Group phenotypic composition: implications for individuals and their groups

Trends in Ecology & Evolution “From Individuals to Groups and Back: The Evolutionary Implications of Group Phenotypic Composition”

Man, I wish that this article was written in a more accessible manner, because what it presents is important. There is a critical feedback between how the composition of groups affects individuals and how individuals affect the composition of their groups; this article tries to make conceptual sense of this feedback and the complex interactions it implies.

The key to understanding this article (well, as much as I could) is understanding the equations presented in Box 1. A major contention of these authors is that group phenotypic composition (GPC) can influence and be influenced by the evolutionary process in a variety of ways; the equations in Box 1 are designed to break down these different potential influences such that researchers might design studies to measure how GPC impacts the evolution of social species.

I will try to put these four equations into very clear terms. Keep in mind that I am struggling (perhaps alongside you?) to make full sense out of all these terms, so make sure to check that what I am suggesting seems consistent with what the article claims:

Equation I:
As I interpret this equation, it is just another way of looking at the concepts that underly the Price Equation: we want to understand not just the fitness effects of an individual in a group (β_z) but also the fitness effects of being in that group (β_y). What the article reinforces is that neither of these measures is valuable without also understanding how changes in GPC affect individual fitness (β_yz). In other words, the simplistic approach of considering an individual’s fitness in the group and the overall group’s fitness ignores an important interaction that is captured by considering how changes in GPC alter individual fitness.

Equation II:
As far as I can tell, this is just a check on variation: if there is not sufficient variation in population of individuals (VAR_z) or between groups of individuals (COV_zy) then selection at these levels (individual = β_z, group = β_y) cannot “act”.

Equation III:
Things get more complex because the GPC can impact how individuals express their phenotype (Ψ_zy) and individuals can impact GPC (Ψ_yz). These equations capture these effects on phenotypic expression, which are not captured in Equation I.

Equation IV:
These are kind of the “take home equations” that combine all of the ideas explored in Equations I-III. Response to selection at both the individual and group levels depends on these two interactions (GPC effects on individuals via Ψ_zy, individual effects on GPC via Ψ_yz) so long as there is a genetic covariance between the phenotypes of individuals and their GPC (G_zy).

Again, I make no guarantees that I have broken down these equations correctly. But I am trying to state the importance of each equation in as simple terms as I can, because that’s the only way I can make meaning out of what these authors are saying. Please comment below if I got anything wrong, or even wrong-ish.

Articles like this are just begging for better graphics. I understand that much of the technical literature assumes a high degree of background, but when you write a review article the goal ought to be to reach beyond one’s particular sub-sub-field to inspire those in other fields to consider what’s been discovered in your own. I think this article wants to get more empirically-focused behavioral ecologists (and others) to pursue experiments that explicitly consider GPC effects, but the way it is written limits the potential for reaching that goal.

Figure 1 (reproduced above) attempts to capture some important ideas of the article, but most of what it presents is easy enough to understand in verbal terms. What’s harder to interpret are the many interactions and effects found in the four equations these authors present; understanding these interactions and effects is the key to understanding the major ideas of this article. It would have been really valuable to have additional images that depicted the meaning of these different model terms!