PhD: Patterns, processes, rates, and constraints, in the evolution of morphological disparity

From Aristotle onwards, it has been apparent that there are discontinuities in organismal form, such that species are clumped within morphological ‘design space’ [1]. These clumps at once help us in attempts to classify but often frustrate our efforts to resolve phylogeny.

There is vigorous debate as to why organisms are irregularly distributed in this way. Do clusters reflect peaks in a rugged fitness landscape? Are the intervening regions of design space impossible to colonise – either because of physical or developmental constraints – or are they unoccupied because insufficient time has elapsed? Were they once occupied and subsequently vacated in the wake of past extinctions? Is the range of designs (morphological disparity) realised progressively through time, or in an early burst of innovation followed by stasis? The answers to these questions have important implications, particularly whether the same processes shape macroevolution throughout deep time (uniformitarianism) or whether these forces change [2].

To date, analyses have been based largely on empirical case studies of extinct organisms. Little attempt has been made to establish a theoretical or empirical foundation for interpreting empirical disparity analyses. This is unsurprising, since methods for simulating the evolution of morphological data have only recently become available [3]. This project will augment these tools in order to simulate phenotypic evolution under the diversity of evolutionary models that are inferred to have shaped the morphological disparity of real clades. These include intrinsic factors (such as changes in rates of character evolution and the scale of innovation, phylogenetic burden, and lineage duration) as well as extrinsic factors (such as competition and mass extinction events). These putative drivers and limits will be examined in turn and in combination to determine whether they yield the patterns of design space occupation that are observed in real clades. The candidate will also explore how the structural and phylogenetic relationships among characters contributes to morphospace occupation.

While the intention is to sample broadly across organismal diversity and throughout the Phanerozoic. there is also scope to focus on particular groups, depending upon the successful candidate’s interest.

Ultimately, the results of the simulation studies will identify the relative roles of different evolutionary processes in shaping the occupation of design space, and provide a new theoretical framework within which to evaluate the results of empirical disparity analyses. They will also speak to a seminal controversy in macroevolution, namely the uniformitarian/non-uniformitarian debate.

References
1. D. H. Erwin, Disparity: morphologic pattern and developmental context. Palaeontology 50, 57-73 (2007).
2. D. H. Erwin, Evolutionary uniformitarianism. Developmental biology 357, 27-34 (2011).
3. J. E. O'Reilly et al., Bayesian methods outperform parsimony but at the expense of precision in the estimation of phylogeny from discrete morphological data. Biology Letters 12, 20160081 (2016).
4. J. W. Oyston, M. Hughes, S. Gerber, M. A. Wills, Why should we investigate the morphological disparity of plant clades? Annals of botany, (2015).