The random walk of evolution

March 11, 2011 § 2 Comments

One of the reasons that some people have a hard time accepting evolution is that the organisms we can study today are so darn complicated that it’s hard to see how they could have arisen from many small steps.  Take chemotaxis in bacteria, for example.  The motor for movement, the flagellum, has frequently been cited as an example of so-called “irreducible complexity“; proponents of intelligent design claim that it could not have evolved because every one of its 40 or so parts is needed for the motor to function at all.  To me, this argument has a strange sort of circularity about it: the underlying assumption seems to be that the flagellum needed to be exactly the way it is, and so the problem that evolution faced was how to build it in such a way that each step improved motor activity. If you can’t imagine a path in which motor activity smoothly improved from zero to current-day, evolution must be wrong; and because you started out by imagining what is in effect an engineering design process, you end up concluding that the only way it could have succeeded is through the actions of a designer.  But what is wrong instead is the idea that evolution takes linear paths that lead ever upward, and that have always been focused on the improvement of the function we see today.  It’s so much easier to imagine evolution as a march of progress than to try to get your head around over 3 billion years of a meandering multibranched process of accident, failure and occasional success.  In particular, it’s hard to keep in mind that the advantage that leads to increased evolutionary success can be very very small; often so small that it’s extremely hard to measure.

A recent paper (Wei et al. 2011. The population dynamics of bacteria in physically structured habitats and the adaptive virtue of random motility. PNAS doi/10.1073/pnas.1013499108) set out to disentangle a different question in the evolution of chemotaxis, namely the question of what drove the evolution of motility in the first place.  This is a less trivial question than it may appear at first sight.  The genes that allow the bacterium to sense and respond to chemical cues in the environment are separate from those that allow movement, so either this is another example of irreducible complexity or (my choice) one of these two functions is useful without the other.  Does it do you any good to sense food if you can’t move towards it?  Maybe not, though I wouldn’t rule out the possibility that at some distant time in the past it might have been advantageous for bacteria to be able to grow selectively in the direction of food.  (If nobody else can move either, why not?)  But it seems more likely, from several lines of evidence, that being able to move is the function that evolved first, and that chemotaxis evolved as a way to bias the motion towards useful directions. « Read the rest of this entry »

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