The Nematode Vulva Indicates That Evolution Is More Selective Than Stochastic

The question is basic: Is evolutionary change largely random or is it more often shaped by selective forces? The former is linked to what is called Neutral Theory, and it has a lot of support, to the extent that it most likely true. The latter is part of what is sometimes known as the Adaptationist Program, and it is certainly correct. New research on the Development of the Nematode Vulva is sure to cloud the issue even further...

First, a word on this confusing introduction. We know that when we observe life, we do not see a really wide range of degree of adaptation among closely related forms to a particular basic problem. For instance, there are dozens of species of cats, and they all hunt using their teeth and claws, and while each species tends to specialize on prey (with respect, especially to body size) there is no sensible way to rank the various cat species as to how good they are at hunting. Individual cats can be ranked, of course, and this sort of potential variation is part of the raw material for the selective forces that result in cats generally being well adapted. If the features related to cats being able to hunt were not shaped primarily by selection, but instead by random effects, then there would be observable variation among species.

At the same time, when we look at just the genetics of any set of organisms, we see a a surprisingly random-looking pattern. If there is a close link between adaptation and genetics, the known pattern of the rise of species, often in relatively sudden adaptive radiations punctuating long periods of very little observable change in morphology should cause the genes to vary in a non-stochastic way. In other words, if we took ten species that can be organized in a large phylogeny, the very different histories each lineage went through should result in differing rates of genetic change along those lineages. But this does not seem to happen. While there are variations in rate of genetic change, these changes do not seem to map onto adaptive features of phylogenies. An existing fish species is just as genetically different from an existing frog species as it is from an existing mammalian carnivore species. This suggests that genetic change is not patterned by presumed adaptive structure of the phylogeny.

Many biologists chose to be a Neutral Theorist or an Adaptationist. In my view, each of these approaches is strongly supported by data that serves to falsify the other. This must mean that both views are wrong, or at least, grossly oversimplified. (Of course, I've oversimplified them here in my description, but it is likely that the less simplified versions that you will find in the literature are still oversimplified.)

Now, all of this is about traits (phenotypes) and genes. The paper in question is about neither. It is about development. Development is, of course, the process behind the traits. Indeed, in my view, traits are figments of our scientific imagination. Traits are processual entities both in terms of how they get there (on or of a particular organism) and how they change over time (in an evolutionary sense). Do not fetishize the traits, or do so at your own risk.

The paper concludes ...

that developmental evolution is primarily governed by selection and/or selection-independent constraints, not stochastic processes such as drift in unconstrained phenotypic space.