Sunday, May 22, 2011

Leaping Lizards

At a party last night, I ran into a biologist who studies lizards. So we got to talking, as you do, about bipedalism. The habit of running on two legs has arisen in several different lineages of lizards, but why did it evolve? Speed, energetic efficiency, heat loss, vision, or that all-purpose explanation sexual selection? or maybe, like us and the birds, they've got something better to do with their front limbs?


None of the above, says the biologist. Sure, there are bipedal lizards. But very likely, bipedalism in lizards did not evolve.

Wait, how's that?

Aerts et al., Bipedalism in Lizards:
The exact advantages of bipedal locomotion in lizards remain debated. Earlier claims that bipedalism would increase maximal running speed or would be energetically advantageous have been questioned. Here, we use ‘whole body’ mechanical modelling to provide an alternative solution to the riddle. The starting point is the intermittent running style combined with the need for a high manoeuvrability characterizing many small lizard species. Manoeuvrability benefits from a caudal [rearward] shift of the centre of mass of the body (body-COM), because forces to change the heading and to align the body to this new heading do not conflict with each other. The caudally situated body-COM, however, might result in a lift of the front part of the body when accelerating ... [leading to] observable distances passively covered bipedally as a consequence of the acceleration. In this way, no functional explanation of the phenomenon of lizard bipedalism is required and bipedalism can probably be considered non-adaptive in many cases.
In other words, if you, being a lizard, need to change direction quickly when you're running, it's better to have your center of mass situated in the back, near your pelvis. That makes it easier to swing the front of your body around when you turn. But a side effect of having your center of gravity toward your back end is that your front end tends to rise when you accelerate sharply, as, being a lizard, you often do. (You, being a person now, have experienced this if you've ridden a bike up a steep hill. Conversely, brake suddenly, the back end of the bike goes up.) Air resistance adds to this effect, as does the fact that one of the ways the center of gravity is moved backwards is an overdevelopment of the rear legs relative to the front ones. The result is that lizards evolved for junk in the trunk end up sometimes running on their rear legs, even if that was not selected for at all.

(The linked article is based on experiments with a mechanical model of a lizard. According to dude at the party, the same conclusions are suggested by observations of lizard bipedalism in nature.)

I'm writing about this partly just because it's cool (go science!) but also because it's a nice illustration of an aspect of evolution that's not widely understood, especially, perhaps, by some of its more aggressive proselytes. Darwinism is certainly correct, on some level: on the level that the appearance of design in an organism in no way implies the existence of a designer. But the statement that complex adaptive traits are the result of natural selection, while true, tells us much less than it seems to at first glance, because it's seldom obvious what constitutes a "trait"; even more seldom what universe of alternatives it was selected from. In this case, we, proud bipeds, see a lizard running on its hind legs and think, that's a trait; whereas, dancers and gymnasts perhaps aside, we're not much conscious of where our center of mass is. But what we see as a trait isn't necessarily what evolution sees; not everything in Borges' encyclopedia is selected on. Perhaps the majority of what we see as traits are, as in this case, spandrels.

Needless to say, this is especially true when the organisms are human beings and the alleged trait is something psychological, especially something relating to sex and gender roles. A true evolutionary explanation should provide both concrete evidence (not just a just-so story) for the selective advantage of the supposed trait, and an account of the specific developmental pathways through which it arises; at least it should have one of the two. But in many of the "evolutionary" stories that people get most excited about, both are entirely lacking. Certainly when it comes to higher brain functions, with the exception of vision, the only statement genuinely grounded in evolutionary biology is, "We don't know."

1 comment:

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