150 years after Charles Darwin’s The Origin of Species there are many open questions in evolutionary biology. For example I have been wavering between the possibility that on the molecular genomic scale evolutionary process is predominantly neutral and stochastic, and a new possibility that selection is pervasive (a new possibility that is actually old). The nice aspect of this area of study today is that empirical data can be brought to bear upon ancient arguments. Previously the dialogues were fruitless in terms of actually resolving opposing viewpoints, as interlocutors dug into their presuppositions, and the same theoretical paradigms flew through data free debates.
One area where this was clear was in relation to the of whether evolutionary process is deterministic or stochastic on the most general level. More prosaically, is evolution an experiment which one can broadly reproduce genetic and functional outcomes over and over? Stephen Jay Gould was the most famous proponent of the position that evolution is not a reproducible experiment. Rather, it is a contingent historical process. You can’t rewind the clock and expect the result to resemble what came before. In contrast Richard Dawkins has tended to defend the stance that there are broad inevitabilities as evolution explores the adaptive space of possibilities (see The Ancestor’s Tale for his detailed position). Over the past few years Joe Thornton’s lab has been looking at this issue by examining the evolutionary genetic trajectory of steroid receptors. This may seem abstruse, but obviously these are functionally significant, and, they’ve been able to utilize ingenious biophysical methods to “rerun” evolution. The general conclusion seems to be that steroid receptors as we understand them are subject to path dependence in a fashion where the outcome is sensitive to unlikely sequences of mutations.
The group now has a paper in Nature, Historical contingency and its biophysical basis in glucocorticoid receptor evolution (ungated). Let me jump straight to the conclusion:
If evolutionary history could be replayed from the ancestral starting point, the same kind of permissive substitutions would be unlikely to occur. The transition to GR’s present form and function would probably be inaccessible, and different outcomes would almost certainly ensue. Cortisol-specific signalling might evolve by a different mechanism in the GR, or by an entirely different protein, or not at all; in each case, GR—or the vertebrate endocrine system more generally—would be substantially different. Because GR is the only ancestral protein for which alternative evolutionary trajectories to historically derived functions have been explored, the generality of our findings is unknown. The specific biophysical constraints, and in turn the degree and nature of contingency, that shape the evolution of other proteins are likely to depend on the particular architecture of each protein and the unique historical mechanisms by which its functions evolved.
The issue about generality is important. Read the whole paper and you’ll be struck by the level of experimental detail that went into making the inferences they arrive at. There’s a reason that these papers get into glamour journals. But is this just a story about a particular class of proteins, or the story of evolution writ large? When molecular evolutionary neutralism became ascendant in the 1980s some responded that though stochasticity might dominate on the sequence level of A, C, G, and T, there was no such randomness when it came to morphology. The power of this argument seems most evident when it comes to the body plans of some metazoans which are clearly dictated by the laws of physics. Marine mammals have evolved toward a morphology which has clear parallels with that of species of the fish lineage which occupy the same niche. Similarly, the elephantine legs of ancient sauropods were no coincidence. As land animals become large their massive bulk becomes unwieldy for more gracile body plans, and there is a tendency toward stout builds, as the cross-section of bone and muscle attempts to race up to the massively increased volume and mass.
The same tension seems to be at play in these sorts of results, which focus on the contingencies of a specific piece of biological machinery. Is there only one way to construct a particular a component due to biophysical constraints? Perhaps. But what does this tell us about the construction of the whole organism, which is the stitching together of innumerable biomolecular parts? In science fiction is not peculiar to imagine worlds where gross morphology is broadly recognizable, but none of the organisms are edible to humans because of divergences in biochemistry. Ultimately as implied in the paper above other groups have to reproduce this sort of work on other families of proteins to see how ubiquitous contingency really is.
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