I'm vaguely aware of the vast new field of epigenetics, defined in various ways, but all definitions are based in the central concept that environmental forces can affect gene behavior, either turning genes on or off. I thought this recent summary by Helen Fisher was a nice statement of the importance of this new field:
..two basic mechanisms are known: one involves molecules known as methyl-groups that latch on to DNA to suppress and silence gene expression; the other involves molecules known as acetyl-groups which activate and enhance gene expression...Moroccan Amazighs or Berbers, people with highly similar genetic profiles reside in three different environments: some roam the deserts as nomads; some farm the mountain slopes; some live in the towns and cities along the Moroccan coast. Depending on where they live, up to one-third of their genes are differentially expressed.
...Genes hold the instructions; epigenetic factors direct how those instructions are carried out. As we age, these epigenetic processes continue to modify and build who we are. Fifty-year-old twins, for example, show three times more epigenetic modifications than do three-year-old twins; and twins reared apart show more epigenetic alterations than those who grow up together. Genes are not destiny; but neither is the environment...some epigenetic instructions are passed from one
generation to the next. Trans-generational epigenetic modifications are
now documented in plants and fungi, and have been suggested in mice.
The 18th century philosopher, John
Locke, was convinced that the human mind is an empty slate upon which
the environment inscribes personality. With equal self-assurance, others
have been convinced that genes orchestrate our development, illnesses
and life styles. Yet social scientists had failed for decades to explain
the mechanisms governing behavioral variations between twins, family
members and culture groups. And biological scientists had failed to
pinpoint the genetic foundations of many mental illnesses and complex
diseases. The central mechanism to explain these complex issues has been
found...to me as an anthropologist long trying to take a
middle road in a scientific discipline intractably immersed in
nature-versus-nurture warfare, epigenetics is the missing link.