NATURAL SELECTION—-DOBZHANSKY AND ALLEN 363 
Selection consists in differential perpetuation of genetic variants 
in the gene pool of a population. Selective success is reproductive 
success. The Darwinian fitness, or adaptive value of a genotype, is 
measured by the mean contribution of the carriers of this genotype, 
relative to other genotypes, to the gene pool of the succeeding gen- 
erations. The highly fit genotypes are these which transmit their 
genes most efficiently; the less fit ones have a mediocre reproductive 
efficiency; the unfit ones leave no surviving and fertile progeny. 
Under this sober appraisal, the “fittest” is nothing more spectacular 
than a parent of the largest family. He is no longer the mighty 
conqueror who has subdued countless competitors in mortal combat. 
He need not necessarily be even particularly hale and hearty ; strength 
and toughness increase Darwinian fitness only insofar as they con- 
tribute to reproductive success. Mules are at least as vigorous and 
resistant to harsh conditions as their parents, horses and donkeys. 
But the Darwinian fitness of mules is zero, because of their sterility. 
Conversely, a hereditary disease which strikes after the close of the 
reproductive period does not diminish the adaptive value of the geno- 
type. Anexample of thisis Huntington’s chorea. This isa dominant 
disease due to a single gene, the incapacitating effects of which do not 
usuaily appear until its carrier has passed most or all of the reproduc- 
tive period. There has even been a suspicion that the carriers of this 
gene have on the average a greater number of children than their 
normal siblings. The infirmities of old age are easily accounted for 
by the theory of natural selection. What happens to the organism 
after the reproductive age is of no concern to natural selection, or 
only insofar as the condition in old age is correlated with some traits 
which appear during the reproductive age. In a social organism 
like man, natural selection may, however, control survival in later 
years, because what happens to the older members of the family or 
community also affects the welfare of its younger members. The 
tendency of this control might be to shorten the interval between the 
close of the reproductive period and death because, as Haldane has 
pointed out, in some societies the oldsters prove a useless drain on the 
resources of the group. But comparison of the postreproductive years 
in man with those in other primates would probably show that the net 
effect of selection has been to lengthen this period. 
The question whether modern man is subject to natural selection 
can now be answered. Hecertainly is. Natural selection would cease 
only if all human genotypes produced numbers of surviving children 
in exact proportion to the frequencies of these genotypes in the popu- 
lation. This does not, and never did, occur in recorded history. 
Quite apart from the hereditary diseases and malformations for which 
no remedies are known and which decrease the reproductive fitness, 
