NATURAL SELECTION—DOBZHANSKY AND ALLEN 369 
infections prevalent in crowded towns is probably more important 
than it was. So is the ability to learn, to become educated, and to 
live in reasonable accommodation with one’s neighbors. Natural 
selection now works in what some may call unnatural conditions, but 
it is still natural selection. 
RELAXATION OF SELECTION 
The hoary fallacy which is perpetuated by some modern writers 
is that for a genetic variation to be selected it must be important 
enough to decide between the life and death of the creature. In 
reality, even a slight advantage or disadvantage which increases 
the probability of one genotype leaving more offspring than another 
will be effective in the long run. It has recently been found (Aird, 
Bentall, Mehigan, and Roberts, 1954) that the proportion of people 
with blood group O is slightly higher among patients suffering from 
duodenal ulcer than it is in the general population. This does not 
mean either that everybody with O blood gets a duodenal ulcer, or 
that those with other blood groups are immune. But the possibility 
that the frequencies of O bloods in human populations may be in- 
_ fluenced by the greater susceptibility of O persons to duodenal ulcer 
is a real one. 
The fitness, the adaptive capacity of the carriers of a given geno- 
type is continuously changing. Suppose that the contribution of one 
genotype to the gene pool of the following generation is equal to unity. 
The contribution of a different genotype may then be represented as 
1—s. The values is the difference in reproductive success between the 
two genotypes and is called the selection coefficient. Now, the magni- 
tude of the selection coefficient depends upon the environment. Selec- 
tion coefficients grow larger as selection becomes more stringent and 
they diminish as selection is relaxed. When s is zero, the genotypes 
are equal in fitness, and selection does not operate upon them. 
There can be no doubt that modern technology, and especially 
modern medicine, have greatly mitigated the disadvantages of many 
genetic weaknesses and disabilities: In other words, in an environ- 
ment which includes modern technology and medicine, selection coeffi- 
cients operating against certain human genotypes are smaller than 
in a primitive environment. But this amounts to saying that the 
fitness of the carriers of these genotypes has increased. A person 
afflicted with hereditary diabetes mellitus can live reasonably happily 
and may even raise a family if his environment includes proper doses 
of insulin administered at proper intervals. Genetically considered, 
a disability that can be corrected by environmental means so that it 
no longer causes an impairment of reproductive efficiency ceases to 
be a disability when a suitable environment is provided. 
