Lab and Field Experiments of Group Selection

Late as usual.  I think I will not be able to keep up the weekly posting.  This week I want to talk a little about what why we do group selection experiments, and what they tell us about group selection in nature.

In a standard laboratory group selection experiment several sets of metapopulations are set up.  The different metapopulations are subjected to group selection for a particular trait.  For example, an easy to describe early, actually the first, group selection experiment is that of Wade (1977. Evolution 31: 134-153.).  In this experiment he set up four treatments, although I will only discuss three of them, a group selection for high population size, a group selection for low population size, and a no group selection treatment (the fourth treatment is a random group selection treatment).  Each treatment was a metapopulation of 48 populations of 16 beetles each.  These populations were raised for 37 days (one generation), and then censused for population size.  In each metapopulation the populations were ranked by their population size.  In the group selection for high population size treatment the largest population was taken, and as many populations as possible were set up from that population.  When that population was exhausted the next largest was taken until the metapopulation of 48 populations was obtained.  For the group selection for low population size treatment the smallest population was used to establish as many populations as possible, then the second smallest etc. until the necessary 48 populations were obtained.  Finally in the no group selection treatment each population established exactly one population in the next generation.

wade experimental design

The Wade 1977 experiment.  Shown here are three treatments, group selection for large population size, group selection for low population size, and no group selection.  Redrawn from Wade (1977. Evolution 31: 134-153.)

Wade got what we now know to be a typical result:  There was a significant response to selection both for increased and decreased population size.

Wade response to selection

The response to selection in the Wade 1977 experiment, reported as a deviation from controls. Redrawn from Wade (1977. Evolution 31: 134-153.)

This was the first group selection experiment ever, so perhaps you can find some flaws in the design (they are there), but his general result of a solid response to group selection has been replicated many times at this point.  Below is a list of group (or community) selection studies I compiled some time ago that all show a significant response to group selection.  There would be more if I brought it up to date.

Other studies of group selection

So what these experiments tell us is that group selection can bring about a response to selection.  They actually don’t tell us whether or not there is group selection, rather, what they tell us is whether group selection can bring about a response to selection.  In other words, what they tell us is that there is heritability at the group level.  They don’t tell us whether group selection is acting in nature, rather they tell us that if there is group selection in nature we can expect that that selection will cause adaptive evolution to occur.

Historically this was a bit of a conundrum in the early days of group selection experiments.  The earliest criticisms of group selection, such as Maynard Smith’s famous “hay stack model” (J. Maynard Smith 1964. Nature 201:1145) posited on theoretical grounds that group selection should not work.  Experiments such as Wades quickly demonstrated that group selection does work, and that it works far better than anybody could have expected.  This simply shifted the debate.  Now that we knew it could cause evolutionary change, the argument against group selection simply became that it didn’t occur in nature.

Unfortunately, no amount of laboratory studies will ever show whether or not group selection is an important force in nature.  I am not sure who said it, but I once heard it said that theory was the realm of all conceivable worlds, the lab is the realm of all possible worlds, and nature is the realm of the real world.  Yes, group selection experiments work, but it would never be convincing as long as the selective force was the investigator.

To study the importance of group selection in nature, it is important to actually go into nature and measure the effects of selection on natural populations.  When doing this we typically do not know the heritability of the traits on which we are measuring selection.  The reason for this is two-fold.   First, we know the genetic basis for traits in model organisms, but our model organisms are not typically what we study in the field.  Second, in natural populations we tend to be interested in complex behavioral traits that are often poor candidates for genetic studies.  In any case, this is why the distinction made in quantitative genetics between selection and the response to selection is so important.  It is indeed legitimate to study selection even if we do not have a way to find out what the evolutionary consequences of that selection are.  As a general rule, we study genetics and heritability in the lab, and selection in nature.

The first study to actually demonstrate group selection actually occurring in nature was done by Breden and Wade (1989. Am. Nat. 134:35-50).  They did a series of experiments on cannibalism in the imported willow leaf beetle, Plagiodera versicolor.  With out going into detail, what they found was that there was a general advantage at the individual level to being a cannibal: cannibals grew faster, and generally survived better.  However, they found a positive relationship between group size and survival.  Fairly obviously, eating your brethren negatively impacts your group size.

B n W grp size by survival

Copied from (Breden and Wade 1989. Am. Nat. 134:35-50)

This relationship between group size and survival is not surprising.  Leaf beetle larvae are group foragers, and collectively defend against predators.  Larger groups are more efficient foragers (perhaps because they can better break the leaf cuticle, and more resistant to predation (they exude noxious chemicals, and large groups are more toxic than small groups).  On the other hand cannibals benefit from the very high nutrient and energy content of their victims.

willowleaf4_600px

Imported willow leaf beetles are group foragers.  Larger groups survive better than small groups.  Photo by J. Hahn, University of Minnesota http://www.extension.umn.edu/garden/diagnose/plant/deciduous/willow/leavesholes.html

As I said, this was the first study of group selection in natural populations, and it successfully found group selection.  On the one hand this is heartening:  You look for group selection and there it is.  On the other hand, there is no question that Breden and Wade went out and looked for a good candidate species to find group selection.  Thus, this is perhaps a biased sample.  The real problem with this study, however, is that the methods that they used were very species specific, and they do not provide a protocol that is easily generalized to the study of group selection on any trait in any species.

Regardless, this emphasizes my point nicely:  Lab studies of group selection successfully answered the question of whether group selection works.  It does.  This field study, and others I will discuss show that there is group selection in nature.  Collectively these experiments suggest that the role of group selection in evolution is something that should not be ignored.

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