A phenotypic view of evolution Evolution in Structured Populations

Selection and Individuality

Apparently I have shifted to an every other week post. It is not that I am lazy, just that my life is a bit chaotic. A week ago Saturday my daughter got married, and this week I am flying back to Brazil. Somewhere in the haze last weeks post simply didn’t happen.


blame the lack of post last week on the haze of getting out of town for a year. Besides, how else am I going to work Jimi Hendrix into a post? (from http://markmywordssite.com/2011/05/05/purple-haze-all-in-my-brain/)

In any case, I felt that I left you last time with a rather unsatisfying answer: An individual is that which you define to be an individual. This week I want to argue that some designations of “individual” are better than others. To see this we need to continue to work with contextual analysis.

To start, lets imagine we assign fitness at the lowest possible level, that is the cell. Ten points if you can tell me why it is NOT the gene! Twenty points if you can tell me why the lowest level might be the organism! – hint: What is an orgamism?. In that case we are calling the cell the “individual”.


Selection at the level of the Gene (Simmons) makes no sense. (from http://musicrowgirl.com/tag/gene-simmons/)

First lets look at cancer. If we imagine that fitness is just the rate at which cells divide (and ignoring organismal mortality for the moment) then cancer cells have a higher fitness than “normal” cells because they divide more rapidly. Thus in contextual analysis we can imagine a case of pure cellular (individual) selection then:

sel and ind eq 1

Here is the kicker: If we assume that there is ONLY cellular selection and we do a bit of math I prefer not to show you then we quickly discover that:

sel and ind eq 2


sel and ind eq 3

what this tells us is that, because we assumed it to be that way, there is selection at the level of the Individual cell, but no selection at the level of the organism. Thus, in this simple case the qualitative result is that selection is acting at the cell level.

If we assign fitness at the level of the organism then we are de facto defining the organism to be an individual. Here we get a very different answer. In particular, since there is no assignment of fitness at the cell levelsel and ind eq 4 and sel and ind eq 5are undefined. Similarly we cannot do the partial regression sel and ind eq 6, and can only do the simple regression, sel and ind eq 7, which emphatically does not equal zero, and in fact had we measured it, sel and ind eq 8.

Whether we assign fitness at the level of the cell or the level of the organism, we still measure evolution by natural selection; however, what happens is that our interpretation of how selection is acting qualitatively changes as we move between levels.

Now, lets consider selection on a second trait. In this case it is something that only acts at the organismal level, nevertheless we will assign fitness at the level of the cell. Because, by assumption, selection is only acting at the whole organismal level

sel and ind eq 9

On the other hand

sel and ind eq 10

Again, because we considered only a situation with selection at the organismal level, contextual analysis says there is group (organismal) selection, but no individual (cellular) selection.

Now, if we were to assign fitness a the level of the organism instead of the cell we would discover that sel and ind eq 7 had not changed, and that we still detected exactly the same strength of selection at the organismal level as we did when we assigned fitness at the level of the cell. In other words, in this second case when we changed levels at which we assigned fitness we got no qualitative change in our interpretation of how selection was acting.

So, what does this have to do with individuality? I would defend the statement I made in the last post that the individual is at some level arbitrary, and it is the level at which you assign fitness, however I would argue that some choices are better than others. In particular, if we imagine that we assign fitness at the lowest conceivable level, for selection on most traits we would find some higher level at which “contextual” selection is acting. If we assign fitness at the level of the cell we would find that for a large number of traits there is a strong contextual component of fitness that is at the level of the organism. I would argue that for selection on any given trait the logical level at which to assign individuality is the level at which there is a strong contextual component to fitness.

Logically, how do we find this “natural” level of individuality? The easy way is simply to do the contextual analysis and find the level at which contextual selection is acting. A corollary to this is that when you move through that level at which contextual analysis is acting you will see a qualitative change in your interpretation of how selection is acting. In the example above when selection was acting at the cell level, assigning selection at a higher level qualitatively changed our interpretation. That is when we assigned fitness a the level of the organism what had formerly been interpreted as cellular level selection becomes organismal level selection. Importantly, when we had a situation in which selection was acting at the organismal level then moving up the level of individuality from the cell to the organism does not qualitatively alter our interpretation of how selection is acting, and we see that in this later case the cell is not the natural individual, whereas the organism is.

This actually has some important philosophical implications. First, it appears that selection in some sense defines individuality. Since selection acts on traits that means that these selection defined individuals will be trait dependent. Thus, the selection defined individual for cancer or differential cell proliferation will in most cases be the cell, whereas selection on running speed will most likely end with defining the individual as an organism. Now we ask the musical question of whether a bee colony is an individual. From a selection standpoint the answer is it depends. For the probability of a worker producing an egg we might decide the bee (the organism) is the individual, whereas for a trait such as the probability of the colony surviving the winter we may well decide that the colony is the individual.


Japanese honey bees mob and “cook” an Asian hornet. The Asian hornet hunts honey bees, however if the colony detects one they will mob it, surrounding the hornet and raising its body temperature until it dies. European honey bees do not have this behavior. For this trait the hive is the individual. (http://www.nbcnews.com/id/20823983/ns/technology_and_science-science/t/surprise-strategy-bees-smother-enemies/#.VCw2n-dfnzg)

The final question is why is multilevel selection a better term than say, “group selection”. The answer, of course, is that selection is acting at only one level then that level is the selection defined individual. In the simplistic world of selection acting at only one level there is only individual selection, but sometimes the “individual” will be a group. The only time it makes sense to talk about group selection in this view is when selection is acting at more than one level. In this case the lowest level at which selection is acting would be “individual selection”, and selection on contexts larger than the individual would be “group selection”.


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