Social Insects and Evolutionary Convergence
Thinking about nonhuman cultures has mostly concentrated in studies of primates. Though work on cetaceans is changing this, the fundamental basis for these considerations developed from our close “cousins.” Chimpanzees, in particular, prompted efforts at rethinking the singular association of culture with humans because of their manifold forms of tool use. This line of reflection was further fueled by the recognition that common forms of culture across these species lines must reflect it being evolutionarily conserved among primates. A far more radical approach involves social insects, where the common capacity for eusociality—cooperative care, overlapping generations, and divisions of labor in colonies or nests—is contrastingly the result of evolutionary convergence. Wasps, bees, and ants all evolved social behavior independently. These remarkable convergences are highlighted in a recent Annual Review of Entomology article, “Molecular Evolution of Insect: Sociality: An Eco-Evo-Devo Perspective,” by Amy Toth and Sandra M. Rehan. As they summarize, “Eusociality evolved once within the ants, once within the Crabronidae (and sociality is rare and not well developed in this group), once or twice within the Vespidae, and four times in the bees [twice in the family Apidae and twice in the family Halictidae].” There is much a tap in this article, particularly the fusion of multiple levels of analysis entailed by the “eco-evo-devo perspective” they articulate. But initially I find it useful to consider how these varying temporal processes of convergence prompt them to define sociality in common across these families.
Toth and Rehan discuss four levels of social complexity—subsocial, incipiently social, primitively social, and advanced eusocial—delineated to contextualize and compare evolutionary transitions highlight convergent features shared across social lineages. “However,” they observe, “each lineage has specific characteristics for many social traits, and therefore behavioral phenotypes differ in some important ways across lineages.” Because these result from different evolutionary trajectories, “only through broader comparative studies can we truly understand whether there are common patterns across independent origins of sociality.” Such comparative undertaking hinge upon defining and characterizing sociality such that its contours and limits are clear across insect taxon. These efforts, in turn, may provide a basis for defamiliarizing the concept of sociality as developed regarding mammals.
The evolutionary perspective renders up sociality in great relief. It entails a shift from solitary, laboring creatures—each conducting elements of nest construction, foraging, and egg-laying tasks individually—to ones that approach these activities collectively. This profound transition results in colony-level selection, which acts to produce novel social traits. This is a complicated matter: “Different selective forces and molecular mechanisms may be involved during different stages of social evolution.” The effort to tease these apart involve ongoing research and synthetic conceptual formulations, as reviewed and rendered by Toth and Rehan. But it also entails thinking about what counts as social, not just what constitutes sociality. How we recognize it, in this perspective, depends on calibrating and synthesizing distinct levels of analysis.
As we think through the interactions of genetics and environments, how do we know what we’re seeing is the emergence of the social? This uncertainty is glimpsed in the matter of genomic imprinting: “In social interactions between two full-sister hymenopterans, patrigenes should promote altruistic behavior (favoring passing on of the same patrigenes), whereas matrigenes should promote selfishness. The prediction is reversed in social systems with multiple paternity where sisters have different fathers, in which caste patrigenes should favor selfishness.” And it’s further evident the effort to discern environmental drivers of gene expression and genomic evolution in social insects.
Basically, their model assumes that multiple factors are working on the molecular level: “Social insects are affected by other individuals of the same species in the colony (social environment) and myriad additional factors, including symbionts, parasites, pathogens, predators, competitors, and abiotic effects (external environment). These factors can feed back on gene expression patterns and, via indirect genetic effects, also impact DNA sequence evolution.” Recent work on epigenetics in humans suggest a similar model is relevant for us and other primates, but there remains intense anxiety among social scientists about linking genetics and sociality. Regardless, the reconceptualizations of communication and intelligence involved with assessing social insect, will certainly prompt a broader reassessment of sociality. The auditory, chemical, and visual channels social insects use to exchange and generate information may be, in some regard, far more complex than our own; while the dispersed intelligence of the “social brain”—facilitated and necessitated through overlapping interactions in densely nests—may also revise what we regard as thinking. There’s more here to mull. As they conclude, “Looking to the future, an appreciation of both the diversity of genetic mechanisms and the diversity of social forms is necessary to build a more comprehensive understanding of the molecular evolution of sociality.” Humans’ version of the social in this wide ‘universe’ of sociality may look quite different as a result.
Amy L. Toth and Sandra M. Rehan, “Molecular Evolution of Insect Sociality: An Eco-Evo-Devo Perspective,” Annual Review of Entomology 62 (January 2017): 419–42, https://doi.org/10.1146/annurev-ento-031616-035601.
The evolution of eusociality is a perennial issue in evolutionary biology, and genomic advances have fueled steadily growing interest in the genetic changes underlying social evolution. Along with a recent flurry of research on comparative and evolutionary genomics in different eusocial insect groups (bees, ants, wasps, and termites), several mechanistic explanations have emerged to describe the molecular evolution of eusociality from solitary behavior. These include solitary physiological ground plans, genetic toolkits of deeply conserved genes, evolutionary changes in protein-coding genes, cis regulation, and the structure of gene networks, epigenetics, and novel genes. Despite this proliferation of ideas, there has been little synthesis, even though these ideas are not mutually exclusive and may in fact be complementary. We review available data on molecular evolution of insect sociality and highlight key biotic and abiotic factors influencing social insect genomes. We then suggest both phylogenetic and ecological evolutionary developmental biology (eco-evo-devo) perspectives for a more synthetic view of molecular evolution in insect societies.