Recognition of phenotypic 'traits' implies that the pheno-type is perceived as composed of somewhat discrete, or modular, unit structures, and behavior is no exception. Particular behaviors may be turned off and on, repeated, and expressed in different combinations within individuals, or in different individuals, populations, or related species. Ethologists have long recognized this in their analyses of behavior in terms of ethograms or 'displays' that are described in terms of sequences of behavioral unit traits. Modularity is a product of regulation by a particular off-on decision mechanism that marks the beginning of a developmental (e.g., neuromuscular response) pathway leading to a 'modular' trait. Adequate criteria of modularity are not always observed in discussions of 'behavioral modules' in evolutionary psychology and ethology.
The on-off switch mechanism that defines modular structure is the locus of environmental and genetic influence on the timing and degree of sensitivity of a behavioral response. For example, research on honeybees shows that the proboscis (tongue) extension reflex of worker bees is more readily stimulated by greater concentrations of sugar (sucrose) solutions. The threshold for this response is different in different genetic strains of honeybees - it is readily affected by artificial selection. It is also affected by the feeding and foraging experience of individual bees, age (accompanied by hormonal changes), time of year, and the quality of nectar being collected by nestmates. That is, the threshold for the nectar response of the bee is affected by both environmental and genetic factors. Particular chromosomal regions (QTLs, or quantitative trait loci) that affect this threshold have been localized and shown to be associated with gene products (neurochemicals such as octopamine and dopamine, and protein kinases) active in sensory pathways and affecting the response thresholds of the bees. In addition, identified genes affect hormonal networks, such as the insulin signaling cascade that modulates the regulation of juvenile hormone and vitel-logenin, which in turn influence the response thresholds of the bees. Control of this modular, plastic behavioral trait - the honeybee feeding response - is obviously polygenic (hormonal and sensory systems being genetically complex), one possible reason for its ready response to artificial selection. While this may seem an overly detailed mechanistic analysis for a seemingly small plastic modular trait, in fact the sucrose response threshold is believed to be a fundamental determinant of the division of tasks among the workers of a honeybee colony, for it influences the pollen and nectar foraging readiness, and the learning ability, of individual bees.
Modularity means that unit traits are potentially subject to deletion and duplication of expression, and to reorganization during development and evolution, because of the sensitivity of their switch-controlled development to both environmental and genetic change. This is illustrated by studies of courtship behavior in grasshoppers (Figure 2): modularity bestows plasticity on the sequence in which behaviors are performed, which can be adjusted by individuals in response to conditions. This same modularity, which allows dissociability within individuals, renders behavioral subunits subject to rearrangement during evolution (Figure 3).
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