What's species got to do with it?

Today I'm one of the speakers at a one-day symposium (sponsored by the Swedish government and organized by Patrick Keeling) in honour of the 300th birthday of Linnnaeus. The focus of the symposium is "Species", so I've been working to integrate issues surrounding species to my talk on "Do bacteria have sex?".

The two pictures at the left illustrate how genetic exchange transforms what would otherwise be a clonal population into a species. Think of each skinny line as representing a gene or allele. In the clonal population, all the alleles in any one genome stay together (the broad green bars represent genomes). Alleles change only by rare mutation, so each allele becomes specialized to work well with only the particular alleles of other genes that are present in its own genome.

But in populations with sexual reproduction, all the alleles of the genomes' genes are reshuffled each generation. The two sets of alleles we inherited from our two parents are recombined into new single sets in the gametes we contribute to our children. That's why I've represented the sexual species by a mesh, like macrame, with different strands coming together and then separating to join with other strands. This frequent changing of 'genetic environment' means that individual alleles can't specialize, but must be generalists, able to work well with all the alleles of all the other genes in the population. This is what creates the genetic cohesion that makes a biological species.