The common picture, painted all too often by commentators of various stripes, depicts a world in which mutations run a harrowing gauntlet of selection that is likely to foolishly discard both the gems and the proto-gems of biological function. Oh sure, the cream eventually rises to the top, but only through the magic of seemingly endless eons and limitless opportunities. I hope that most readers of the Panda's Thumb are annoyed by this crude caricature, but it's the standard tale, and when the narrator only has a paragraph, it's the one we're most likely to hear.Evolution is, of course, much more complex, with much genetic interaction in the form of linkage disequilibrium and polygenesis just to name a few of the processes that occur. Add to this that there is genetic drift in populations that often results in gene fixation and flow between them that allows for the persistence of deleterious genes. Now it seems that evolutionary "buffering" occurs.
It is well known that, in the course of genetic replication, there is, often, duplication of genes. This extends to developmental genes as well. As Steve notes, most developmental genes are regulated by other genes called enhancers. These are also susceptible to replication. Recounting recent work by Frankel and colleagues on flies, he writes:
What if the redundant enhancers can also buffer against genetic disasters? The experiment was straightforward: they deleted one copy of a major developmental control gene (called wingless). Those animals are just fine, until they lose the buffering of the shavenbaby [a developmental gene] shadow enhancers. Without the redundant system, the loss of one wingless gene leads to a significant change in developmental patterning. The conclusion, I think, is quite interesting: the impact of the shadow enhancers only becomes apparent when the system is stressed, by environmental challenges and even by genetic problems elsewhere in the genome.In other words, these developmental duplications allow for the genome to withstand some seriously bad mutations. This increases genetic diversity and genetic load in a population, which allows for greater adaptability to environmental change and insult. Neat stuff.