Thursday, January 24, 2008

Evolution: Non-directional (The Hox Gene, part 1)

One of Gould's recurrent themes is that evolution is not necessarily a process of increasing complexity. Parasites present a useful example of this. It's all about streamlining.
Gould: parasites "often adapt to their surroundings by evolving an extremely simplified anatomy, sometimes little more than a glob of absorptive and reproductive tissue".

Protozoa are mobile, single-cellular animals. Metazoa are complex, multicellular animals.

Then there are Mesozoa - literally, "middle animals". The major group, Dicyemida, are parasites residing in squids and octopuses.
They comprise one central cell, with about 10 to 40 cells arranged in an outer layer. The debate around these animals had been where to place them in the evolutionary map. A 1999 article in Nature (Kobayashi et al) announced the discovery within them of a Hox gene. These are known only in higher order metazoa, specifically those with three cell layers (triploblasts - they include an inner body cavity).

Thus it has been concluded that these creatures evolved to the most efficient design necessary to survive in that environmental niche. Evolution is about developing the best fit to environment. At times, that constitutes an "arms race" between species; other times it involves a paring back.

Survival is what counts, not persistently marching up a metaphorical hill. The metaphor is more akin to negotiating an endless, multi-path maze. There are many directions that work, some are dead ends, and sometimes there's a park bench to rest at - there's no specific goal other than to be.

Gould, SJ (2002): I Have Landed; Jonathan Cape, London.

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