It is just fact that diversity exists within species. It is agreed that mutations and change occur in life. The question is: do these changes we are aware of within species, account for the variety of life? Richard Dawkins puts it this way, that within the relatively short time of existence of human life, we have bred dogs to such a great variety. From wolves and great danes to the tiny chihuahua. He extrapolates this and postulates that if we can account for the variety of dogs, then within the billions of years of life on Earth, surely the variety of life can be accounted for by similar processes, ie. natural selection.
However, to take a closer look, we notice that all dog species have 2 eyes, 4 legs, 2 ears, a tail, canine teeth and paws etc. Some things have not changed. And to use his logic, if it didn't change for these thousands of years, what empirical evidence can I provide that they will change in millions of years?
All of the evidence for evolution from one species to another are simply similarities. For example, when we humans develop in our mother's wombs, we, believe it or not, possess gills. Evolutionists say that because other forms of life have gills, that means one is probably descended from the other. (It is also possible that there could be 2 branches of the tree of life which evolved gills independently).
They automatically assume that likeness implies that they are related. But scientists know that likeness doesn't always mean that 2 things are related. Eg, both bats and birds have wings, but evolutionists contend that one did not evolve from the other.
Also, instead of providing how evolution happened assuming it did, evolutionists provide a means of how evolution could have happened. I trust you see the difference.
I believe we can all agree that atheists must believe in complete and naturalistic darwinian evolution in order to maintain their atheism. Theists on the other hand have the option of believing in evolution or not.
I must leave with this note. Both creationism and evolutionsim begin with premises concerning God. The former believes that God created the universe and tries to understand how God 'did it'. The latter holds that God is not needed to answer questions regarding our natural world, and seeks how the natural laws and processes govern the universe. I ask why not come to the scientific evidence without preconceptions of God and instead of reading 'creationistic' or 'atheistic' into the evidence, lets be real and complete evidentialists and draw our conclusions from the scientific evidence. If science says evolution, then evolution it is. If science says not evolution, then it is not evolution.
(of course, we should not think that all knowledge is to be known through science. Science has always been one of the channels of knowledge amongst philosophy etc)
Also, a question from me (a real question, not a rhetorical question):
If you put a male dog and a female dog on a planet and this planet is such that natural selection do not exist, if I came back to it in a million yrs, I should see a great variety of dogs. If the same planet 'had' natural selection, then wouldn't we see a lesser variety of dogs(natural selection would wipe out the inferior dogs)?