Monday, June 08, 2009

What Is Natural Selection?

 
If you ever thought that the concept of natural selection was easy then you must read Understanding Natural Selection: Essential Concepts and Common Misconceptions by T. Ryan Gregory. The article appears in the latest issue of Evolution: Education and Outreach and it contains many references to studies of how students perceive evolution.

It's a very good article. Ryan has thought of many misconceptions that I would have missed and he has documented the existence of those misconceptions in the classroom—among students and teachers alike!. He also does a really good job of explaining natural selection, with one important omission.

One thing that the article didn't mention is that natural selection is a stochastic process. A given allele may be demonstrably beneficial to survival but that doesn't mean it will sweep to fixation.

One of the misconceptions that we most frequently encounter in the blogosphere is the idea that whenever a beneficial effect is demonstrated, or conceived, it will automatically take over. The reality is that the probability of fixation is related to the population size and the coefficient of selection. Imagine that a new mutation gives rise to an allele conferring an advantage of 1% on the individual carrying it. The probability of fixation in the population is approximately 2%—in other words, the allele is lost 98% of the time.

Most people think that beneficial alleles will always become fixed in a population. That's one of the most important misconceptions about natural selection and it's a shame that it was left out of the article.

This misconception is behind much of adaptationist thinking. To them it seems to be sufficient to postulate a benefit, no matter how small, and it automatically follows that the entire population/species will acquire it. The reality is that such adaptionist thinking requires two separate components: (1) the existence of a possible beneficial allele and, (2) the demonstration that the postulated benefit is of sufficient potency to lead to fixation with a high probability.1

There's one other misconception that's missing. Many people think that natural selection only occurs when the environment changes. This is formally equivalent to a belief that, in a stable environment, all species become perfectly adapted so that no further adaptation can take place. There's no evidence to support this concept. It requires that most species are sitting at the top of an adaptive peak.

In order to become perfectly adapted, all possible beneficial mutations would have had to arise and be fixed in the population. I believe that this hasn't happened in most cases so that all species are still capable of further adaption even if the environment has been stable for a million years.

The corollary to this misconception is that species that haven't changed much in outward appearance over a long period of time (e.g. living fossils) have not evolved because their environment didn't change. In fact, we see plenty of change at the molecular level in these so-called "living fossils" and some of that change is almost certainly adaptation.

If you combine Ryan's article with the ideas that natural selection has a stochastic component and that perfect adaptation is rarely achieved, then you will understand natural selection better than most people.

If, in addition, you learn that there's more to evolution than natural selection then will truly be able to say you understand evolution.


1. For example, it's possible to imagine that there might be an allele conferring a particular behavior that's beneficial. Say, females not communicating with male relatives when they are ovulating. But that's not sufficient. Evolutionary psychologists must also explain why the adaptive benefit is sufficient to lead to fixation and preservation in the population. Is it a 0.001% benefit or a 25% benefit?

4 comments :

  1. That the relation between the size of population, the fitness advantage/disadvantage of a given trait, and the speed with which it gets fixed is not well understood by many who should know better is also demonstrated by the number of people who claim that the evolution of humans was inevitable. While humans (and great apes in general) are exactly the type of species with small effective population size in which these considerations matter the most

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  2. Thanks for the heads up on Ryan's paper! I will be contrarian in regards to the idea that adaptionists

    postulate a benefit, no matter how small, and it automatically follows that the entire population/species will acquire it.

    In regards to the idea that sperm taste bad to select against oral sex, I agree that is the case. However, if there are 100,000 mutations that each have a small selective advantage, a few will be fixed. The question is what is the rate of advantageous mutations in a given environment. I don't know, adaptionists don't know, and either does anyone else.

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  3. Great post! I do like these topics that get to the crux of the big topics that all biologists are (or at least should be) interested in.

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  4. This was a great paper. BTW--for Larry or any other readers: is there a similarly detailed paper like this on random mutations?

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