While the reproductive rate must be looked upon as a characteristic
which has its adaptations like other characteristics, it has one
peculiarity--its increase is always opposed by lethal selection. The
chances of life are reduced by reproducing, inasmuch as more danger is
entailed by the extra activities of courtship, and later, in bearing and
caring for the young, since these duties reduce the normal wariness of
individual life. The reproductive rate, therefore, always remains at the
lowest point which will suffice for the reproductive needs of the
species. For this reason alone the non-sustentative form of selection
might be expected to be the predominant kind.
J. T. Gulick and Karl Pearson have pointed out that there is a normal
conflict between natural selection and fecundal selection. Fecundal
selection is said by them to be constantly tending to increase the
reproductive rate, because fecundity is partly a matter of heredity, and
the fecund parents leave more offspring with the same characteristic.
Lethal selection, on the contrary, constantly asserts its power to
reduce the reproductive rate, because the reproductive demands on the
parents reduce their chances of life by interference with their natural
ability of self-protection.
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