Waddington's (1942) notion of canalization has been widely invoked in developmental psychology to conceptualize species-typical regularities in behavioral development as genetically determined. In contrast, a developmental systems view, such as the one described in the present article, sees the genes as only one component in a hierarchy of influences, all of which contribute to canalize behavioral development. A key issue is that genetic activity does not by itself produce finished traits; differentiation occurs as a consequence of events above as well as below the cellular level, necessarily involving factors in addition to genetic influences to canalize behavioral development. In exploring the possible experiential canalization of development, it was found that the mallard duck embryo's contact call plays a canalizing role in species-specific perceptual development (Gottlieb, 1991). Thus, normally occurring experience, in concert with genetic and other activities, can canalize behavioral development.
The notion that phenotypic traits, including behavior, can be predetermined has slowly given way in biology and psychology over the last two decades. This shift in thinking is due in large part to the growing evidence for the fundamental role of developmental processes in the generation of the stability and variations in phenotype that researchers in developmental and evolutionary sciences seek to understand. Here I review the tenets of a metatheoretical model of development called probabilistic epigenesis (PE) and explore its implications for furthering our understanding of developmental and evolutionary processes. The PE framework emphasizes the reciprocity of influences within and between levels of an organism's developmental manifold (genetic activity, neural activity, behavior, and the physical, social, and cultural influences of the external environment) and the ubiquity of gene-environment interaction in the realization of all phenotypes.
The central dogma of molecular biology holds that "information" flows from the genes to the structure of the proteins that the genes bring about through the formula DNA-->RNA-->Protein. In this view, a set of master genes activates the DNA necessary to produce the appropriate proteins that the organism needs during development. In contrast to this view, probabilistic epigenesis holds that necessarily there are signals from the internal and external environment that activate DNA to produce the appropriate proteins. To support this view, a substantial body of evidence is reviewed showing that external environmental influences on gene activation are normally occurring events in a large variety of organisms, including humans. This demonstrates how genes and environments work together to produce functional organisms, thus extending the author's model of probabilistic epigenesis.
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