Approaches to Macroevolution: 2. Sorting of Variation, Some Overarching Issues, and General Conclusions

Abstract

Approaches to macroevolution require integration of its two fundamental components, within a hierarchical framework. Following a companion paper on the origin of variation, I here discuss sorting within an evolutionary hierarchy. Species sorting-sometimes termed species selection in the broad sense, meaning differential origination and extinction owing to intrinsic biological properties-can be split into strict-sense species selection, in which rate differentials are governed by emergent, species-level traits such as geographic range size, and effect macroevolution, in which rates are governed by organism-level traits such as body size; both processes can create hitchhiking effects, indirectly causing the proliferation or decline of other traits. Several methods can operationalize the concept of emergence, so that rigorous separation of these processes is increasingly feasible. A macroevolutionary tradeoff, underlain by the intrinsic traits that influence evolutionary dynamics, causes speciation and extinction rates to covary in many clades, resulting in evolutionary volatility of some clades and more subdued behavior of others; the few clades that break the tradeoff can achieve especially prolific diversification. In addition to intrinsic biological traits at multiple levels, extrinsic events can drive the waxing and waning of clades, and the interaction of traits and events are difficult but important to disentangle. Evolutionary trends can arise in many ways, and at any hierarchical level; descriptive models can be fitted to clade trajectories in phenotypic or functional spaces, but they may not be diagnostic regarding processes, and close attention must be paid to both leading and trailing edges of apparent trends. Biotic interactions can have negative or positive effects on taxonomic diversity within a clade, but cannot be readily extrapolated from the nature of such interactions at the organismic level. The relationships among macroevolutionary currencies through time (taxonomic richness, morphologic disparity, functional variety) are crucial for understanding the nature of evolutionary diversification. A novel approach to diversity-disparity analysis shows that taxonomic diversifications can lag behind, occur in concert with, or precede, increases in disparity. Some overarching issues relating to both the origin and sorting of clades and phenotypes include the macroevolutionary role of mass extinctions, the potential differences between plant and animal macroevolution, whether macroevolutionary processes have changed through geologic time, and the growing human impact on present-day macroevolution. Many challenges remain, but progress is being made on two of the key ones: (a) the integration of variation-generating mechanisms and the multilevel sorting processes that act on that variation, and (b) the integration of paleontological and neontological approaches to historical biology.

Keywords: Diversification; Evolutionary trends; Hierarchy; Mass extinction; Multilevel selection; Paleobiology; Radiation; Species selection; Species sorting.

Fig. 1

Clade-level patterns of phenotypic change, modeled with the punctuated cladogenesis tempo and mode for clarity. A Preferential speciation in the direction of the overall trend. B Higher speciation rates on the right, so that diversity accumulates on that side, producing a trend. C Lower extinction rates on the right, also producing a trend. D Clade originates near a lower limit, so that diffusion yields an increase in maximum and mean value to the right. E Unbounded, unbiased speciation, producing an increase in the maximum value and a decrease in the minimum. E Clade originates near a lower limit, so that diffusion yields an increase in maximum and mean value to the right. F Narrowing of variation by a decrease in the maximum value, and an increase in the minimum. Modified from Jablonski (2010a), based on Gould (1982) and Stanley (1979)

Fig. 2

Body-size evolution in Late Cretaceous bivalves and gastropods, showing a variety of patterns in a bivariate space measuring the change in the maximum and minimum size for each genus-level clade; sizes are log₂-transformed, so that an increase or decrease of one unit represents a doubling or halving of body size, respectively. Icons in each quadrant show idealized clade profiles, e.g. upper right is increase is both maximum and minimum size and thus directional size increase (Cope’s rule), and upper left is an increase in the maximum and decrease in the minimum and thus an increase in size range, with percentages giving proportion of clades falling in each quadrant. Lineages falling on the diagonal begin and end their histories with a single species (but may be richer in between). This approach is especially useful at lower taxonomic levels where numbers per clade are too low for rigorous fitting of evolutionary models; see Hopkins for a multivariate version, with the axes being principal coordinate scores for morphological data. Modified after Jablonski (1997)

Fig. 3

Three simple models of antagonistic clade interactions. A Double-wedge dynamic: the expansion of Clade 2 drives Clade 1 to extinction; shown here under Sepkoski’s (1984) logistic assumption. B Interference dynamic: both clades reciprocally damp diversification; unimpeded diversification rate of Clade 1 seen before advent of Clade 2, unimpeded diversification rate of Clade 2 seen after extinction of Clade 1. C Incumbency dynamic: Clade 1 precludes the diversification or introduction of Clade 2 until the extinction of Clade 1 allows Clade 2 to diversify. From Jablonski (2008c)

Fig. 4

Left, diversity-disparity space for analyzing the relation between taxonomic and morphological diversification. Type 1: Morphology outstrips taxonomic diversification; Type 2: Morphology concordant with taxonomic diversification; Type 3: Morphology trails behind taxonomic diversification. Right, three empirical trajectories, for Cambrian-Ordovician blastozan echinoderms, Jurassic-Cretaceous aporrhaid gastropods, and Ordovician-Carboniferous blastoidean echinoderms. Data from Foote (1993, 1996), and Roy (1994). Boostrapped confidence limits not shown here, but blastoids lie significantly above the 1:1 line, blastozoans lie significantly below it, and aporrhaids never leave it