There are six mechanisms that produce variation in the gene system:
1. Built-in variation in the gene pool
2. Reproductive exchange
3. Independent assortment during meiosis
4. Crossing over during meiosis
5. Transposable elements
6. Recombination of chromosomes
The faith required to
believe that any one of these mechanisms, let
alone all of them, came about by chance is extraordinary. If design is
the option chosen, then obviously variation of organisms is a hallmark
of Creation. God did not then create immutable, unchangeable species,
but rather an enormous capacity for change.
Classification
With these mechanisms, animals have an enormous capacity for change.
The
question is no longer whether change can take place or not, but rather
how much change and where the limits are. The modern animal classification system is based on the way species
look. However, scientists have discovered that the classification of
certain species such as mice, rabbits and whales is incorrect when
compared to their genetic make-up. For example, some mice look similar,
but have very different molecular make-up, and therefore are more
closely related to gerbils than mice, while still classified as mice.i There are also
contradictions in molecular and morphological classification of
rodents, rabbits, and primates; and even conflicting classifications in
whales.ii,iii These inconsistencies suggest that we should not classify animals based
only on their appearances.
Reproductive Isolation
Some species have a built-in mechanism that prevents crossbreeding, called reproductive isolation. Some say that species evolved this mechanism over a long period of time to keep the integrity of the species. However, reproductive isolation could develop over a short period of time if the existing genome was reshuffled. The flexibility of the genome allows for rapid change. This flexibility has nothing to do with evolution, but rather with built-in variation ability.
Here is an example of reproductive isolation that developed quickly.
Mosquitoes that fed only on birds entered a 
tunnel system in London.
The mosquitoes rapidly changed from feeding on birds to feeding on the
rats in the tunnel. These mosquitoes are now incapable of breeding with
the mosquitoes that remained above ground.
Dr. Jenny Graves of La Trobe University says this about the “jumping genes” that could cause reproductive isolation: “We thought it took millions of years of long-term selection for a jumping gene to be activated. We’ve now shown that it can happen in five minutes after fertilization.”iv
Within the gene pool of each animal is a huge capacity for variety. And as we have seen, this variety can occur very quickly, and does not need millions of years to evolve.
The Biblical concept of “kinds” must be reexamined based on this information. Read about the true definition of “kinds” in "Species" versus "Kind"
i P.C. Chevret et al., "Molecular evidence that the spiny mouse (Acomys) is more closely related to gerbils (Gerbillinae) than to true mice (Murinae)," Proceedings of the National Academy of Sciences 90 (1993):3433-3436.
ii D. Graur, "Molecular phylogeny and the higher classification of eutherian mammals" Trends in Ecology and Evolution 8 (1993):141-147.
iii M.C. Milinkovitch, et al., "Revised phylogeny of whales suggested by mitochondrial ribosomal DNA sequences," Nature 361 (1993):346-348.
iv La Trobe Bulletin (September 1998): 7-8.