Experimental evolution of aging, growth, and reproduction in fruitflies

Abstract

We report in this paper an evolutionary experiment on Drosophila that tested life-history theory and the evolutionary theory of aging. As theory predicts, higher extrinsic mortality rates did lead to the evolution of higher intrinsic mortality rates, to shorter lifespans, and to decreased age and size at eclosion; peak fecundity also shifted earlier in life. These results confirm the key role of extrinsic mortality rates in the evolution of growth, maturation, reproduction, and aging, and they do so with a selection regime that maintained selection on fertility throughout life while holding population densities constant.

Figure 1

Intrinsic and total mortality in the population cages. Intrinsic mortality is indicated by the irregular lower lines, one line per replicate. Negative mortalities indicate that too many flies accidentally were added in the last check. Total mortality (intrinsic plus extrinsic mortality) is given by the thick straight line. The symbols indicate when there were not enough flies available to achieve the target mortality because not enough eggs had been laid 14 days previously. □, Replicate 1; ○, replicate 2; ▵, replicate 3. Filled symbols indicate females, and open symbols indicate males. (Upper) HAM. The symbols above the total-mortality line indicate that 4 times a replicate was lost because of mislabeling and was replaced from backup stocks 2 weeks behind the current history of selection. At the start of the experiment and after larval density was increased in January 1996, there were sometimes not enough replacement flies to achieve the target mortality (23 times for females, 2.1% of the checks; 20 times for males, 1.8% of the checks). (Lower) LAM. Before January 1996, natural mortality rates twice exceeded 20%. After January 1996, when the target mortality was 10%, there were more old flies in the cages, and intrinsic mortality increased. From January 1996 to July 1998, intrinsic mortality was 11.3% for females (vs. 2.8% in HAM) and 10.3% for males (vs. 3.6% in HAM).

Figure 2

Changes in dry weight (a), time to eclosion (b), and early fecundity (c), 1993–1997. *, P < 0.05; **, P < 0.01; ***, P < 0.001. The bars at the end of the series in a and b are the standard errors of three sets of measurements made in the spring of 1998. Divergence between HAM and LAM (LAM minus HAM) in dry weight (d), time to eclosion (e), and early fecundity (f), 1993–1997.

Figure 3

Intrinsic mortality rates per 3-day interval. Number dying in each period determines statistical power. Results show averages for the two treatments. Mortality curves were measured April–July 1998.

Figure 4

Correlations of early fecundity with subsequent age-specific mortality defined on 5-day age classes. Open symbols, HAM; closed symbols, LAM. Traits were measured in November 1996.