Natural Selection and the Parameters of Population Growth

The intrinsic rate of natural increase, r, is a constant hat measures the ability of a population to grow under a given set of conditions. The Malthusian parameter, m, is a measure of genotypic rate of increase. Although the mathematical formulations for r and m are the same, their biological characteristics are different except in isogenic populations. Selection will always favor genotypes with the highest m under a given set of conditions, but such selection may result in a reduction of r for that popu]ation. The mechanism by which this process occurs is discussed and simulated, and it is concluded that evolutionary reductions in r are most likely to have occurred under conditions of density dependence. It is also suggested that the recent terms r and K selection have mixed ecological parlance into evolutionary processes in a potentially confusing manner. Population eeology and population genetics beeame self-conseious fields of study at approximately the same time ( Pearl 1925, Lotka 1956, Fisher 1958, Gause 193A, Haldane 1924 ). This eoineidence should have led to fruitful eollaboration, but no sueh eollaboration took plaee. It is not our purpose to seareh for eauses of the failure, but to diseuss some of the anomalous eonsequenees. Among the most obvious of the anomalies is the eonfliet between theoretieal deduetions that natural seleetion tends always to maximize the intrinsie rate of inerease or Malthusian parameter (Fisher 1958), and the observations of evolutionary history, in whieh some deseendent speeies seem elearly to have lower rates of inerease than did aneestral ones. This is exemplified in the repeated observations of increasing size over geologieal time, eombined with the observation that larger bodied species have lower rates of inerease than snlaller ones. Careful attempts to understand fully some of the recent literature ( MacArthur 1960, 1962, MacArthur and Wilson 1967, Murphy 1968) in population and evolutionary biology have convinced us of the necessity of bringing the various ideas and usages together, so that proper distinctions can be made and some common misconceptions corrected. We thank R. D. Alexander, E. O. Wilson, S. G. Tilley, and P. DeBenedictis for many helpful comments on the manuscript. We, however, accept full responsibility for . . any remalnlng errors.