Publishers of the Journal of Heredity
Join the AGA

The Interactant Covariance: Who interacts with whom?

**This post is a part of the series on the 2020 AGA Presidential Symposium – Genes as Environment: Indirect Genetic Effects on Evolution, Agriculture, & Medicine**



About the Blog Author: The following is a brief commentary on Brodie et al. (2021) – Phenotypic Assortment Changes The Landscape Of Selection by University of Virginia graduate student Sarah McPeek. Sarah’s research in Butch Brodie’s lab focuses on IGE in the context of plant-insect interactions. She loves crawling under logs and searching for forked fungus beetles with the Beetle Crew. Follow Sarah’s work @sarahjmcpeek and read more science writing at





The Brodie Lab Beetle Crew spends a lot of time watching beetles on logs in the woods. For over a decade, the Crew has investigated patterns of social interactions in a wild metapopulation of forked fungus beetles (Bolitotherus cornutus) at Mountain Lake Biological Station in southern Appalachia. Each log subpopulation gets a name and each beetle receives a unique fluorescent ID label so the Crew can record patterns of interactions across multiple years of beetles’ adult lives. The crew is especially interested in tracking who interacts with whom: who stays close to who on fungal shelves? Who courts and mates with who? Who challenges who over mating opportunities? Do individuals interact with others who are like them, or do they prefer individuals who are unlike them?

Figure 1 Wild forked fungus beetles interacting on a log in the woods around Mountain Lake Biological Station, Giles County VA.

This latter question refers to phenotypic assortment or Cij’, the covariance between a focal individual’s phenotype and the phenotypes of its social partners (Wolf et al. 1999, McGlothlin et al. 2010). While this interactant covariance does not necessarily translate into either indirect genetic effects (IGE) or social selection (βs), Brodie et al. (2021) explain how Cij’ has profound implications for each. First, phenotypic assortment will modulate the strength and consistency of IGE experienced by individuals in groups. Second, and most interesting to Brodie et al. (2021), phenotypic assortment mediates the strength, targets, and direction of social selection. To use body size as an example, larger individuals may outcompete smaller individuals for resources and mating opportunities. Therefore, we might expect to see selection against smaller body sizes in large-bodied groups. If Cij’ is strongly negative, meaning larger individuals preferentially associate with smaller individuals, Cij’ can strengthen and reinforce the direction of social selection (e.g., Formica et al. 2011). Alternatively, if smaller individuals associate with other smaller individuals, or if there is no phenotypic assortment by size, then Cij’ can weaken or even erase the social selection gradient βs (e.g., Santostefano et al. 2020).

Figure 2 Inside the Beetlearies, the experimental cages that enable the Brodie lab to manipulate the beetles’ social and physical environment and measure effects on phenotypic assortment and social selection.

Whether Cij’ impacts phenotypic evolution hinges on whether individuals in groups assort consistently across space and time. In their paper, Brodie et al. (2021) present multiple years of observational field studies of forked fungus beetles, documenting log-level consistency of Cij’ for male body size and horn size in the wild. While log subpopulations remain consistent for multiple years, Brodie et al. (2021) also found that Cij’ varies widely among logs, which could lead to different patterns of social selection among subpopulations. The Crew also creates experimental populations where they manipulate properties of the beetles’ environment and measure effects on Cij’. These experiments show that an individual’s physical and social environment affect patterns of Cij’. Specifically, Brodie et al. (2021) find that phenotypic assortment on male horn size shifts across environmental contexts. Hence, changes in local ecology can also affect Cij’, which will in turn affect the strength and consistency of social selection a population experiences through time.

The Beetle Crew’s work on forked fungus beetles presents an intriguing picture of how Cij’ may accelerate, forestall, or subdivide patterns of phenotypic evolution in social populations. Researchers of social trait evolution resolve a more complete picture of the realized force and scale of social selection when they also measure Cij’. Exploring patterns of Cij’ through time may further explain why some populations respond rapidly to social selection while others fail to respond at all. Overall, measuring Cij’ may be key to translating a largely theoretical trait-based approach to IGE and social selection into a predictive empirical science of social trait evolution.



Brodie, E. D., Cook, P. A., Costello, R. A., & Formica, V. A. (2021). Phenotypic Assortment Changes the Landscape of Selection. Journal of Heredity. 113: 91-101.

Formica, V. A., McGlothlin, J. W., Wood, C. W., Augat, M. E., Butterfield, R. E., Barnard, M. E., & Brodie III, E. D. (2011). Phenotypic assortment mediates the effect of social selection in a wild beetle population. Evolution: International Journal of Organic Evolution, 65(10), 2771-2781.

McGlothlin, J. W., Moore, A. J., Wolf, J. B., & Brodie III, E. D. (2010). Interacting phenotypes and the evolutionary process. III. Social evolution. Evolution: International Journal of Organic Evolution, 64(9), 2558-2574.

Santostefano, F., Garant, D., Bergeron, P., Montiglio, P. O., & Reale, D. (2020). Social selection acts on behavior and body mass but does not contribute to the total selection differential in eastern chipmunks. Evolution, 74(1), 89-102.

Wolf, J. B., Brodie III, E. D., & Moore, A. J. (1999). Interacting phenotypes and the evolutionary process. II. Selection resulting from social interactions. The American Naturalist, 153(3), 254-266.

Subscribe to Our Blog