Convergent evolution as natural experiment: the tape of life reconsidered

Abstract

Stephen Jay Gould argued that replaying the ‘tape of life’ would result in radically different evolutionary outcomes. Recently, biologists and philosophers of science have paid increasing attention to the theoretical importance of convergent evolution—the independent origination of similar biological forms and functions—which many interpret as evidence against Gould's thesis. In this paper, we examine the evidentiary relevance of convergent evolution for the radical contingency debate. We show that under the right conditions, episodes of convergent evolution can constitute valid natural experiments that support inferences regarding the deep counterfactual stability of macroevolutionary outcomes. However, we argue that proponents of convergence have problematically lumped causally heterogeneous phenomena into a single evidentiary basket, in effect treating all convergent events as if they are of equivalent theoretical import. As a result, the ‘critique from convergent evolution’ fails to engage with key claims of the radical contingency thesis. To remedy this, we develop ways to break down the heterogeneous set of convergent events based on the nature of the generalizations they support. Adopting this more nuanced approach to convergent evolution allows us to differentiate iterated evolutionary outcomes that are probably common among alternative evolutionary histories and subject to law-like generalizations, from those that do little to undermine and may even support, the Gouldian view of life.

Morphological Evolution Repeatedly Caused by Mutations in Signaling Ligand Genes

“…Thus, while it represents perhaps a genuine DGT component, it remains ambiguous if endothelin pathway genes can be a mutational target of evolution in a natural context. To truly assess the role of signaling ligand genes in morphological evolution, it is useful to focus on radiating lineages that allow a trait-by-trait dissection by forward genetics (i.e., taking advantage of natural variation between closely related lineages -populations and sister species) and, sometimes, natural experiments of replicated evolution (Kopp 2009;Powell and Mariscal 2015). In the next sections, we focus primarily on stickleback fishes and Heliconius butterflies, for which numerous linkage mapping efforts have been uncovering the genetic basis of several morphological adaptations.…”

The domain relativity of evolutionary contingency

“…Particularly relevant here are the insights of Powell and Mariscal (2015). They argue that convergences must be sorted along several axes: specificity (meaning roughly degree of abstraction), independence (the degree to which convergences rely on shared developmental resources) and scope (the range of conditions under which a convergence is to be found).…”

Quantification provides a conceptual basis for convergent evolution

“…A contrasting view, made famous by Stephen Gould (based on the same data from the Burgess Shale that Conway Morris used to come to the opposite conclusion) is that life is strongly influenced by stochastic events (so-called 'contingency'), which make phenotypes relatively unpredictable (Gould, 2000; see discussion in Powell & Mariscal, 2015). To repeat Gould's now familiar analogy, if we re-ran the tape of life, very different forms of biodiversity would evolve each time.…”