Evolution Is Not An Arrow!

I think sometimes people get the very wrong impression that evolution moves organisms along a fitness axis slowly, inexorably, but inevitably in one direction, towards increasing fitness.  A number of analogies spring to mind:  a stair step; a hill; an arrow.  This is entirely wrong.  In fact, evolution is closer to a random walk on an inclined plane.   Wikimedia has an example of a random walk animation:

Now imagine this random walk is on a slightly sloped plane, where the slope represents the slight increase in fitness from a certain trait (camouflage, nutrition, whatever).  There is a good chance that our desired outcome (increased fitness) will be the result of walking randomly along that sloped surface… but sometimes it won’t be, because it randomly heads in the wrong direction.  We can often detect only the cases where the slope of that plane is very steep, so that randomness contributes only a small amount to the outcome.

It’s a word I’ve used twice in one day, but evolution is not what you would call deterministic… it’s stochastic (in the sloppy sense of that word that a biologist can use).  That is, you can’t predict where the actual mix of alleles will end up based purely on knowing the fitness slope, or what alleles carry a slight advantage.  It’s not truly random, but the information for the outcome isn’t contained in the setup.

I see this mistake all the time among proponents of the scientific view:  “The birds have optimized beaks because it increases their fitness” is not QUITE the same thing as “The fitness increase of specialized beaks should be favored over time by selection”.  The difference is in the inevitability of fitness optimization.  You can’t assume that every good trait will be preserved.  Some of them just get lost by random effects.

I’ll leave you with a good quote:

Sometimes a 3-1 favorite loses. That’s why they call it gambling, and that’s why they keep flipping over the cards.
Richard Roeper

The same is true for fitness landscapes…  or if you prefer: “The race is not always to the swift, nor the battle to the strong, but that’s the way to bet”  : )  Darwin couldn’t have put it better.
-C