Trends in time

The most extensive data on temporal trends in macroevolution are from the fossil record, in particular, that of shelly marine invertebrates, which has been extensively documented. The data tend not to be analysed at the species level because of various inaccuracies and biases that are accentuated at low taxonomic levels.

The gross history of this group (Figure 14.3) shows a generally increasing trend in family richness punctuated by sudden decreases, in particular, the five big mass extinctions occurring at the end Ordovician, late Devonian, end Permian, end Triassic, and end Cretaceous. These episodes were the end of many groups. Over time, however, virtually every possible history of family richness is demonstrated, from sudden increase and decline, to slow increase and slow decline. Decreases in diversity following the mass extinctions are then followed by increases in diversity, and this increase often takes the form of a replacement of one taxon by another, a trend that suggests that interactions between taxa are important in macroevolutionary dynamics, and further that rates of diversification are limited in a logistic sense.

Extinction rates and speciation rates both exhibit a general decline over time. This decline coincides with the replacement of taxa that exhibit

Fig. 14.3 How diversity has changed over time (P—Palaeozoic, M—Mesozoic, C—Cenozoic), according to the fossil record. The well-known marine family record shows three possible equilibria (e1, e2, e3) corresponding to the dominance of three faunas (Ca—Cambrian, P—Palaeozoic, and M—Modern). The equilibria are punctuated by the five big mass extinction events (O—Ordovician, D—Devonian, Pe—Permian, Tr—Triassic, and K—Cretaceous). The terrestrial record in contrast looks much more exponential. After Benton (1995) with permission from AAAS.

Fig. 14.3 How diversity has changed over time (P—Palaeozoic, M—Mesozoic, C—Cenozoic), according to the fossil record. The well-known marine family record shows three possible equilibria (e1, e2, e3) corresponding to the dominance of three faunas (Ca—Cambrian, P—Palaeozoic, and M—Modern). The equilibria are punctuated by the five big mass extinction events (O—Ordovician, D—Devonian, Pe—Permian, Tr—Triassic, and K—Cretaceous). The terrestrial record in contrast looks much more exponential. After Benton (1995) with permission from AAAS.

high rates of diversification and extinction and that are present at the start of the Phanaerozoic (the Cambrian fauna of trilobites (Figure 14.4), Monoplacophora and graptolites, the emergence of a second fauna that dominates until the end of the Permian (the Palaeozoic fauna of crinoids, cephalopods, and ostracods). This fauna displays intermediate speciation and extinction rates. Finally, a modern fauna of gastropods, bivalve molluscs, and echinoids dominates to the present with low rates of speciation and extinction (Sepkoski 1999). A logistic model of clade growth of these three faunas, with interaction purely through the logistic feedback term, with these estimated diversification rates can predict the pattern of replacement very accurately. For best accuracy the mass extinctions need to be imposed on the system, but the pattern of faunal replacement and general trends in

Fig. 14.4 The trilobite Dicranurus monstrosus, from the Devonian of Morocco, length about 4.5 cm minus spines. Trilobites were a component of the 'Cambrian Fauna' one of the three great marine faunas in the fossil record, characterized by a high origination rate and high extinction rate. Photo courtesy of Richard Fortey and Claire Mellish, © The Natural History Museum, London.

Fig. 14.4 The trilobite Dicranurus monstrosus, from the Devonian of Morocco, length about 4.5 cm minus spines. Trilobites were a component of the 'Cambrian Fauna' one of the three great marine faunas in the fossil record, characterized by a high origination rate and high extinction rate. Photo courtesy of Richard Fortey and Claire Mellish, © The Natural History Museum, London.

species richness occurs even without these. In fact the net effect of the mass extinctions is to delay the replacements rather than to speed it up as is generally assumed (Kitchell and Carr 1985). Thus, the general large-scale trends for the marine realm suggest logistic growth of clades with interaction and replacement (Figure 14.3). Consistent with this, the patterns within clades show a slow decline in rates of origination over time. There is also some suggestion from the shape of phylogenies of extant taxa that rates of diversification tend to decline over time. The pattern of origination of bird families is not consistent with a constant rate model of diversification, and suggests, instead, that there has been a decline over time (Harvey et al.1991). However, the terrestrial fossil family-curve looks much more exponential (Figure 14.3), and it also has been suggested that an exponential species-curve may underlie the logistic marine family-curve (Benton 1995). It does appear, therefore, that not all groups have diversified in a logistic fashion, and that different biotic realms may impose different constraints to diversification.

What of disparity? Again, variable patterns are seen in the fossil record. In some groups, such as the Palaeozoic trilobites, crinoids, and insects, diversity rose initially followed at length by a rise in disparity. In many groups however, including the Cambrian arthropods as a whole, many plant taxa, Carboniferous ammonoids, and Paleozoic gastropods, disparity peaks early and taxonomic richness rises afterwards (Foote 1997). This latter pattern is perhaps the most common (cf. Schluter 2000, pp. 59-60), and is prima facie consistent with the null pattern predicted by Gavrilets (1999). Foote (1997), however, argues that for several reasons geometric constraints are unlikely to be the only reason for the frequency of this pattern.

The relationship between disparity and taxonomic diversity can also be addressed by comparisons of recent radiations. Schluter (2000) used comparisons of replicate radiations in finches, mammals, cichlid fishes,warblers, and Anolis lizards to show that morphological diversity was greater in the older clades, showing a continuing rise in disparity with time. However, many of these clades do not show increased species richness with age, suggesting that taxonomic richness reaches limits first, as paralleled in some fossil groups. Studies of extant radiations also suggest that the evolutionary rates both of morphological and taxonomic richness are higher when ecological opportunity is greatest (Schluter 2000). For example, island radiations often display greater morphological diversity and species richness than their mainland counterparts.

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