Evolutionary trends in specialization

Given that we can hypothesize circumstances that might favour specialization over generalization and vice versa, how might the frequency of specialization and generalism vary over evolutionary time? There are three commonly cited hypotheses. First, specialists may be more prone to extinction than general-ists. Second, specialists may be more prone to speciation than generalists. Third, lineages may change anagenetically from generalist to specialist more frequently than the reverse.The first hypothesis is intuitive.A generalist seems more likely to maintain a positive rate of increase in the face of environmental change, and a species with a large geographic range or population size would seem less likely to be prone to extinction causing events. The prediction is supported by metapopulation models (see Chapter 13), where the greater the density of habitable patches, the less likely the population will go extinct. The second hypothesis is motivated by the observation that a large number of species, especially those in adaptive radiations are ecologically specialized.

The third hypothesis, an anagenetic trend to specialization, was hypothesized by Simpson (1953) to be the dominant trend in adaptive radiation. It could come about for a number of reasons: resource partitioning through competition, adaptation might be quicker through specialists than generalists (see above); specialization might allow speciation through host switching (see above); specialization might canalize evolution through suites of co-adapted traits acting as constraints (Schluter 2000).

Does specialization tend to increase the rate of extinction? Here the evidence strongly supports the hypothesis. Species persistence times in the fossil record are higher for generalist clades in crinoid worms, marine gastropods, marine bivalves, shrews, and antelopes. In recent (anthropogenic) extinctions, there is a tendency for increased extinction risk with decreasing geographic range in primates, carnivores, and birds, and increased extinction risk with increasing dietary specialization in hoverflies, reptiles, birds, and primates (Purvis et al. 2000), and with habitat specialization in a number of taxa (Fisher and Owens 2004). Does specialization tend to increase the rate of cladogenesis? Although there is some evidence consistent with this observation, there is also potential counter-evidence. For example, Owens et al. (1999) found that bird species richness is positively correlated with dietary and habitat generalism. This,however, might be due to a lower extinction rate or a higher speciation rate, or both.

As regards transitions, trends are seen across phylogenies for increases in specialization over time, increases in generalization and for variable transition rates between the two. Schluter (2000) in a survey of 20 plant, invertebrate, and vertebrate phylogenies found no overall tendency for the ancestor of the radiation to be a specialist rather than a generalist or vice versa. Some taxa, such as the Galapagos finches, had a generalist ancestor, while others, such as the Hawaiian silverswords, had a specialized ancestor. Only in one group, the genus Aquilegia, was there a tendency from a generalist to specialist state. In the other groups there was no detectable trend.

The evidence to date suggests that generalization reduces extinction rates, that degree of specialization variably affects speciation rates, and that transitions between specialists and generalists are also variable. The next chapter asks what all these species are specialized at doing, and how that evolves.

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