be vastly removed from each other e.g. dogs and whales.
So asking why a dog doesn’t give birth to a cat or other species is intellectually dishonest at best and idiotic at worst.
Speciation is like a giant family tree. You have different branches representing different species of animals. Trance them back far enough and you’ll find a common ancestor between them.
Most modern mammals share a common ancestor with an insect eating, rat-like creature that lived 66 million years ago. Likewise mammals, birds and reptiles share a common ancestor.
So remember next time someone brings up evolution and speciation, tell them, “Yes, you are a monkey’s uncle.” Several times removed of course.