Evolution of Brain Structures and Adaptive Behaviors in Humans and Other Animals: Role of Polymorphic Genetic Variations

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Abstract
One important goal of neuroscientists is to eventually understand complex behavioral adaptations of humans and other animals as explained by the structure and function of their brain tissues and principles of evolutionary biology. To this end, the dominant model in evolutionary psychology assumes that complex adaptations involving brain or any other tissue are precisely crafted for a specific function and are produced by genes whose alleles all have equal fitness (genetic homomorphism) because sexual recombination would disrupt adaptations produced by genes whose alleles have unequal fitness (genetic polymorphism). This genetically zoomorphic model maintains that humans and other animals have evolved universal neuronal circuits and behavioral adaptations, which in their sum constitute a species-typical nature (a universal human nature for Homo sapiens), and that almost all variation in adaptive behavior among same-sex individuals is due to environmental effects. Alternatively, we assert that a review of currently available data in neuroscience, biology, and psychology strongly suggests that complex adaptations involving brain and other tissues have many imperfections, different functions in different environments, and much polymorphism genetic variation. Our genetically polymorphic model accounts for all these data and predicts that humans and other animal species have evolved many nonuniversal complex behaviors (multiple species-typical behaviors or “human natures”) that differ genetically between males and females, as well as within males and within females. Variability in same-sex behavior is therefore due to both environmental and genetic variation. Our genetically polymorphic model reconciles fundamental assumptions of evolutionary psychology with basic principles of evolutionary biology, behavioral genetics, and neuroscience.