Ever since then, all descendant vertebrates have had the forward end of the digestive system and the forward end of the respiratory system very much involved with each other. This manifests itself in the human body with a crossing of the two systems in the throat.

In other words natural selection will always be in greatest opposition to the decline of the most senescence-prone system. Williams was also well known for his work on the evolution of sex and was an advocate of evolutionary medicine. According to this original formulation

"if the adverse genic effects appeared earlier in one system than any other they would be removed by selection from that system more readily than from any other.

Williams was a professor emeritus of biology at the State University of New York at Stony Brook. George Christopher Williams (May 12 1926 – September 8 2010) was an American evolutionary biologist. D.