More specifically, natural selection. Populations of microorganisms are not uniform. There is always some spectrum of properties. For example, certain virions of HIV may have a mutated protease. A lucky mutation (lucky for from the point of view of the virus, of course) makes a drug molecule incompatible with the receptor. Other viruses do not proliferate, but the lucky one does and starts a new population, passing on the favorable mutation. The effect of this mutation may be then amplified by subsequent mutations/natural selection.
More specifically, natural selection. Populations of microorganisms are not uniform. There is always some spectrum of properties. For example, certain virions of HIV may have a mutated protease. A lucky mutation (lucky for from the point of view of the virus, of course) makes a drug molecule incompatible with the receptor. Other viruses do not proliferate, but the lucky one does and starts a new population, passing on the favorable mutation. The effect of this mutation may be then amplified by subsequent mutations/natural selection.
But the question wasn't about natural selection. It was about speciation.
I don't see that any creationists deny variation within species or natural selection. The folks at AiG and CRI don't, which you'd know if you ever bothered to read what they have to say.
The issue most creationists have with evolution is that enough change is possible to create entirely new species. The kinds of major changes that evolutionists insist occurred to get from bacteria, to trilobites, to dinosaurs, to birds and mammals.
That includes major changes in the number of chromosomes in many cases, and I have yet to be informed of any changes in the number of chromosomes which has not had a deleterious effect on the individual in which it occurs.
What you are talking about, darwin referred to as diversification. We see this in cats, dogs, all kinds of species will adapt to conform with their natural environment.
The same phenomenon is responsible for drug resistance. However, this is different then say a virii transforming into a bacillus, which is what evolution insists we ought to see.
Darwin assumes that diversification eventually leads to the transformation of species, but cannot prove his assertion.