Abstract
A number of authors have pointed to “convergent evolution” as evidence for the central role of natural selection in shaping predictable trajectories of macroevolution. However, there are numerous conceptual and empirical difficulties that arise in broadly appealing to the frequency of homoplasy as evidence for a non-contingently constrained adaptational design space. Most important is the need to distinguish between convergent (externally constrained) and parallel (internally constrained) evolution, and to consider how the respective frequencies of these significantly different sources of homoplasy affect a strong adaptationist view of life. In this paper, I critically evaluate Simon Conway Morris’s use of the homoplasy literature to support his argument for a non-contingent, counterfactually stable account of macroevolutionary pattern. In so doing, I offer a conception of parallelism which avoids the charge that it differs from convergence merely in degree and not in kind. I argue that although organisms sharing a homoplastic trait will also share varying degrees of homology, it is the underlying developmental homology with respect to the generators directly causally responsible for the homoplastic event that defines parallel evolution and non-arbitrarily distinguishes it from convergence. The notion of “screening-off” is used to distinguish the proximal generators of a homoplastic trait from its more distal genetic causes (such as a master control gene).
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Notes
Gene regulatory networks are hierarchical, with earlier linkages having more pleiotropic effects than the more distal fine-grained terminal processes. The former upstream sub-circuits, which Davidson and Erwin (2006) have termed “kernels,” specify the more general domain of the developing organism. Because kernels are “recursively” wired, interference with any single kernel gene will destroy its function altogether, resulting in phenotypic catastrophe.
Haas preferred to distinguish convergence and parallelism geometrically (rather than causally)—with the former entailing that two lineages resemble one another more than their ancestors did, and the latter referring to cases in which two lineages evolve in the same direction without resembling one another any more than their ancestors did. For example, a trend of increasing body size in grasshopper and walrus clades would represent a parallelism for Haas, although Simpson would presumably demur since the parallel increases are probably not linked to shared developmental homology.
The misexpression of the Pax-6 transcription factor has been shown to lead to the formation of differentiated ectopic eyes in both vertebrates and invertebrates. This data may seem to represent a counterexample to the claim that Pax-6 is casually insufficient for the formation of the macroscopic eye. However, this objection is neutralized by the contingent fact that the abnormal expression of Pax-6 simply triggers a cascade of downstream developmental events which are directly responsible for the substance and structure of the ectopic eye.
SCM does draw upon some examples of genuine convergence that could be the subject of robust macrobiological generalizations, such as those pertaining to sensory modalities (2003, Ch. 7); unfortunately, he offers no principled method for comparing the philosophical or inductive significance of different types of homoplasy. Additionally, he fails to show that any of the evolutionary endpoints that he infers from the distribution of homoplasy are associated with either diversification or persistence (or some other measure of evolutionary success), which (in my view) undercuts the notion that such outcomes represent stable islands of form amidst a roiling sea of stochasticity.
I would like to thank Robert Brandon, Dan McShea, Alex Rosenberg, members of the Duke Philosophy of Biology Reading Group, and (especially) an anonymous referee for helpful comments on an earlier draft of this paper. Thanks also to Victoria Campbell for kindly sketching the illustrations in Fig. 1. Finally, I am indebted to the late Stephen Jay Gould, whose popular works encouraged me to explore philosophical issues in the history of life.
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Powell, R. Is convergence more than an analogy? Homoplasy and its implications for macroevolutionary predictability. Biol Philos 22, 565–578 (2007). https://doi.org/10.1007/s10539-006-9057-3
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DOI: https://doi.org/10.1007/s10539-006-9057-3