UC Davis

This chapter focuses on adjustability of organism/environment interactions in the phylogenetic and immediate time frames. Learning is described as adjustment of instinct in the immediate time frame, with natural selection in the phylogenetic time frame setting the specifications for neuronal plasticity that restrains learning ability. An example of slow phylogenetic adjustment of behavior is described in which California ground squirrel recognition of their two snake predators, the rattlesnake and gopher snake, can persist under relaxed selection spanning thousands of generations. At the neuronal level of organization in African jewel fish and European honeybees, an experiential model of neuronal adjustment is presented in which longer, more adjustable dendritic spines can theoretically exhibit the electrotonic properties of shorter spines in transferring ionic conductances from spine heads to parent dendrites. Short spines are abundant on newly formed pyriform interneurons in jewel fish optic tectum at an age in which instinct predominates. In adult fish, the longer dendritic spines tend to undergo morphological changes with social experience. Similarly in honeybees, the shorter dendritic spines on Kenyon cells in the calyces of the corpora pedunculata undergo prominent morphological changes during the innately mediated nursing and hive-maintenance stage of behavioral development. In contrast, the longer spines undergo morphological changes with the greater plasticity demands of initial flight and complex foraging activity. The most important implication of these studies is that learning and instinct might be mediated by the differential adjustability of the same neuronal processes.