Contrary to popular belief, Christians do not believe in the fixity of species. It is not the amount of change that is in dispute, but the type of change. Like a banging drum, Creationists repeatedly have to explain the difference between Evolution, the mythical process by which fundamentally different animals came into existence, and natural selection, the well observed process by which already existing animals are divided into species to help them fulfill God's command to fill the Earth.
Natural selection is, by definition, a culling process. You can only select from what exists. If a thing does not exist, it cannot be selected. Therefore, natural selection alone can only cause extinction. It cannot cause Evolution. Evolution, by contrast, purports to explain how that which is selected originated.
In the above diagram, we see four varieties of finches. The most notable difference between them is their beaks. Number 1 has a thick, powerful beak, whereas number 4 has a very thin beak, insufficient for breaking nuts. Natural selection does not explain where the beak came from in the first place, but it does explain how, in an environment that favours thick beaked finches, 1 and 2 would be more likely to thrive, whereas 3 and 4 would likely struggle, and even die. This would lead to a smaller variety of finches as the thin beaked species would go extinct.
The lower diagram is the classical depiction of human Evolution. You have a chimp at one end, a human at the other end, and three completely imaginary ancestors that, to this day, are absent from the fossil record. Sorry Charlie... This diagram is what Evolution is really about. It is not about the life and death of what already is, but the brand new creation of what can be. While the ongoing process of natural selection alone would result in nothing existing, Evolution supposedly explains how things came to exist. No amount of killing apes (or "ape-like" creatures) is going to create a human, because apes just do not have the genetic variability to produce humans.
You can see, then, that one does not have to believe in Evolution to accept natural selection. As a matter of fact, Creationist Edward Blyth spotted natural selection before Darwin. There is strong evidence to suggest that Darwin may have even plagiarised Blyth. Natural selection and Evolution are different things. Natural selection is a culling process, Evolution is a creative process. Natural selection is essential to the Creationist model, Evolution is antithetical to the Creationist model. Natural selection is scientifically observable, Evolution is scientifically destitute.
The question arises, then: Why do Evolutionists, when asked for evidence of Evolution, instead offer evidence of natural selection? The answer is obvious: real Evolution doesn't sell well on its own. No one has ever observed any Evolutionary change. No one saw a human descend from an ape. No one ever saw a horse evolve from a hyrax. No one ever saw a bird come from a dinosaur. We've seen birds become birds, horses become horses, and humans become humans, but that's just not very impressive. Evolution, therefore, can be naturally selected against by rational thinkers.