Proximate sources of marine biodiversity
When temperature and other kinds of barrier divide formerly continuous populations and confine them to more restricted geographical areas, there is an evolutionary reaction that will, over time, result in the formation of endemic species. In such cases, an allopatric speciation process is considered to have taken place because reproductive isolation was caused by physical means instead of by natural selection. In contrast, when populations exist in a very high-diversity area and remain undivided by physical events, they exhibit a tendency to speciate by means of sympatry (or parapatry). This process, sometimes called competitive or ecological speciation, does involve reproductive isolation by means of natural selection. Populations that exist in geographical provinces bounded by physical barriers add to the overall diversity through the production of endemic species. This increase by species packing is relatively slow due to the very gradual tempo of the allopatric speciation process. Populations existing in centres of origin add to the general diversity through the production of species that are dominant in terms of their ability to spread over large parts of the world. It is proposed that such species are usually formed by sympatric speciation, a process that can be c. 20 times faster than species formation by allopatry. It is not suggested that sympatry is exclusive to centres of origin, nor that allopatry is confined to peripheral provinces. Both processes are widespread, but there do appear to be distinctive geographical concentrations. Considering that numbers of widespread species produced by centres of origin may eventually become subdivided by barriers, and thus give rise to descendants by allopatry, it is difficult to say how much of our present species diversity has come from one source or the other. Both speciation by sympatry from centres of origin and speciation by allopatry in peripheral provinces appear to be important sources of marine biodiversity.