Birth of species

Sympatric speciation is like the measles; everyone gets it, and we all get over it.

attributed to Theodosius Dobzhansky

Until now we have been concerned almost exclusively with evolutionary change within lineages—anagenesis.Anagenesis creates differences between lineages and hence the diversity of form and function that we observe as a major feature of our planet. It is, however, just one of the two major evolutionary processes, the other being cladogenesis. Cladogenesis is the creation of new lineages, speciation. Cladogenesis and anagenesis are jointly responsible for life's diversity: without cladogenesis, anagenesis could not realize life's potential for phenotypic diversity. Without anagenesis, as we shall see, speciation could not occur.

Biologists have identified four distinct phylogenetic processes that have created new lineages in the history of life (Figure 12.1), of which we have already discussed two. The first is the assembly of biological entities from non-biological chemical processes (Chapters 2 and 3). We have no evidence that this has occurred more than once. The second process is the fusion of two lineages into a single biological entity via symbiosis (Chapters 2, 10, and 11). This has occurred a limited number of times but has often had important consequences. The third process is hybridization: two individuals

Origins of life Symbiosis Hybridization Lineage splitting

Four phylogenetic processes giving rise to new lineages.

from different species produce viable and fertile offspring that become reproductively isolated from their progenitors. The fourth process is when one species divides into two or more. It is these last two processes that will form the subject of this chapter.

In general biologists studying speciation are interested in three types of change: reproductive isolation, genetic differentiation, and ecological differentiation. Sometimes these act in concert, sometimes one results automatically in another, and sometimes one or more are considerably delayed. For example, in Chapter 1 we considered the case of Lake Victoria hap-lochromine cichlids, where reproductive isolation occurred first via changes in mate choice and coloration, followed by ecological change, while genetic differentiation leading to irreversible separation of lineages has yet to occur in many lineages. In the case of hybridization, all three processes (reproductive isolation, genetic differentiation, and ecological differentiation) may be a simultaneous consequence of the formation of hybrids.

The architects of the great Neo-Darwinian Synthesis (such as Dobzhansky, Fisher, Mayr, Simpson, Wright) laid down the groundplan for thinking about speciation. Mayr, in particular, was of the view that speciation of the fourth kind (a lineage splitting into two or more lineages) occurred allopatrically in so-called peripheral isolates, via the founder-effect, a largely non-adaptive process in which small initial isolates created a novel starting gene pool for an incipient species. Genetic differentiation could then be enhanced by divergent natural selection. He sternly denounced the possibility of sympatric specia-tion.Wright also advocated a large role for non-adaptive processes in his 'shifting balance' theory, whereby a small population could traverse a non-adaptive valley by genetic drift, and then diverge through natural selection up a different adaptive peak. Adaptive processes were simply not seen as necessary by most for speciation in allopatry, sympatric speciation was deemed unlikely at best, and natural selection could finish what other processes had started.

Since then we have gradually entered a phase in which the conceptual barriers to sympatric speciation have been eroded, the role of adaptive evolutionary change in the speciation process has become more widely appreciated, and sexual selection increasingly plays a role in our views of speciation. The relevance of speciation by hybridization has had a similarly chequered history, being more popular today than formerly. We should not fall into the trap of believing that the current vogue of research into these areas represents in some sense a greater truth. When a field swings from one extreme to another, and has yet to settle down, predicting where the equilibrium will be found is difficult and risky. There is, however, a sensible prevailing tendency to consider alternative processes more widely, and to ask not if, but in what circumstances, a process may be relevant. Data are still relatively thin on this latter point, but not totally absent, and there are good prospects for quantitative answers in the near future.

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