Originally posted by O Great One
Evolution is defined as a change in allele frequency over time?
Ok, then let's say I have 7 white roses and 5 red roses and I kill all of the white roses. That would be considered evolution because the red roses increased from 41.67% of the population to 100% of the population. How can that be evolution when I started out with roses and I ended up with roses and nothing new has come into existence?
Ambitwister already responded well. So, let me just ramble on a bit more...
First, I assume we're all talking like the white & red roses are the same species. If not, then we have a bad example to begin with.
Some evolutionary mechanisms decrease genetic diversity (selection, etc.) and some increase genetic diversity (mutation, recombination, etc.) Extinction is not an evolutionary mechanism for the victims, but it does greatly affect the evolution of the survivors.
Your killing of the white roses would be artificial selection (if nature did it, it would be natural selection). The red and white roses groups did not have 100% identical genetic codes. Under normal circumstances, red & white interbred and their genes would have be distributed throughout the population. After your imposed extinction of all white roses, the genes associated with red would suddenly get the upper hand. If red was recessive, then any mutation on that gene may get diluted out in the overall population. But now that mutation can spread more freely.
The bottom line is that evolution includes both the creation of new species as well as changes in existing species (even without speciation). Speciation may represent the culmination of a series of smaller changes (e.g., a point where a sub-population no longer breeds with the parent population), but there is always a background of slow change. The make-up of human "races" (x percent black, y percent white, etc.) change from generation to generation, but we're still one species. If some event wipes out one race (e.g., disease), then the diversity within that group (e.g., hair type, particular bone structure, whatever) may be lost or marginalized. The evolutionary path has been redirected.
At the risk of further rambling...not only do such selection events redirect the evolution of the surviving population, but they can also open up previously filled niches that allow other species to adapt and change.
The idea that evolution is a specific "march of progress" toward a particular goal needs to be dropped.