You are here
Summary of problems:
Some similarity of shape might be explained by "natural laws," more commonly referred to among evolutionary biologists as constraints. Such constraints arise through restrictions on the range of variation capable of being produce. Brian Goodwin researches ways in which fundamental mathematical principles place limits on evolution. Such constraints will not be evolutionarily informative, since natural selection can only operate on available variation. EE wrongly claims that Goodwin's research is an alternative to homology, when it actually relies on homology and common descent.
The authors of EE move their inaccurate presentation of homology forward from irrelevant discussions of 19th misunderstandings of evolution to the 20th century in this brief account of Brian Goodwin's process structuralism. They cite Goodwin's work as a challenge to evolution, but this misrepresents his views. Goodwin views evolution through a different lens, but does not deny universal common descent nor the power of the full range of naturalistic evolutionary processes. Goodwin suggests that there are biological rules constraining the sorts of growth patterns that are possible. These limits to the available forms of morphological variation explain various recurring evolutionary patterns. Natural selection acts on a restricted number of possibilities, which explains why evolution has produced less diversity than he might otherwise expect. The authors misrepresent Goodwin's work. Goodwin debates the processes important in evolution, not whether evolution has occurred or whether organisms share common ancestry, let alone the validity of homology.
Goodwin's recent discussion of the vertebrate limb provides a clear sense of his concerns:
An extraordinary thing about our limbs is that they are essentially the same as those of all other tetrapods … Given th[e] diversity of uses [in various tetrapods], one might have expected that natural selection would have designed each limb to optimally serve its functions. Why doesn't the bat's wing start with two bones to anchor it firmly to the shoulder? Why does the horse have that tiny extra bone running like a splint down the side of its main "toe," with another similar one on the other side of the toe? What possible function can they serve? Why not get rid of them altogether? Given their extraordinary utility and the fact that [ancestral tetrapod] Ichthyostega had seven, why don't we have six digits on each hand and banish that rather useless little toe that is so prone to getting stubbed? The answers to these questions usually take the form: Natural selection has to make do with what is given by ancestral form, molding it as best it can to a variety of purposes. But then we are left with the problem: Where does this ancestral form come from, and why is it as it is? Is it just a historical accident, or is there a deeper reason for the basic pattern of tetrapod limbs that provides a rational unity of structure underneath the diversity of functional expression?
Goodwin does not object to the Darwinian explanation, he wishes to supplement it, and to explain why certain forms of variation are available to natural selection, and others are not. A bat could only evolve a more efficient wing hinge (like that found in insects) if there were a way to produce a second anchor point, and Goodwin's objective is to explain why that variation does not come about, but the variation which we do see in tetrapod limbs could and did come to be. This work provides a mathematical basis for research into the ways in which developmental tool kits (discussed in the following claim) operate to organize morphology.
Explore Evolution incorrectly claims that Goodwin "explain[s] homology in another way," saying that "homology does not reflect a process of historical change, but instead reflects constraints imposed by the laws of nature" (EE, p. 43). The passage above clearly demonstrates this to be false. He not only grants, but celebrates, the historical explanation that evolutionary explanations offer. His work offers a set of explanations on top of those historical explanations.
Goodwin's ideas remain controversial, and the links between his mathematical models and the underlying biological processes remain incomplete. Earlier attempts at the sort of unification he is offering have failed on those same grounds. Alan Turing, the father of modern computer science, did some of the earliest work seeking mathematical models which would explain morphological change in terms of simple mathematical concepts. While his math was correct, later work on the molecular basis of development revealed that his models were oversimplified and biologically incorrect. They remain useful in other settings (including manufacturing), and provide a philosophical basis for Goodwin's approach.
For reasons discussed in the previous claim and below in the section on convergence, the sort of similarity that Goodwin's models might predict would not be sufficient to explain homology. His discussion of eye evolution acknowledges and relies on this shortcoming. The consistency of the basic form of the eye is evidence of developmental and functional constraints, but differences in developmental pathways and morphology (structure) provide evidence of multiple origins of the eye in multiple lineages. Eyes, he explains have "evolved independently in at least 40 different lineages. Eyes seem to pop up all over the evolutionary map, and each time they present the same challenge … How could random, independent events ever generate such an inherently improbable, coherently organized process as that required to generate a functional visual system in the first place? What I suggest is that eyes are not improbable at all. The basic processes of animal morphogenesis lead in a perfectly natural way to the fundamental structure of the eye" (Goodwin, 2001, p. 162). EE summarizes Goodwin by claiming that "we should expect to see similarities in the anatomical structures of even different types of organisms" (EE, p. 43). There is a difference between similarity of form (Goodwin's topic) and similarity of structure (the topic of interest in studying homology). In conflating these topics, Explore Evolution confuses the issue of homology, misrepresents the scientist they cite, and skips a chance to help students understand cutting edge research in biology. This is not acceptable for a textbook.