Convergent evolution… tell me more
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Is there a General Theory of Biology?
Convergence may be ubiquitous, as indeed the Map of Life demonstrates, but does it have a deeper significance, even something that could point to a General Theory in Biology? This is a tantalizing prospect, but resolution of both question and answer may still lie far in the future. The fact that we can document convergence at all levels of biology, from enzymes to societies, not only provides powerful evidence for adaptation, but begs the question of what the inter-relationships in the biological hierarchy might be. This is a question to be addressed through studying convergence as well as all other aspects of biology. At present we simply don’t know enough to do more than speculate.
The problem of examining inter-relationships in the biological hierarchy may not, however, be as severe as it first appears. For example, if complex structures are at least to some extent constructed from pre-existing molecular building blocks (as is patently the case), then the emergence of a given complex structure is if anything more likely. This is even more true if several unrelated building blocks can meet exactly the same evolutionary need. In this context the crystallins of the eyes are an excellent example. Crystallins show rampant convergence, with lens proteins being recruited or ‘co-opted’ from a wide range of proteins whose original functions varied from heat-shock buffering to alcohol dehydrogenation.
Back to the Bang
Recruitment or co-option of pre-existing elements of the biological matrix to form molecules and structures with new functions, begs the question of what we call inherency. Inherency refers to the extent to which features of the evolving world were effectively pre-ordained at an earlier time. This question obviously implies an indefinite regress: if not in terms of crystallin proteins, then perhaps back to amino acids and the genetic code, or even pre-biotic processes in interstellar clouds. But if so, why stop there? Maybe we need to go back to the exploding stars from which carbon and the other elements necessary for life were derived. Or back to the actual process of nucleosynthesis. And if that is not far enough back, then what about the Big Bang? So precise are the initial conditions necessary to produce a habitable universe that it seems perfectly sensible to argue that the emergence of intelligence (which is convergent) was inevitable from the instant of the Big Bang. That at least is one view, and convergence certainly argues for a far greater degree of determinism in the evolutionary process than has previously been acknowledged. But that doesn’t mean that evolution is completely predictable, nor does the study of convergence suggest that. Many unique forms have certainly evolved.
Repeated major transitions
There is, however, a general sense in evolutionary biology that there are major points of transition (e.g. the evolution of eukaryotes, sex, multi-cellularity, societies). Can we then identify unique fulcrum points that, if they failed to materialise, were diverted or re-directed then this would have led to a completely different direction of evolution? To be sure this counterfactual world would show its own convergences, but would it not still be quite unlike what evolution on Earth has produced? This is difficult to test, because after all n = 1: we only have one Earth with one history. However, the very ubiquity of convergence means that in many cases a given major transition, one of those key fulcrum points in the evolution of life, has not only been repeatedly passed, but in the same direction. Even if we can identify a transition that appears to have evolved only once, and here the origin of mitochondria may be a good example, there may be general evolutionary trends that point to the same transition being approached from other directions. In one living group of amoeba primary endosymbiosis has been recognised, with the free-living amoeba apparently in the act of acquiring a cyanobacteria with chloroplast-like properties. Further highlighting the tendency for endosymbiosis and organelle capture, many examples of secondary endosymbiosis also exist, in which a eukaryote that has integrated a bacterium by primary endosymbiosis is itself engulfed by another cell.
Imagining absent forms
From an awareness of convergence and optimality we can begin to investigate whether there are any types of biological organisation that models indicate could have evolved, but have failed to do so in reality. This is a remarkably neglected area of study, and has been almost entirely restricted to suggesting whole organisms that might invade an existing ecology or following the decimation of an ecosystem would re-occupy it. The approach of predicting future forms has been brilliantly explored by Dougal Dixon in his book, After Man: A Zoology of the Future, in which he makes hypotheses on what flora and fauna will live 50 million years from today. The new animals he imagines are suitably and engagingly bizarre, but the differences are really little more than skin-deep. Superficial differences in relation to current forms are what we would expect, although a more rigorous application of convergent thinking might further assist predictability and verisimilitude.
As far as the more general problem of trying to imagine things that “ought” to have evolved, but apparently have failed to it is certainly possible to identify “absences” at a local level. For example, Mack (2003) presents a very intriguing analysis of “gaps” in certain floras, where a particular type of plant might reasonably be expected to be present but for some historical reason has failed to manifest itself. This certainly is very relevant to the question of convergence, not least when we consider convergence between biological communities. Identification of local “gaps” also has a much more immediate and practical consequence because where they exist there is a corresponding danger of ecosystem invasion by other “alien” species.
On broader scales, however, it seems more difficult to identify obvious “vacant lots”. Being large animals ourselves, the exercise of imagining “what ought to exist” may be slightly easier for animals than thinking at the level of plants (notwithstanding Mack (2003)), let alone single-celled organisms or even biochemistries. All one can say is that convergences do indeed appear to act as a reliable guide to what forms will appear, from the community to molecular level. Additionally, formal or more quantitative descriptions of the occupation of “morphospace” may help to infer apparently viable solutions that have never evolved. One exercise of this kind is Thomas & Reif’s “skeleton space” model (1993), where the authors conclude that evolution is forced to return to certain patterns of organic design due to the nature of the materials and functions involved. In the case of animal skeleton structure not only has “space” been thoroughly filled but also in many cases a given “box” has been “visited” repeatedly, and those that are vacant are clearly functionally impossible.
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