Does Natural Selection Continue to Operate 
in Modern Mankind?’ 
By Tueoposius DoBZHANSKY 
Professor of Zoology, Columbia University 
and 
GorDOoN ALLEN 
New York State Psychiatric Institute 
THE PROBLEM 
Tue purvose of the present article is to examine the validity of the 
assertion, frequently made in medical, biological, and sociological 
writings, that natural selection has been relaxed or even done away 
with altogether in modern mankind, particularly in advanced indus- 
trial societies. With this assertion as a premise, dire predictions of 
biological decadence of the human species have been uttered, espe- 
cially in popular scientific literature. It is of course not our intention 
in this article to grapple with this immense problem in its entirety, 
and we mean neither to affirm nor to refute the predictions of de- 
cadence. We feel, however, that the thinking in this field may gain 
in clarity from a reexamination of the concepts of natural selection 
and adaptation, particularly as they apply to man. Such a reexami- 
nation is the more needed since these concepts have not remained stable 
even in biology since they were advanced by Darwin. Particularly 
rapid change has taken place in recent years in connection with the 
development of population genetics. 
Natural selection is regarded in modern biology as the directing 
agent of organic evolution. The process of mutation yields the genetic 
variants which are the raw materials of evolutionary change. Sexual 
reproduction then gives rise to innumerable gene combinations or 
genotypes. However, which mutants arise, and when, have nothing 
to do with their possible usefulness or harmfulness to the species. 
Natural selection, nevertheless, so maneuvers the genetic variability 
that living species become fitted to their habitats and to their modes 
1 Reprinted by permission from American Anthropologist, vol. 58, No. 4, August 1956. 
492520—59—_24 359 
