The Nematode Vulva and the Nature of Evolution

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.

They approach this conclusion by studying development of more than 40 characteristics of vulva development "including cell fates, fate induction, cell competence, division patterns, morphogenesis, and related aspects of gonad development" in 51 species. This produced a taxon-dense character-rich phylogeny. With this, it would be possible to test for patterns that are directional vs. directionless.

They found "an astonishing amount of variation and an even larger number of evolutionary changes, suggesting a high degree of homoplasy (convergences and reversals). Surprisingly, only two characters showed unbiased evolution. Evolution of all other characters was biased."

Having shown that there are biases in the evolution of developmental process in this system, the authors ask the question: What kind of bias is this?

They suggest that there are two kinds of constraints. One is selection independent constraints, which would primarily consist of developmental/phylogenetic constraints. The other, of course, is selection. Telling them apart can be difficult. One approach that they try is the following:

One test for bias by positive selection is to study the mechanisms that underlie convergent changes. If convergent changes to the same character state occurred in two lineages but by different mechanisms, a bias by selection is indicated.

The authors suggest that some of the features they examined show convergence from multiple directions, suggesting selection as the cause of the bias. However, they are cautious and do not draw strong conclusions from these observations, opting instead to propose that more research is needed.

In conclusion, our study shows that evolution of vulva development is strongly biased and only few aspects are likely to change in an unconstrained stochastic fashion. Our phylogeny for rhabditid species, including the model systems C. elegans and P. pacificus, provides a foundation for evolutionary analyses of other characters as well. If the patterns that we observed in the vulva system are found more generally, then most of developmental system drift is driven by deterministic and not stochastic processes.

Kiontke, Karin , Antoine Barriere, Irina Kolotuev, Benjamin Podbilewicz, Ralf Sommer, David H.A. Fitch, and Marie-Anne Felix. (2007) Trends, Stasis, and Drift in the Evolution of Nematode Vulva Development. Current Biology. Current Biology, Vol 17, 1925-1937, 20 November 2007