Trends in space

Are there geographic patterns in macroevolutionary processes? The evidence suggests so. Jablonski (1993) showed that the first occurrence of marine invertebrate fossil groups was more often in the tropics than would be expected by chance. This points to a higher origination rates of higher taxa there, perhaps due to fostering of important morphological innovations that could give rise to radiations. These innovations also occur in shallow water environments more often than deep water environments. Stehli etal. (1972) also showed that extant species of Foraminifera do not vary in age according to their present day latitudes. This suggests that extinction rates do not vary with latitude, and consequently invoking higher speciation rates in the tropics to explain their higher diversity there. In contrast, the ages of higher taxa of corals, bivalves,

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.

and barnacles decline towards the tropics. This suggests that speciation rates are higher there (Sepkoski 1999). Studies of extant taxa also suggest similar trends. Comparisons of the species richness of tropical and temperate sister taxa in swallowtail butterflies and passerine birds suggests that the net rate of diversification has been higher in the tropics (Cardillo 1999) (Figure 14.5).The average age of avian tribes per latitudinal band increases towards the tropics (Gaston and Blackburn 1996), suggesting a loss of older tribes towards higher latitudes. In contrast, northern hemisphere freshwater fish species show a steep decline in diversity north of 50 degrees, in areas recently glaciated. Interestingly, however, the mitochondrial DNA distance between sister taxa is lower at these high latitudes, suggesting a recent rise in speciation rate with latitude following colonization of these lakes, a trend consistent with the notion of ecological opportunity (Bernatchez and Wilson 1998).

Finally, reconstructing the history of geographic ranges in groups from phylogenies of living species has suggested that some radiations have originated in the tropics (Bleiweiss 1998,Bohm and Mayhew 2005 ).What has happened to morphology in these communities? A few studies have compared the morphospace occupied by temperate and tropical communities. Ricklefs and O'Rourke (1975) found that disparity increased in tropical moth communities, with new morphologies not found in temperate communities. Similar trends have been found in tropical marine communities (Roy and Foote 1997).

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