By whatever parameters phenotypic plasticity is described, whether in terms of reaction norms or other measures based on cross-generational or populational phenotypic variation, phenotypic plasticity is a result of 'developmental' plasticity - environmental sensitivity of formative mechanisms, including the neuromuscular mechanisms that produce behavior, during the ontogeny of a trait within individuals. Studies of phenotypic plasticity in quantitative and population genetics are seldom concerned with the mechanisms that underlie the environmental sensitivity of individuals nor with the mechanism-related genetic architecture of plasticity itself. But some early leaders in the study of phenotypic plasticity, such as Scharloo, recognized that it is not sufficient simply to use statistical methods to describe and analyze variation and to reveal selection, but that research must also examine the physiological and developmental background of quantitative variation in phenotypic traits. For behavioral traits, this background may include the development of morphological features responsible for movements, and the hormonal and neural mechanisms that sense environmental conditions, regulate movements, and coordinate the performance of complex behaviors. The term 'developmental plasticity' invites attention to the developmental nature of individual behaviors - including patterns of gene expression -to complement the quantitative and population genetic studies usually associated with the term 'phenotypic plasticity'.
Developmental plasticity includes responsiveness throughout development, from the time an egg begins to undergo phenotypic change, through maturity and old age, until responsiveness ends with death. The scope of developmental plasticity includes responses to factors in the environments within cells. Intracellular environments that affect behavior can include gene products and biochemical changes associated with neurohormonal processes, processes that may be affected by interactions among cells, as well as by environments outside the organism. Effects of the external environment may involve transmission via neural or hormonal pathways to affect gene expression or gene-product activation associated with particular behaviors. The behavioral response of a female to an environmental factor such as the stimuli produced by a courting male, for example, may involve change in hormonal physiology or responses of muscle cells in the female reproductive tract that promote the fertilization success of the male. That is, variables in the external environment stimulate changes in the internal environment that illuminate the nature and functions of particular behaviors whose significance would otherwise be incompletely understood.
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