Monday, August 28, 2006

Evo-devo and natural variation   posted by JP @ 8/28/2006 01:41:00 PM

I just finished Carl Zimmer's excellent book At the Water's Edge, a general audience recounting of two major events in macroevolution--the evolution of tetrapods and of whales. One of the major recurring themes is how past events in evolution constrain the probability space of the future. This passage (in a section describing Hox genes and limb development) stopped me cold:

Neil Shubin has found one of the most striking demonstrations of how the innovation of the limb imposes a new order. In 1991 a freak freeze turned a pond in Marin County, California, to ice. It killed hundreds of rough-skinned newts and perfectly preserved their corpses. Shubin, who was working at the time at Berekely, got hold of the newts while they were still frozen, and in the years that followed was able to study 452 of their limbs. In this one gathering of a single species, he discovered an orgy or variation: almost a third of the newts had some dramatic oddity in their limbs...Yet the variation was actually very limited and biased. Fusions and extra bones always occurred along the path of the branching limb axis that Shubin and Alberch had identified in 1986.

My two thoughts/questions:

1. One-third of the individuals in this small population had dramatic variation in their limbs (bone fusions, extra bones). That's a lot. Does anyone know of numbers like this for humans? Are we as polymorphic as this in terms of structure?

2. The fact that all these mutations occured along the branching limb axis set essentually by Hox genes is striking, and emphasizes the role of development in determining natural variation (ok, yeah, evo-devo people have been saying this for years. duly noted). Have these sorts of results been found in other organs (like say, the brain)? I think this is going on my list of books to read.