THE COSTS OF REPRODUCTION AND DIETARY RESTRICTION - PARALLELS BETWEEN INSECTS AND MAMMALS

Dietary restriction increases life span in mammals. This essay connects the dietary restriction response to evolutionary life history theory and experiments related to it. Evolutionary biologists have shown mathematically that aging is an inevitable consequence of age-specific natural selection acting on species with somata separate from germ lines. Empirical tests of this prediction currently point to its general validity. Two specific genetic mechanisms are known which could underlie the evolution of aging under these conditions: age-specificity of gene effects and antagonistic pleiotropy between early and late ages. The antagonistic pleiotropy theory assumes that some genes with beneficial effects on early life fitness will have deleterious effects upon fitness in later life. Experimental work in insects, particularly selection experiments in Drosophila melanogaster, has tested these ideas. The negative genetic correlation between longevity and reproductive effort produced by selection has been shown to be paralleled, in some cases, by environmental manipulation. Thus the increase in life span caused by dietary restriction might be explained as an incidental consequence of lower reproductive effort. This response also could have been an adaptation,that enhanced fitness in some species that faced uncertain food supplies, a condition that may have evolved independently in a wide variety of taxa. Several schools of research, besides that of the evolutionary biologists concerned with genetic correlations, have produced corroborations of this hypothesis in insects and mammals: the gerontological work on life span extension, reproductive physiologists concerned with factors that affect fertility, and various life history studies.