Primer Selection and the origin of species

Why are there so many species on earth? Answering this question requires an understanding of how species form. An obvious place to start looking for answers is Darwin's 'On the Origin of Species by Means of Natural Selection' (1859). But his title is deceptive: Darwin's book is about adaptation and the origin of varieties and has surprisingly little to say about selection and " the origin of species — that mystery of mysteries ". To be fair to Darwin, it was not for another 80 years or so that the modern view of the species was developed. The 'biological species concept' defines a species as one or more populations of potentially interbreeding organisms that are reproductively isolated from other such groups. Humans and chimps are today separate species not only because we are genetically and phenotypically distinct, but because we are reproductively isolated. Neither finds the other attractive when choosing a mate ('premating isolation') and very likely, hybrids are inviable or sterile ('postmating isolation'). Reproductive isolation is therefore the most salient evolved feature of a species, at least in sexual organisms. Even 'good' species may hybridize once in a while, but to meet the species criterion the flow of genes between them must be negligible. The study of speciation is therefore the study of how reproductive isolation evolves, premating or postmating, between populations. Natural selection is the differential survival or reproductive success of individuals differing in phenotype within a population. Sexual selection, by contrast, is the differential mating success of phenotypically different