Those who say that the variety of species today could not have evolved by chance do not understand evolution by natural selection.
This page will be very simple. It's not meant to explain the theory of evolution; I do that elsewhere. Here I simply mean to show that the two step process that is evolution by natural selection so that you can see it is not chance.
In this context, "negative" mutations are those that hurt an organism's chances for reproduction, and "positive" mutations are those that make the organism more likely to reproduce, and thus pass on its genes.
The process is not chance.
What are the odds that nature will continuously produce many mutations and that some of those will be beneficial to survival?
From observation, we can say that those odds are 100%. That's not chance, that's certainty.
Natural selection takes it from there. The individual with the beneficial mutation passes its variation on to its descendants. Its descendants are also more likely to survive in their environment than their peers, and soon the mutation has spread through the whole species. Or worse, the rest of the species dies, and only the lineage with the favorable variant survives.
Does that really happen?
Every day. The worst examples of this are diseases that we've been treating. AIDS, tuberculosis, staph infections, etc. We kill off all the viruses or bacteria except the ones with genetic variants that make them immune to our medicines. Suddenly our medicines are ineffective.
That's terrible for us, but it illustrates evolution by natural selection. As far as the bacteria or viruses are concerned, things are wonderful. They're surviving better!
The term "natural selection" was coined as a contrast to "artificial selection." Artificial selection is what farmers and breeders do, picking the traits they want in cattle, dogs, doves, or whatever else they might be breeding. Artificial selection is very effective at changing species rapidly.
Evolution by natural selection does this by survival and reproduction. Those mutations that harm survival lead to lower reproduction. Those that increase survival lead to greater reproduction and change in the species