For those readers who share my undue interest in the intersection of evolution and philosophy, this post by John Wilkins from a few months back may be of interest. He surveys a number of other posts and tries to make the case that strict neo-scholastic essentialism was not in fact the prevailing view in biology prior to Darwin. Instead, he argues, it arose in response to Darwin's theory, and Ernst Mayr was primarily responsible for perpetuating the myth that essentialism had been the only real alternative and precursor to Darwinism.
Certainly, the 1840s-1860s were hardly a heyday of Aristotelian or Thomistic thought, and he's right that many 18th and 19th century philosophers and scientists subscribed to a loose typology in regards to species rather than a strict essentialism.
Still, I wonder if one of the reasons that essentialism has become one of the primary critiques of evolution (perhaps David or Shelray of Cosmos-Liturgy-Sex would know a bit more about this than I -- feel free to correct) is that essentialism touches on some very real truths that people intuitively recognize exist, and are not well explained by evolution or (more specifically) a basic materialist philosophical system ostensibly based upon it.
The evidence seems clear to me, though others differ, that the evolution of species does indeed take place. And yet, even if at a certain biological level a species is simply that population of creatures which currently possess a set of characteristics and can only breed successfully with one another, there's another sense in which the characteristics of those populations are significant -- at least if we are to make any sense of the world.
For instance, for the concept of medical science to make any sense, we must take it that certain things are 'healthy' and others 'unhealthy' -- and yet that suggests an inherent judgment as to how individuals within a given species ought to work.