Trends across taxa

What of differences between taxa? There are consistent messages from both fossil studies and studies of extant taxa. Across taxa, speciation rates and extinction rates tend to be positively correlated (Stanley 1979; Sepkoski

More species Tropical

Less species Temperate

Less species Temperate

Fig. 14.5 Correlating diversification rates with latitude. Sister clades, sharing a common ancestor and being the same age, are compared. One of the clades is largely tropical, the other temperate. The difference in their species richness is also compared. If over a number of such pairs of sister clades, the more tropical clade has the most species, then latitude affects the net rate of diversification.

Fig. 14.6 A weevil, family Curculionidae, walking on a pencil. Weevils are the most species-rich family, in the most species-rich order, class, phylum, and kingdom. Photo © Peter Mayhew.

1999). One possible reason is that extinction and speciation are both promoted by the same characteristics. Data from Jablonski (1986) on North American Late Cretaceous gastropods illustrate a potential mechanism: those taxa which have planktonic larvae, and hence are good dispersers, have wide geographic ranges, and experience low extinction rates but also low speciation rates. In contrast, taxa with direct development (no planktonic stage) have small geographic ranges and tend to have high extinction rates, but also high speciation rates. In many cases, however, we can identify taxa as differing in rates of some cladogenetic process without knowing the reason because the taxa are unreplicated; any feature that differs between the two taxa could be responsible. For example, of the four major primate groups (lemurs and their kin, new world monkeys, old world monkeys, and homi-noids), only the old world monkeys differs from the others in net rate of diversification, and furthermore, this is due to a higher rate of speciation rather than lower rate of extinction (Purvis etal. 1995). This can be inferred from the relative shapes of the phylogenies of the groups (Figure 14.2),which for the old world monkeys shows a recent burst of branching indicating a high rate of speciation over extinction. Similarly, based on their ages and extant species richness, the beetles (Figure 14.6) show a higher net rate of diversification than their sister group (Mayhew 2002), though there are many possible reasons. Other such 'significant radiations' include the canids (Bininda-Emonds et al. 1999), and the passerine and wading birds (Figure 14.7) (Harvey etal. 1991).

Fig. 14.7 A snipe, Gallinago gallinago, a member of the order Charadriformes; one of two bird orders that are significant radiations. Photo courtesy of Stephane Moniotte.

One way of pinning down the reasons for differences in rates of cladogen-esis is if replicate radiations have occurred in different groups with similar characteristics (Figure 14.5). One can then compare the species richness of these groups with those of their sister taxa which lack the characteristic or 'key innovation' in question. A large number of such studies have now been done. A number of these are consistent with theory of speciation and adaptive radition. For example, promiscuous insect groups have diversified more rapidly than their sister groups with other mating systems (Arnqvist et al. 2000). Promiscuous insects are those in which a single female is mated by many males, and the genetic interests of male and female are predicted to be divergent (see Chapter 7), leading to rapid evolution of sexual traits in an intraspecific arms race. This rapid evolution could lead to rapid evolution of reproductive isolation. Another trait that is linked with elevated species richness is sexual selection. In passerine birds, a higher incidence of sexually dimorphic coloration, indicative of sexual selection, is associated with elevated species richness (Barraclough et al. 1995). Nectar spurs in plants may have a similar effect by association with specific pollinators which promote reproductive isolation (Hodges and Arnold 1995). Other traits seem more likely to be linked with ecological opportunity. The link between species richness and phytophagy, for example, seems likely to be related to this (Mitter etal. 1988; Farrell 1998), as does latex and resin canals in plants, which provide defence against herbivores (Farrell et al. 1991) allowing 'escape and radiation' (Chapter 11).

To get at extinction correlates we can also draw on the sobering recent dataset on anthropogenic extinction. How do species that have recently gone extinct differ from their close relatives that have not? In a number of groups, correlates of extinction risk also match the theory of extinction (Fisher and Owens 2004) (Chapter 13). In primates, carnivores and birds, small geographic range is correlated with extinction risk, as is dietary or habitat specialization in hoverflies, reptiles, birds, and primates, and low population density in reptiles, carnivores, and primates. Large body size gives a higher risk of extinction in birds, primates, and hoverflies.

There are few surprises here. What is particularly interesting, however, is the interaction between the source of extinction threat and traits in birds. Birds that are small are likely to be at risk from exploitation or introductions, but not from habitat destruction. However, birds with specialized habitats are more at risk of extinction from habitat destruction, but not from exploitation or introductions (Owens and Bennett 2000).These threat interactions may account for some of the contradictory results obtained from some studies of extinction. For example, in mammals species in taxonomi-cally isolated groups are most at risk of extinction whereas in angiosperms it is species in species-rich groups. Body size has been reported to have positive effects on extinction risk in primates and birds, but negative effects on all mammals, and no effect in a large number of other studies. Generation time has positive effects in carnivores, negative effects on birds, and no effects on primates, reptiles, and hoverflies (Purvis etal. 2000).

The combination of fossil evidence and data from extant taxa then is starting to make headway into the dynamics of clade characteristics and differences between them. Taxonomic diversity and morphological diversity are both promoted by ecological opportunity but they may not follow directly parallel paths. In addition a number of characteristics affect speciation and extinction rates that match the predictions of speciation and extinction theory. In the next chapter we will examine large-scale patterns from a more ecological perspective.

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