Another pure essay post. I was surprised that last weeks post didn’t generate any controversy. I guess that that proves that the only people who read my posts are people who agree with me. Sigh. As the song goes “I’d love to change the world, but I don’t know what to do, so I’ll leave it up to you”
This week I want talk about indirect genetic effects in comparison to contextual traits, something about which I have not been particularly clear. In general it can be dismissed in two sentences. Individual and contextual traits are part of the phenotypic compartment, and indirect genetic effects are part of the inheritance compartment. As such they are independent concepts.
Whether a trait is an “individual trait” or a “contextual trait” depends entirely on what it is measured. Thus, if it is a characteristic of an individual (height, weight, sprint speed) it is an individual trait. If it is a characteristic of the group, neighborhood or other aspect of the context that an organism finds itself in, then it is a contextual trait. One point here is that one of the whole points of contextual analysis is that we are treating “as if” they were traits of the individual, so perhaps from a rather odd perspective there really is no difference between individual and contextual traits.
Indirect effects on the other hand occur when genes in one individual affect the expression of a trait in another individual. This is an idea that has been around for a long time, certainly it is an underlying theme in Griffing’s work (e.g., Griffing 1977 Selection for populations of interacting genotypes. In: Proceedings of the International Congress on Quantitative Genetics, August 16-21, 1976. E. Pollak, Kempthorne O, andBailey TB (eds.) Iowa State University Press, Ames Iowa., and references there-in), however the modern development of the idea, and the term “indirect genetic effects” can be traced to (Moore, Brodie, and Wolf 1997 Evolution 51, 1352-62.). Indirect effects will almost certainly affect contextual traits, but in many circumstances they will also affect individual traits. And that is the point of this essay: individual traits can be influenced by both the genetics of the individual and the genetics of other individuals with whom they interact. Similarly, contextual traits can be influenced by the genetics of the focal individual, and by the genetics of other individuals with whom they interact.
Thus just because a trait is clearly measured on the individual and correctly called an “individual” trait, does not mean that the genes reside in the individual expressing the trait. A really good example is Griffing’s study of biomass in Arabidopsis. If you recall, in this study Griffing grew pairs of plants together in sterile agar, and measured dry weight of the plants after they were harvested Clearly, biomass is a trait measured on an individual, and must be considered an individual trait. Just as clearly in his study the trait biomass was determined both by the “direct effects”, that is the effects of in individuals genes on itself, and indirect genetic effects, the effects of its interacting partner on its phenotype.
At this point I am basically done with the issue I wanted to raise today, but it is worth discussing this point a bit more. Just as with contextual traits, the realized heritability of individual traits will potentially depend both on the mating structure of the population and on the interaction structure. Thus, even apparently pure individual traits can have there heritabilities change when the interaction structure changes.
Nobody has ever done a detailed manipulative study of the effects of interaction structure on the heritability of individual traits. This is too bad, because it potentially has some profound implications. I will give you one: One of the truisms of evolutionary theory is that you can get a response selecting on just about anything. However, In my thesis I worked with Arabidopsis selecting on leaf area (Goodnight 1985 Evol. 39, 545-58). In this study I actually got a negative response to individual selection, a result that was predicted by Griffing (1977 In: Proceedings of the International Congress on Quantitative Genetics, August 16-21, 1976. E. Pollak, Kempthorne O, andBailey TB (eds.)). Further, the one apparent exception to the idea that you can select on anything is competitive ability. There have been a lot of experimental studies of the evolution of competitive ability that have failed to get a response (e.g., Futuyma 1970 American Naturalist 104, 239-52.). Perhaps now we can put that old saw that you can select on anything into a new light. Perhaps you can select on anything when you put the organisms in an environment where competitive interactions among individuals are minimized. Apparently in both my study and Futuyma’s study the indirect genetic effects outweighed the direct genetic effects and prevented a response to selection from occurring.