In the 1990s, William Muir, an agricultural researcher, set out to develop more efficient egg-laying hens. His initial approach involved picking the highest-yielding hen from each cage to generate the next lineage. However, after numerous breeding cycles, this technique led to a strain of exceedingly aggressive hens. These hens excelled in productivity by dominating and suppressing their fellow cage mates, which ultimately resulted in a decline in overall egg output at the cage level.
Muir then pursued an alternative method, opting to select the most productive cages in their entirety and breeding all hens from those cages. This led to a remarkable productivity surge—by 160%—in just five generations, with the hens also becoming more docile and cooperative.
This experiment marks a pivotal point in the discussion surrounding evolutionary biology. Conventional perspectives on natural selection emphasize competition at the individual level, suggesting that traits proliferate because they enable individual organisms to surpass their rivals. Nevertheless, Muir’s findings revealed that selection can function at the group level as well. By focusing on entire cages instead of just standout performers, productivity, behavior, temperament, and even the social interactions within the flock were impacted.
The landscape of evolutionary biology and the notion of multilevel selection is evolving. A recent bibliometric review published in Frontiers in Ecology and Evolution examined almost 3,000 scientific papers and identified 280 studies that support multilevel selection. This theory suggests that natural selection occurs concurrently across various levels of biological organization.
Anne Clark from Binghamton University, a co-author of the review, remarked that while tracking trait frequency increases due to natural selection isn’t incorrect, grasping the mechanistic differences necessitates a look at various levels. The significance of group competition or cooperation may differ across diverse contexts.
Historically, the concept of group selection encountered resistance from prominent evolutionary theorists who viewed it as an outdated notion eclipsed by individual-level selection. Detractors contended that self-serving individuals in groups would consistently outpace altruists, negating any advantages at the group level.
The latest research disputes this limited viewpoint, showcasing the intricacy of selection mechanisms. For example, cancer, often perceived as a failure of individual-level selection, consists of intricate interactions within and among tumors and diseases. Likewise, the swift replication and dissemination of pathogens may benefit from variable selection levels based on population context.
Michael Wade conducted pioneering experimental research on group selection using flour beetles in the 1970s. By selecting for group size, he noted swift divergence in group traits within just a handful of generations, all without necessitating individual altruism.
Recent studies with yeast also illustrated the emergence of macroscopic multicellularity through artificial group-level selection pressures. This occurrence challenges traditional views on evolutionary transitions and underscores the potential of group selection.
These revelations bear practical implications for agriculture and beyond. Group selection has become a common practice in breeding programs, exemplified by the development of favorable root traits in wheat across China. Muir’s chicken experiment was not an isolated incident.
Clark perceives broader ramifications for human structures such as classrooms and workplaces. She proposes that systems focusing exclusively on individual incentives may overlook the advantages of group dynamics. Emphasizing empirical evidence and data, rather than engaging in debates about selection units, is recommended as a constructive path forward for evolutionary biology. The broader acceptance of multilevel selection may hinge on the perspectives held by various factions within the field.