Contingent Predictability in Evolution: Key Traits and Diversification

Abstract

Key innovations have often been invoked to explain the exceptional diversification of particular groups. However, there are few convincing examples of traits that are repeatedly and consistently associated with increased diversification. The paucity of such cases may reflect the contingent nature of the diversifying effect of key traits. These contingencies can be viewed as statistical interactions between the trait and at least three kinds of factors: (1) other taxa, (2) other traits of the group itself, and (3) the physical environment. I describe tentative examples in each of these categories:(1) a dampening of the diversification of clades with image-forming eyes by groups that earlier evolved such eyes, (2) an effect of growth form (woody or herbaceous) on the diversifying effect of biotic seed dispersal in angiosperms, and (3) an effect of atmospheric CO 2 level on the diversifying effect of C 4 photosynthesis in monocots. These examples suggest the need for more complex analyses of the relationship between possible key traits and diversification. They also suggest that radiations may be predictable given certain circumstances, thus supporting a view of evolution as both predictable and contingent. Ironically, a certain degree of predictability may be critical to arguments for evolutionary contingency.

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“…This may reflect the context-dependence of a key trait's effect on diversification [24]. There are many potential factors that could influence how a key trait affects diversification.…”

Fossil evidence for key innovations in the evolution of insect diversity

“…There is also little support for wing folding, and in fact Polyneoptera show a marginally non-significant decline in origination rates relative to Palaeoptera (electronic supplementary material, table S2), as well as a quite similar richness. It is however likely that the diversity of Holometabola is in some way contingent [16] on this innovation given the richness of species, such as beetles, that depend on it.…”

“…Ultimate causes of macroevolutionary change can include extrinsic factors such as environmental change [14,15], as well as intrinsic ones such as evolutionary novelties [16]. Key innovations are novel phenotypic characters such as morphologies, behaviours or developmental strategies that enhance species richness [8,17], seen through an increase in net diversification rate.…”

Anti-predator defence drives parallel morphological evolution in flea beetles

“…The number of species in Galerucinae and Alticinae by far exceeds that in other subfamilies of Chrysomelidae, in particular, considering their comparatively recent origin, while nearly two thirds of the species exhibit the metafemoral spring (electronic supplementary material, table S7). In this respect, the metafemoral spring could be considered an evolutionary 'key innovation' [58] that links the origin of a trait to high species diversity in a clade. However, imbalance of diversification rates associated with the appearance of this structure was not significant in the (highly conservative) Slowinsky -Guyer test, while high species richness is also evident in the sister clade that ancestrally lacks the metafemoral spring, i.e.…”