Saturday, July 21, 2007

Evolution by mutations of large effect...or not?   posted by p-ter @ 7/21/2007 09:17:00 AM

The model of evolution that came out of the "Modern Synthesis" (see the figure on this site) predicts that evolution by natural selection occurs as a sort of stepwise optimization algorithm-- that is, a population will approach some fitness "peak" in a series of small steps, rather than all at once. This sounds rather reasonable-- making small changes to an organism is more "conservative", in some sense, than drastic restructuring with a single mutation, and thus less likely to knock out critical pathways. Yet there have seemed to be some examples of a single locus controlling major morphological differences between species. One example comes from Drosphila, where two species--D. melanogaster and D. sechellia--show rather different devlopmental patterns at a certain larval stage. This difference has been mapped (via inter-species crosses-- unfortunately not something that can be done in, say, humans and chimps) to a single locus referred to as shavenbaby (this being Drosophila, gene naming conventions are, well, non-existent).

However, it was unclear whether the changes in the regulation of shavenbaby were due to a single mutation of large effect, or several mutations of smaller effect. The latter would be predicted by the Fisherian model, and indeed, new research has found the precise genetic changes underlying the difference, showing that there are several mutations in different enhancers that individually generate smaller changes in shavenbaby expression.

The position of the gene in the developmental network is shown in the figure (from here), and the authors speculate that the reason for multiple regulatory changes at this single locus is due to the architecture of the network. In their words:
Given that laboratory-induced mutations in dozens of genes alter trichome patterns [the developmental pattern in question here], it is striking that multiple mutations at a single locus account for the entire evolved difference. Svb seems peculiar in the network of genetic interactions that establish the trichome pattern, because it sits at the nexus of the upstream patterning genes and the downstream effector genes. Although trichome pattern could be changed by altering any of several upstream genes, these changes would probably produce pleiotropic effects on other developmental processes. In contrast, none of the known downstream genes is sufficient on its own to prevent or promote trichome formation. Thus, changes at svb enhancers may provide the only available genetic mechanism to evolve trichome patterns without pleiotropic consequences.
Mutations cannot be fully studied in isolation-- they do not exist outside the context of genetic and biochemical networks.

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