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New York, NY, USA. A multi-national team of biologists has concluded that developmental evolution is deterministic and orderly, rather than random, based on a study of different species of roundworms.
The researchers were interested in how development evolves in organs which themselves do not change. To do so, they examined the vulva—the female's copulatory and egg-laying organ — in nearly 50 species of roundworms.
Because the vulva does not significantly change across species, one might predict that there would be little variation in vulva development. The findings are reported in the latest issue of the journal Current Biology.
Variation of vulval development among rhabditid nematodes.
Image courtesy of New York University.
In executing the study, the research team analyzed more than 40 characteristics of vulva development, including cell death, cell division patterns, and related aspects of gonad development.
They plotted the evolution of these traits on a new phylogenetic tree, which illustrates how species are related to one another and provides a map as to how evolutionary changes are occurring.
Their results showed an even greater number of evolutionary changes in vulva development than the researchers had expected. The researchers found an astonishing amount of developmental variation. They then reasoned that this variation, since it did not affect the final adult vulva, should have evolved in a stochastic, or random, fashion.
For example, they concluded that the number of cell divisions needed in vulva development declined over time — instead of randomly increasing and decreasing.
In addition, the team noted that the number of rings used to form the vulva consistently declined during the evolutionary process. These results demonstrate that, even where we might expect evolution to be random, it is not. The authors argue that evolutionary changes among these species were unidirectional in nearly all instances.
Trends, Stasis, and Drift in the Evolution of Nematode Vulva Development. Karin Kiontke, Antoine Barrière, Irina Kolotuev, Benjamin Podbilewicz, Ralf Sommer, David H.A. Fitch, and Marie-Anne Félix. Current Biology 2007 17: 1925-1937.
Summary
Background. A surprising amount of developmental variation has been observed for otherwise highly conserved features, a phenomenon known as developmental system drift. Either stochastic processes (e.g., drift and absence of selection-independent constraints) or deterministic processes (e.g., selection or constraints) could be the predominate mechanism for the evolution of such variation. We tested whether evolutionary patterns of change were unbiased or biased, as predicted by the stochastic or deterministic hypotheses, respectively. As a model, we used the nematode vulva, a highly conserved, essential organ, the development of which has been intensively studied in the model systems Caenorhabditis elegans and Pristionchus pacificus.
Results. For 51 rhabditid species, we analyzed more than 40 characteristics of vulva development, including cell fates, fate induction, cell competence, division patterns, morphogenesis, and related aspects of gonad development. We then defined individual characters and plotted their evolution on a phylogeny inferred for 65 species from three nuclear gene sequences. This taxon-dense phylogeny provides for the first time a highly resolved picture of rhabditid evolution and allows the reconstruction of the number and directionality of changes in the vulva development characters. We 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.
Conclusions. We propose that developmental evolution is primarily governed by selection and/or selection-independent constraints, not stochastic processes such as drift in unconstrained phenotypic space.
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Crazy Sexy Cancer. This is the trailer for a documentary film by Kris Carr about her cancer adventure. It premiered on TLC and Discovery Health (Fall 2007).
Kris also authored two books, Crazy Sexy Cancer Tips and Crazy Sexy Cancer Survivor: More Rebellion and Fire for Your Healing Journey.
Variation of vulval development among rhabditid nematodes. 
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