How many instincts do people have
Evolutionary Biology: Are Instincts Learned?
An increased plasticity, i.e. flexibility of their behavior, is initially a survival advantage in the two examples mentioned. However, if the environmental conditions remain stable, a decrease in plasticity can also offer an advantage. The associated behavior is genetically cemented more and more. The result is stereotypical - in the classic sense instinctive - behavior. How exactly such a genetic assimilation of a previously variable property takes place is, however, still a matter of dispute.
Is the development of instincts influenced by epigenetics?
The key to this could be in epigenetics, say Robinson and Barron. Epigenetic processes have a very direct influence on genes: for example, they prevent the cell from reading certain DNA segments or, on the contrary, enable a cell to produce an important building block. This is done, among other things, by attaching small molecules, the methyl groups. However, epigenetic processes also affect learning. Nerve cells regulate, for example, how willing they are to make new connections. The "sense organs" of the nerve cells, their receptors, are also subject to epigenetic control. According to the two scientists, these processes could play a decisive role in the genetic assimilation of learned behavior and thus the formation of instincts.
A particularly impressive example of how epigenetics affects behavior was provided in 2013 by a research team led by Brian Dias and Kerry Ressler from the Emory University School of Medicine in Atlanta. They had male mice sniff acetophenone, a chemical that smelled of almonds, and shortly thereafter gave their paws a weak electric shock. After three days, the mice had learned to associate the odor with the electric shock and froze in fear when they smelled the almond odor. The males were then mated with unconditioned females. And surprisingly, the majority of the offspring also reacted anxiously to the scent. The effect was even detectable in the following generation of mice, so it must have been inherited. The researchers also succeeded in identifying a physical characteristic for the altered reaction: the mice had altered anatomical structures of acetone-sensitive nerve cells.
What is largely unexplored is how such an epigenetic inheritance could take place. Researchers suspect small RNA molecules that have migrated from the brain cells to the germ line cells of the conditioned fathers. On board sperm, they get into the egg cell and thus into the developing offspring. Only after fertilization, all epigenetic appendages on the DNA are removed and only added again in the course of the differentiation of the individual cells. Whether and how the epigenetic information from the sperm can survive by then can currently only be speculated.
In addition, epigenetic effects disappear after three generations, at least in experiments with mammals. Epigenetics seems, if at all, to be more of a "soft" method of gene adaptation that enables a very flexible adaptation to the environment - in other words, the opposite of classical instinct. This can make sense in evolutionary terms. Just like for the grandchildren of the mice conditioned on acetophenone. For them there were no more electrical stimuli, the smell of almonds was no longer associated with danger. And the sensitivity to this substance was lost again.
If epigenetics is to bring about permanent changes in the genome, the environmental conditions would have to remain stable over generations. Given this prerequisite, it would then be expected that epigenetic changes originally acquired through learning would gradually lead to a selection of a certain gene combination. Genetic assimilation would take place on the principle of multiple repetitions of adaptation, epigenetic fixation and finally genetic fixation, as suggested by Gerd Müller and Stuart Newman in 2005 in the Journal of Experimental Zoology.
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