Speciation: what is a species?

Each of us has a very strong impression of what a species is. We think understand, as Aristotle did, that animals come in well-defined types that we can easily identify. Coming up with a working definition of a species is difficult however:

Is it appearance that defines a species? Clearly this is not always a clear indicator since sexual dimorphism (differences in appearance between males and females) can make two individuals of the same species radically different in appearance. Second, we all recognize that physical differences between various breeds of dogs does not mean that they are members of distinct species.

In fact, we are perhaps too sure of our concept of a species. This perhaps comes from the residue of the pre-evolutionary concept of "natural species". We believe that species are hard-and-fast differences between types of organisms. In popular thought we do not recognize the fact that species definitions change over time.

The problem is that if species can change over time then how do new species arise (a process called speciation). Biologists use a different method to define species, the biological species concept.

The idea is that species can be defined by reproductive behavior: do individuals normally mate and produce fertile offspring? Note that by saying "normally" I am excluding the idea that the individuals are capable of producing fertile offspring, but don't do so in nature. To be considered members of the same species individuals should be members of freely interbreeding populations.

So, species can be defined as "groups of interbreeding natural populations that are reproductively isolated from other such groups"

The concept of "reproductive isolation" is an important one. What it means is that as a result of some mechanism which exists in the natural state, individuals are precluded from breeding. This is clearly the case for individuals of very different species: cats and dogs clearly are not capable of interbreeding; they are clearly members of distinct species which are reproductively isolated by the inability to produce viable zygotes (the zygote is the diploid cell produced by the fusion of two haploid gametes--egg and sperm). This definition of a species is more difficult with aminals who can, under very special and unnatural circumstances, produce fertile offspring