A phenotypic view of evolution Evolution in Structured Populations

Multilevel Selection In Impatiens

Last week’s post was bit of a set up.  To reiterate I made the point that lab selection experiments really tell us about the ability of population to respond to imposed group selection, and the one early study on group selection in nature did not provide a convenient protocol that could be easily extended to other systems.  This brings us to contextual analysis.  It is a statistical method, and as such it is not particularly beholden to any single system.  This can be applied to virtually any natural system in the same manner that an Arnold and Wade (1984 Evolution 38, 720-34; Evolution 38, 720-34) selection analysis can be applied.  As an example, I want to talk about Stevens, Goodnight and Kalisz (1995. Am. Nat. 145, 513-26).

First, as an anecdote to show that motivations are perhaps not as honorable as they should be, the reason we undertook this study is that after we published Goodnight, Schwartz, and Stevens (1992 Am. Nat.140:743-761), Lori Stevens and I were sitting around the lab talking and decided that SOMEBODY was going to use this in a field study.  We decided the first to publish such a study would have to be cited by everybody doing field work on multilevel seleciton, and I didn’t care what we studied, but that somebody was going to be us.  So, we recruited a fine plant biologist, Susan Kalisz, and set off to study multilevel selection in a plant that very obviously had group selection acting.  Sadly, at least in retrospect, we thought that there was some sort of hurry to do this study before we were scooped.  Another funny story worth mentioning is that the work involved spending long periods lying prone on a scaffold hanging over the plants censusing them.  Sadly, both Lori and Susan were quite pregnant, and somehow they thought this was an excuse for not having to do the censuses.  Bottom line:  I spent a lot of that summer being bit by mosquitoes while draped over a board measuring plants with the blood rushing to my head.

We did this study at Kellogg Bird Sanctuary (I was working at the biological station that summer), which was a great place to work.  It had large continuous stands of impatiens in areas that were restricted so there was no danger of having your work disturbed (although one of our sites was destroyed when a duck made her nest in the middle of it).

KBird Sanctuary

Kellogg Biological Station and Bird Sanctuary:  A great place to do research. (Photo from http://activerain.com/states/MI/cities/Augusta/communities/The%20Kellogg%20Bird%20Sanctuary)

In the first year of the study we determined the appropriate neighborhood size, which we decided was basically only the nearest neighbors, those in a ½ meter circle around the focal plants.  We then measured a bunch of traits associated with growth and size, and three “fitness” traits, number of cleistogamous (self pollinated) flowers, number of chasmogamous (open pollinated) flowers, and survival.

Impatiens flowers

Impatiens capensis cleistogamous and chasmogamous flowers. (http://homebuggarden.blogspot.com/2012/02/self-pollinator-of-week-cave-hortulanus.html)

 The problem with the large number of traits we measured is the typical problem with regression analysis:  you run out of degrees of freedom.  Fortunately path analysis provides a way out.  In particular, we did a factor analysis to reduce the large number of traits down to a few manageable factors.  We ended up with three factors.  Second, used the neighborhood means for the factors as the contextual traits.  Finally, we did separate analyses for each of the three measures of fitness.  As a result we had a manageable data set for contextual analysis that conceptually looked like this:

analysis schematic

Analysis schematic.  We censused a large number of traits, then used factor analysis to reduce that number to a manageable number of (three) factors.  The neighborhood mean of those factors was then used as the contextual traits. 

As is virtually always the case with factor analyses the first factor could be roughly called “size”, and that is the one I will focus on.  What we found was that the results of the contextual analysis depended on the measure of fitness we were using.  For open pollinated chasmogamous flowers there was strong individual selection for larger plants, but essentially no group selection.  For survival we used a logistic regression (either a plant was alive or dead), and found that although group and individual selection were in apparent opposition, only the individual selection component of the multiple regression was significant.  Finally, and very interestingly, for the self pollinated cleistogamous flowers we found that group and individual selection were both significant, and of equal magnitude but acting in opposite directions.

impatiens CA results

The strength of selection on Impatiens capensis.  The bar graph is the distribution of factor 1, (which is a measure of size).  For open pollinated (chasmogamous) flowers there is strong individual selection, but no neighborhood selection for large size.  For survival group and individual selection are in opposite directions, but group selection is not significant.  For closed pollinated (cleistogamous) flowers group and individual selection are of equal magnitude, but in opposite directions.

Focusing on the cleistogamous flowers, this is something we have seen before.  In fact, this IS soft selection, which occurs when every group (or neighborhood) produces the same number of progeny, but there is selection acting with groups.  In an earlier post I showed that this was a mix of group and individual selection in which selection at the two levels was of equal magnitude but opposite sign.

Of course there is nothing new under the sun, and our results were consistent with old published results, indeed, they are so consistent that they have been called the “law of final constant yield (Kira, Ogawa Shinozaki 1953 J. Inst. Polytech. Osaka Cy. Univ. D. 4, 1-16, cited in Harper 1977) .  In fact, people have observed constant yields in crop plants over several orders of magnitude of planting densities.  According to Harper the constant yield law is extremely common, if not nearly universal in plants.  What we found in Impatiens is that the constant yield law is soft selection, and what we found in the theoretical work (and the Impatiens) is that soft selection is multilevel selection.

What this is saying is that, at least for yield, multilevel selection is nearly universal in plants.  So, it appears that to answer to the question of how common is group selection in nature:  In plants it is nearly universal.




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