Epigenetics and the evolution of ANIMAL INSTINCTS

We all realize that an animal’s mind is not born as an empty canvas: bottlenose dolphins are born alive and immediately know how to swim, monarch butterflies know how to leave the Mexican mountains after birth and migrate all the way to Canada during the summer following which they fly back to the Mexican mountains in autumn (taking four generations to accomplish this), and honey bees know how to dance without ever having learned these skills. In grade school we learn “well, this is just an instinct.” But, what REALLY is the mechanism in these animals’ brains?

Little is known about how animals acquire the instincts that enable such innate behavior. Instincts are widely held to be ancestral to learned behavior. Some have been elegantly analyzed at the cellular and molecular levels, but general principles do not exist. Based on recent research, authors [see attached article] argue instead that instincts evolve from learning and are therefore served by the same general principles that explain learning.

Consider individuals in an ancestral population that use behavioral plasticity to respond adaptively to their environment. If this adaptive response increases fitness and reproductive success, then natural selection should favor animals that manifest the trait earlier in development or with less practice. Selection (in the nervous system), acting to adjust the timing and extent of plasticity, can thus produce “an instinct”. The selective forces would depend on the environment. In certain environments, behavioral plasticity might be favored, but in other environments, more stereotyped behavior might prove superior. This process need not result in the programming of every single detail of an instinct; all that is required is an initial behavioral bias –– followed by a process of experience-dependent refinement –– and driven by predictable patterns of environmental reinforcement.

Authors propose herein that a unified model of behavior — one that employs the same cellular and molecular mechanisms to explain both instinct and learning — will lead to new lines of investigation. Consider the question of the formation of an instinct in light of what we know about the formation of a memory. Learning involves experience-dependent strengthening of specific synapses. If learning is defined by the notion that “neurons that fire together, wire together,”, then how do instincts get wired during development in the absence of experience-dependent neuronal firing? In the case of human speech and hearing, it is known that neural development is shaped by experience in utero. Could epigenetic mechanisms that regulate changes in gene expression related to long-term memories play similar roles during development to form instincts?

Science 7 Apr 2o17; 356: 26–27

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