Wednesday, April 18, 2007

Canards: Evolution and Selection

"The Age of Darwin" (paid subscription, but not worth reading, trust me) tells us:
The logic of evolution explains why people vie for status, form groups, fall in love and cherish their young. It holds that most everything that exists does so for a purpose. If some trait, like emotion, can cause big problems, then it must also provide bigger benefits, because nature will not expend energy on things that don’t enhance the chance of survival.

While criticizing the NYT's science is like shooting fish in a barrel, this is nonetheless a distressingly common misconception. So let's talk some biology here. [Lengthy refs at the end.]

Evolution is
the process of change. Many evolved traits provide no benefits. Evolution is not 'raising us higher' or going towards an ever-more-complex end. It can go from simplicity to complexity, but it goes the other way. It causes a set of both adaptive and nonadaptive responses to the environment.

So you have an organism, and it has DNA. The DNA codes for all kinds of RNAs and proteins, and there are all kinds of complex regulation going on, including a 'fact-checker' system that goes around seeing if the DNA doesn't match, or if it's gone funny-shaped, or if there are holes.

These problems happen all the time: radiation damage, oxidation; various proteins make mistakes. Normally, if a cell's DNA is screwed up, it won't divide- but if the checkers are messed up, the errors go through. (This is a vastly simplified explanation.) Main message: mutation is random.

Some mutations are harmful, like if you stop cells from making protein, they're dead. We call this selective pressure: there is a need for the function, and doing it better increases chances of thriving. More or less. Say a mutation causes a bird to not fly: it probably won't reproduce. If it flies slower, it won't reproduce as well, because it can't catch mates, or whatever. Its reproductive fitness is hurt.

But imagine a mutation that makes a bird fly a tiny bit slower. Will this make a huge difference? If there is little competition, or if resources (food, twigs, mates) are abundant, it won't. There doesn't have to be a selective pressure on every function.

'Emotion.' Does it do us any good? Well, I'm sure 'fear of a large animal eating me' was a useful trait. One can imagine more examples. But it doesn't have to provide advantages- if the selection is weak. If there's a smallish disadvantage to having a trait, you don't need a huge advantage to balance it out: you don't necessarily need any.


1. Encyclopedia of Evolution. Mark Page, Ed. Oxford UP 2002

SJ Gould:"Suppose... that a trend to smaller average body size among the species of a clade occurs not because smaller bodies confer adaptive advantages on organisms, but because species composed of small organisms tend to manifest properties... that enhance their rate of producing new species. In that case, decrease in average body size would spread as a trend through the clade by 'hitchhiking' on the correlated species-level trait of high speciation rates, and not because small bodies confer Darwinian advantages on organisms. In fact, smaller body size might well be neutral or even slightly detrimental to organisms in competition with [bigger animals, but if there's not a lot of competition, smaller size wins out]".

SC Stearns: "Why is our neurobiology organized in such a way that we can become addicted to certain chemicals? One idea is that the chemical structures of addictive drugs are an unfortunate coincidence. They hijack pathways that evolved to increase fitness... Thus, susceptibility to addictive drugs is a nonadaptive byproduct of structures and processes evolved for other reasons. There are also evolutionary hypotheses for the existence of emotional moods. Depression might be adaptive if it could cause avoidance of risky or dangerous situations... This is a plausible explanation for a moderate level of depression but not of serious depression leading to suicide. If a selection process had repeatedly encountered the problem of deep depression in people young enough to have some remaining reproductive potential, one would expect countermeasures to have evolved..." [Ed.: But they didn't, did they? So no strong-enough-to-take-it-all-the-way-out selective pressure was ever applied.]