It had come to my attention, in the course of exposure to more creationist/I.D. arguments, that there are some who find the puzzle of Stasis to be an issue unmining the theory of evolution, so I'm going to put my 2 cents in regarding this.
First, a quick cover of the Fact vs Theory argument. As far as biologists are concerned, that evolution has happened, and continues to happen, is fact. (Bacterial evolution is pretty obvious in the now resistant strains that are harder to kill off inside of a human body, as well as recent viral mutations in the swine flu virus). the mechanism behind the fact is considered still theory, with Darwin's theory of Natural Selection being the core, if modified, of modern evolution theory.
Stasis in evolution refers to thos species which have been around a very long time with little if any change in over all appearance and biology. So they haven't done much evolving.
it seems to me the answer is relatively simple, if one remembers that evolution itself (according to natural selection) is simply a by product of survivability. If a species' environment, including available food sources, predators, and competitors, remain consistent pressures thent here is little natural selection going on to force change.
Rnadom mutations may pile up and cause genetic drift, but if no mutation creates a shift in balance such that one variation is superior to the original, then at best that new gene will simply be drawn into the normal gene pool, becoming simply a variation of the species, rather than starting off a new track to follow.
As far as I'm concerned, the fact that some species live in stasis is a logic result of natural selection in an evironment where there is little pressure to force that selection.