Thursday, September 17, 2009

How does evolution work?

I'd like to kick this blog off with a very broad question: how does evolution work?  The photo above was taken at Antelope Canyon, in Arizona.  The sandstone walls of the canyon have been carved by erosion over many millenia: uncountable grains of sand, myriad molecules of water, all rubbing against the rock on their journey downstream, gradually shaping the canyon into what it is today.  And what the canyon looks like depends very much on when you visit it; the floor of the canyon, which is just sand left behind by the last flash flood, rises and falls by many feet over short periods of time, or so I am told.  Formations that are hidden by sand one month may be many feet above your head the next month.  Regardless of when you visit it, however, the canyon is stunningly beautiful.

Antelope Canyon is an example of the marriage of the deterministic and the contingent.  On the one hand, the shapes carved in the canyon walls are not predetermined by any mathematical formula.  No computer could predict the particular form that the canyon has today.  A soft spot here and a hard spot there, an eddy here and a fast-flowing current there, have made it the way it is now; but slightly different conditions might have led to completely different contours.  In any case the canyon is not a static entity; if you could visit it a few thousand years from now, it might look quite different.  On the other hand, its shape is not completely random; there are many shapes that will never be formed by its particular dynamics.  Wait as long as you like, but you will never find a tiny replica of the Empire State building cut into the rock.  Sandstone and erosion have their own particular laws, which they always obey.

The question I am interested in, and the one that attracted me to work in the Hendry lab, is: how are the products of evolution — fish and frogs, birds and beavers, you and I — like or unlike Antelope Canyon?  To what extent are the products of evolution predictable and deterministic, a repeatable outcome given environmental conditions?  To what extent are they unique, the irreproducible results of a grand experiment that can never be replicated?  What are the laws that evolution obeys?  What sorts of products will it tend to create over and over, and what sorts of products, like the Empire State Building, are forever beyond its reach?  How quickly can its products change — how long does it take for a new species to form, or for a new trait to evolve?  And — a question that particularly interests me, but that seems completely unapproachable — why are the products of systems at the interface between determinism and contingency, like living organisms or Antelope Canyon, so incredibly beautiful?

Really answering such questions is far beyond the reach of a PhD thesis, or even an entire career; but they are the kinds of questions that inspire me.  All this could be framed in much more academic, jargon-laden ways: gene flow and selection gradients and additive genetic variance.  But it's important, I think, to keep in mind the ultimate reasons we have embarked on this journey: our curiosity about nature, and our joy upon beholding its beauty.  Let's keep the destination in mind, but enjoy the journey, too.

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