ADAPTIVE LIFE HISTORIES EFFECTED BY AGE-SPECIFIC SURVIVAL AND GROWTH-RATE

Life history data for three unexploited populations of brook trout, Salvelinus fontinalis, were used to test the predictions of life history theory that, relative to juveniles, (1) high adult survival favors low reproductive effort and delayed reproduction, and (2) increased juvenile growth rate favors high effort and early reproduction. Field data supported both predictions. The population having the highest adult-to-juvenile survival ratio expended the least effort, reproduced latest in life, and experienced the lowest survival cost of reproduction. Among populations a high juvenile-to-adult growth rate was associated with early reproduction, high reproductive effort, and high reproductive cost. Early reproduction was also associated with increased growth within populations. The adaptive significance of interpopulation variation in life history was assessed by comparing the fitness, r, of observed life histories with those of potentially alternative strategies. Empirically derived fitness functions supported the hypothesis that population differences in life history were adaptive. Observed combinations of age-specific survival and fecundity were those that maximized fitness. Within populations the fitness advantages associated with reproducing early in life favored reduced age at reproduction for the fastest-growing individuals. The results are consistent with the predictions of life history theory and demonstrate empirically how survival and growth rate can independently and interactively influence life history evolution.