Chapter 3

The Developmental and Genetic Bases of Behavior

1. The development of any trait, including a behavioral trait, is the result of an interaction between the genotype of a developing organism and its environment, which consists of not only the food it receives and the metabolic products produced by its cells but also its sensory experiences. Genes can respond to signals from both the social and ecological environments by altering their activity, leading to changes in the gene products available to the developing organism.

2. Despite the nature–nurture misconception (the idea that traits are influenced by genes or the environment), no measurable product of development (a phenotype) can be genetically determined. The statement “in garter snakes there is a gene for eating banana slugs” really means “a particular allele in a garter snake’s genotype codes for a distinctive protein; if the protein is actually made, which requires an interaction between the gene and its environment, the protein may influence the development or operation of specific physiological mechanisms underlying the snake’s ability to recognize slugs as food.”

3. By the same token, the interactive nature of development means that no phenotype can be purely environmentally determined. The statement “a sparrow’s dialect is environmentally determined” really means “acoustic experiences early in the sparrow’s life led to chemical changes in the bird’s brain, which altered the pattern of genetic activity in some parts of its brain. These changes set in motion subsequent genetic and neural changes in the physiological systems that an adult sparrow uses when singing a version of its species’ song.”

4. Because development is interactive, changes in either the genetic information or the environmental inputs available to an individual can potentially alter the course of its development by changing the interplay between genes and environment within that individual. Therefore, the behavioral differences between two individuals can be largely genetically determined or environmentally determined, or both.

5. The genetic mechanisms that affect development can be analyzed evolutionarily. On the one hand, these mechanisms have an evolutionary history. Thanks to the discipline of evo-devo, we now know of many examples of genes that have been retained in modern species from a distant ancestor. These ancient genes often have regulatory effects on the development of important traits. Modest modifications in these genes, or in the target genes whose expression they affect, can result over time in the evolution of major developmental differences, even among closely related species.

6. Because some differences between individuals are hereditary, populations have the potential to evolve by natural selection. The adaptationist level of analysis, which deals with the possible adaptive value of a trait, has revealed that behavioral development has adaptive features, such as developmental homeostasis, the capacity of the developmental process to ignore or overcome certain environmental or genetic shortfalls that might prevent animals from acquiring fitness-enhancing traits.

7. Likewise, developmental switch mechanisms are adaptive because they control alternative developmental pathways leading to alternative phenotypes. Each of the different traits helps individuals succeed within a particular part of the larger environment. Learning mechanisms provide another form of developmental flexibility, enabling individuals to use their experiences to make adaptive adjustments in behavior that help individuals cope with a variable environment.