The evolution of cooperation-costly behavior that benefits others-faces one clear obstacle. Namely, cooperators are always at a competitive disadvantage relative to defectors, individuals that reap the benefits, but evade the cost of cooperation. One solution to this problem involves genetic hitchhiking, where the allele encoding cooperation becomes linked to a beneficial mutation, allowing cooperation to rise in abundance. Here, we explore hitchhiking in the context of adaptation to a stressful environment by cooperators and defectors with spatially limited dispersal. Under such conditions, clustered cooperators reach higher local densities, thereby experiencing more mutational opportunities than defectors. Thus, the allele encoding cooperation has a greater probability of hitchhiking with alleles conferring stress adaptation. We label this probabilistic enhancement the "Hankshaw effect" after the character Sissy Hankshaw, whose anomalously large thumbs made her a singularly effective hitchhiker. Using an agent-based model, we reveal a broad set of conditions that allow the evolution of cooperation through this effect. Additionally, we show that spite, a costly behavior that harms others, can evolve by the Hankshaw effect. While in an unchanging environment these costly social behaviors have transient success, in a dynamic environment, cooperation and spite can persist indefinitely.

The evolution of cooperation-costly behavior that benefits others-faces one clear obstacle. Namely, cooperators are always at a competitive disadvantage relative to defectors, individuals that reap the same social benefits, but evade the personal cost. One solution to this problem involves genetic hitchhiking, where the allele encoding cooperative behavior becomes linked to a beneficial mutation. While traditionally seen as a passive process driven purely by chance, here we explore a more active form of hitchhiking. Specifically, we model hitchhiking in the context of adaptation to a stressful environment by cooperators and defectors with spatially limited dispersal. Under such conditions, clustered cooperators reach higher local densities, thereby experiencing more opportunities for mutations than defectors. Thus, the allele encoding cooperation has a greater probability of hitchhiking with alleles conferring stress adaptation. We label this probabilistic enhancement the "Hankshaw effect" after the character Sissy Hankshaw, whose anomalously large thumbs made her a singularly effective hitchhiker. Using an agent-based model, we demonstrate that there exists a broad set of conditions allowing the evolution of cooperation through the Hankshaw effect. We discuss the feasibility of our theoretical assumptions for natural systems, not only for the case of cooperation, but also for other costly social behaviors such as spite. The primary elements of our model, including genetic hitchhiking and population structure, have been discussed separately in previous models exploring the evolution of cooperation. However, the combination of these elements has not been appreciated as a solution to the problem of cooperation.

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Researchers have called for undergraduate courses to update teaching frameworks based on the Modern Synthesis with insights from molecular biology, by stressing the molecular underpinnings of variation and adaptation. To support this goal, we developed a modified version of the widely used Assessing Conceptual Reasoning of Natural Selection (ACORNS) instrument. The expanded tool, called the E-ACORNS, is explicitly designed to test student understanding of the connections among genotypes, phenotypes, and fitness. E-ACORNS comprises a slight modification to the ACORNS open-response prompts and a new scoring rubric. The rubric is based on five core concepts in evolution by natural selection, with each concept broken into elements at the novice, intermediate, and expert-level understanding. Initial tests of the E-ACORNS showed that (1) upper-level undergraduates can score responses reliably and quickly, and (2) students who were just starting an introductory biology series for majors do not yet grasp the molecular basis of phenotypic variation and its connection to fitness.

By benefitting others at a cost to themselves, cooperators face an ever present threat from defectors-individuals that avail themselves of the cooperative benefit without contributing. A longstanding challenge to evolutionary biology is to understand the mechanisms that support the many instances of cooperation that nevertheless exist. Hammarlund et al. recently demonstrated that cooperation can persist by hitchhiking along with beneficial non-social adaptations. Importantly, cooperators play an active role in this process. In spatially-structured environments, clustered cooperator populations reach greater densities, which creates more mutational opportunities to gain beneficial non-social adaptations. Cooperation rises in abundance by association with these adaptations. However, once adaptive opportunities have been exhausted, the ride abruptly ends as cooperators are displaced by adapted defectors. Using an agent-based model, we demonstrate that the selective feedback that is created as populations construct their local niches can maintain cooperation indefinitely. This cooperator success depends specifically on negative niche construction, which acts as a perpetual source of adaptive opportunities. As populations adapt, they alter their environment in ways that reveal additional opportunities for adaptation. Despite being independent of niche construction in our model, cooperation feeds this cycle. By reaching larger densities, populations of cooperators are better able to adapt to changes in their constructed niche and successfully respond to the constant threat posed by defectors. We relate these findings to previous studies from the niche construction literature and discuss how this model could be extended to provide a greater understanding of how cooperation evolves in the complex environments in which it is found.