Speciation: The process by which biological diversity is generated.
Although Darwin's seminal work remains a triumph of natural history, it fell short of justifying its title. Today, speciation is one of the most active areas of research in evolutionary biology. Much of its current groundwork was laid out during the 1940s, through the efforts of Dobzhansky, Mayr and Simpson. Ernst Mayr in particular has become immutably associated in name with standard concepts of speciation.
Speciation is in itself a troublesome concept without a rigourous definition of species. We can circumvent this bootless venture by reducing the problem in twain: the generation of genetic variation and its maintenance by reproductive isolation. Genetic variation is what makes one individual different in appearance, behavior, and aptitude for survival and reproduction than another; more specifically, it is the determined genetic basis underlying such differences. It is ultimately generated by mutation and preserved against the purifying act of natural selection through several mechanisms, including self-incompatibility, population subdivision, and sexual reproduction. However, most people tend rather to associate reproductive isolation with speciation. This brings us to the predominant definition of a species, the Biological Species Concept (sensu Dobzhansky and Mayr): "a species is a group of populations that are reproductively isolated from other such groups by genetically-based mechanisms that prevent the exchange of genes" (paraphrased from Coyne and Orr, 1998). I will consequently place an emphasis on mechanisms of reproductive isolation in this write-up. The maintenance of genetic variation will be the topic of a subsequent write-up.
Reproductive isolation occurs when matings between individuals belonging to different groups are less successful than matings that occur within groups. Conventionally (i.e.by the biological species concept) two subpopulations are considered to have speciated only when they are completely reproductively isolated. This is owing to the rather low level of gene flow that is sufficiently capable of homogenising the genetic state of two populations exchanging individuals. There are many mechanisms by which action reproductive isolation may be caused, and several may be active concurrently. This includes discriminate mate choice, gametic incompatibility (e.g. rejection of pollen or sperm coat proteins), and genetic incompatibility (e.g. different chromosome number preventing proper segregation; incompatible gene products).
Coyne JA and HA Orr. 1998. The evolutionary genetics of speciation. Philosophical Transactions of the Royal Society of London B 353: 287-305.