PhD: Evolution’s First Rule: Testing for a Driven Trend in Complexity among Tetrapods

As popularly understood, evolution is largely synonymous with increasing complexity. But is this really the case? The maximum complexity of animals is undeniably greater today than when they first evolved⁽¹⁾. However, the most marked increase in complexity occurred during the Cambrian ‘Explosion’ ~542Mya, which lasted perhaps 20My (a step-like rapidity that vexed Darwin). Two things in particular remain unclear, however. The first is whether the Cambrian marked a unique gear-change in the evolution of animal body-plans (e.g. sponges are less complex than jellyfishes, which are less complex than molluscs, vertebrates and so on), or whether an overarching trend for increasing complexity persisted from the Cambrian to the present day. The second is whether this process was purely the result of passive diffusion from some lower bound, or whether there was a widespread and driven trend for parallel change in multiple, independent lineages. If the latter, then increasing complexity may be viewed as an evolutionary ‘rule’ of the widest generality, similar to those already documented for increasing maximum size (Cope’s rule) and early high disparity⁽²⁾. Initial investigations⁽³⁾ reveal a driven pattern of increasing complexity in crustaceans on this timeframe, but the generality and implications of this finding are highly controversial^{(1, 4)}.

This project will focus on tetrapods (limbed vertebrates) because of their excellent fossil record and because their bodies comprise variously differentiated but serially homonomous units (vertebrae; ribs) and similarly patterned anterior and posterior appendicular skeletons (limbs; girdles). The student will use a variety of approaches to describe the distribution and specialization of these elements across all groups of living and fossil tetrapods. S/he will initially focus upon discrete character codings and indices of the differentiation of vertebrae and ribs. These data will allow us to express complexity both in terms of the serial specialization of somites, and by plotting the morphological diversity of vertebrae within a single individual relative to empirical morphospaces encompassing all realized forms. The excellent fossil record of tetrapods means that we will be able to calibrate a supertree of major groups against the appearance of lineages in deep time, and thereby track the parallel and convergent evolution of similar morphologies and similar levels of complexity more broadly. This approach will also highlight constraints upon the evolution of bodyplans, and the manner in which different clades have circumvented these. For example, all mammals (except sloths) have just seven neck vertebrae. Elongation of the neck has been achieved by radically different mechanisms in mammals (e.g., giraffes and indricotheres) compared with birds (e.g. ostriches) and many reptiles (which have much greater developmental flexibility).

The student will be trained in phylogenetics, supertree construction and general scripting within R. They will also acquire skills in morphometrics and the project will entail first hand access to museum materials. All the skills they will learn are highly transferrable and will be useful to them irrespective of their precise career path within evolutionary biology and biological sciences more generally.

^1. D. McShea, R. N. Brandon, Biology's First Law: The Tendency for Diversity and Complexity to Increase in Evolutionary Systems. (University of Chicago Press, Chicago, 2010), pp. 170.
^2. M. Hughes, S. Gerber, M. A. Wills, Clades reach highest morphological disparity early in their evolution. PNAS 110, 13875-13879 (2013).
^3. S. J. Adamowicz, A. Purvis, M. A. Wills, Increasing morphological complexity in multiple parallel lineages of the Crustacea. PNAS 105, 4786-4791 (2008).
^4. B. Ekstig, Complexity, natural selection and the evolution of life and humans. Foundations of Science 20, 175-187 (2015).