Well, Razib said not to do an intro post, but I figured I should at least say that I'm new 'round these parts, and give a bit of background on my interests.

Everyone knows that evolution is a continuous process, where one population is descended from an ancestral population by a string of intermediates who were capable of interbreeding. Nevertheless, evolution seems to inevitably result in discrete units, which we call species. It would seem that since the beginning, evolution by natural selection was proposed as the causal mechanism being "The Origin of Species". But the general agreement is that Darwin mostly explained the origin of biodiversity, not the origin of species. Part of this problem is that it is incredibly difficult to define a species; we all feel like we can tell when two different populations are species, subspecies, races, or whatever you want, but when it comes down to a hard and fast definition, it is incredibly difficult. This has been the life's work of many biologists and philosophers, including scienceblogger John Wilkins. I'm not going to propose that I know the true definition of a species, but rather that there is one definition that is the most conducive to empirical work regarding species.

That definition is the biological species concept (what, did you think I would suggest anything else?). The BSC was first fully articulated by Ernst Mayr in his classic (but in my opinion somewhat boring) book, Systematics and Origin of Species. The current standard definition is that

species are groups of interbreeding natural populations that are reproductively isolated from other such groups.

This definition shows of precisely why the BSC is a useful definition for research: it tells that if we want to understand the origin of species, we ought to understand the origin of reproductive isolation! This has been the key motivation in the study of speciation for many years now, and is typically broken down into two components.

First of all, there is post-zygotic isolation. This is the kind of reproductive isolation people refer to when they say that horses and donkeys aren't the same species because mules are sterile. However, besides hybrid sterility, there are other forms of post-zygotic isolation, including hybrid inviability and hybrids being less fit. The latter is a particularly interesting case, as we have several interesting (and oftentimes bizarre) stories of hybrids showcasing phenotypes intermediate between the parental phenotypes. An interesting example involves the hybridization of two populations of blackcap with different migratory directions in the lab. The hybrids were actually shown to migrate a direction intermediate to the directions of the parental populations. The fitness consequences should be obvious should this mating be widespread in the the wild.

Pre-zygotic isolation most often refers to pre-mating isolation. For example, various Drosophila species won't mate because they don't understand each others' mating dances. This is relatively easy to understand, but the origin of pre-zygotic isolation was the cause of one of the biggest debates in the history of the study of speciation. One one side, there were those that argued that pre-zygotic isolation would accumulate faster if two incipient species came into secondary contact than it would if they remained allopatric. This idea, called reinforcement, has a satisfying logic to it: if there are fitness consequences for creating hybrids, perhaps there would be selection to minimize the amount of hybrids that are made. Unfortunately, for many years, reinforcement did not seem plausible. That is, until a very important paper was published...

I'll talk about that paper next time (might be a while, though... I'm in the middle of finals right now).