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I learned a lot at last week’s Nescent Academy on Quantitative Genetics. I saw a lot of material that I wouldn’t have seen in other forums, like the Ornstein-Uhlenbeck models of genetic change under microevolution and macroevolution. During the last half of the week, which focused on macroevolution, I confirmed my impression that when talking about evolutionary history, genetic drift is really the name of the game. When population geneticists talk about the history of particular genes (for example, a gene implicated in a human disease), they rarely speak about selection. There was also a lot of good information on research findings from natural populations.

There were three points in particular that struck me:

  1. When looking at natural populations, there is abundant genetic variation in almost any trait
  2. Selection is generally weak and generally stabilizing selection
  3. Stochastic processes, such as genetic drift, can account for a lot of diversification seen in nature

These findings were interesting to me because I study selection, usually using deterministic models and because I’ve seen the perspectives of other researchers about the relative roles of selection and drift. I have tended to assume, along with many other researchers, that most diversification is due to selection, and that for any “real” differences to matter over evolutionary time that selection must be involved. Why is this?

Selection and adaptation are appealing concepts and they are simple to understand. Darwin’s (three or four) postulates give us all we need to understand how adaptation comes about. Adaptation is a really nice idea: things become more efficient, better, over time. Not only is that aspect appealing, but it’s easy to understand how it could happen: selection eliminates the less efficient, and promotes the more efficient. All you need after that is inheritance. This is so easy that most people get it the first time. Leaving aside the appeal of this from the social perspective (read the first chapter of The Dialectical Biologist), it’s just easy. I teach evolution and ecology to undergraduates and most of them come in getting the basic idea of selection. It’s not hard.

Take genetic drift on the other hand. If you’re like most biologists who’ve tried to teach about genetic drift, you know that genetic drift is the opposite of selection from a teaching perspective. Genetic drift, like selection, removes variation from populations. Only mutation can bring it in. Under genetic drift alleles just disappear: by random chance they fail to make it into the next generation. This only happens in finite populations, that is every single real population. How? Think of it this way: you know that if you flip a coin a thousand times you will get close to 500 heads and the rest will be tails. Do this instead: flip a coin ten times ten times and record the number of heads you get for each ten coin flips. You could then make a graph depicting the number of times you get five heads, six heads and so on. You should get a nice looking histogram: close your eyes and put your finger on a spot on the graph. Your real population is that spot. It could be the one where you got zero heads.

There are two important things about genetic drift: one is how it leads to diversification, and the other is how it accounts for polymorphism. Drift leads to variation between populations because when populations are separated they randomly undergo drift, possibly with different end results: if there are two alleles A and B at a locus under drift, one population could lose allele B and the other could lose allele A. Repeat that over many loci and your get very different looking animals that don’t recognize each other when they get a chance to make babies. The second property is that when you observe polymorphism (genetic variation), it is probably due to drift. Drift over time removes genetic variation from a population, but before that happens the frequency of the allele in question bounces all over the place by random chance. The time window over which that happens is incredibly large, much longer than that for selection. Therefore the large amounts of genetic variation in natural populations are probably due to weak selection, strong drift and lots of mutation across the genome.

Here’s my explanation of the above findings: selection is always happening, but is generally weak. Selection is weak both because drift is happening at the same time, and because life is just not as hard as Darwin and Malthus had in mind. Selection in nature is usually stabilizing selection, meaning that there is some intermediate value that is favored most, and extreme values are selected against: the typical example is birth weight in industrialized societies. Small babies are prone to infection and pulmonary dysfunction and large babies are at greater risk for perinatal complications. However, in most cases, it appears that deviations from the optima that we can detect in nature are not heavily penalized. Especially in large-bodied, iteroparous organisms like birds, ungulates and primates, life just doesn’t seem that hard. This means drift has plenty to work with. Most of the organic diversity we see is probably due to drift randomly sending populations closer to new optima where stabilizing selection takes over again. This is basically Sewall Wright‘s shifting balance theory.

Wright in 1954

Wright in 1954 (Photo credit: Wikipedia)

This positively demonic process could account for most of the organic diversity we see out there, but it is not an appealing idea. I think most intellectuals go through a phase where they attribute everything to randomness — and I’m not suggesting we all get on that bus — but there’s also a Conspiracy to remove slack from the world. People generally don’t like the idea of random forces to explain things. Especially since a lot of biologists, including myself, don’t understand genetic drift (how could you?) it’s really hard to get behind the idea. However, especially when analyzing real populations, such as the evolutionary history of humans, and testing ideas about sexual selection, we have to consider the role of drift. Much of the persistent, between population variation we see that looks adaptive could be due to genetic drift.

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