In recent papers, new data were presented on the late-age reproduction experiment initiated by Luckinbill and Clare in 1981: when early- and late-reproduced lines were compared simultaneously 10 years after the end of the original experiment, differences in the mean life span are observed between the lines. Yet the conditions in which these measurements were done are highly questionable. More fundamentally, using these data, the analysis of the selection process is impossible and conclusions about the determinism of life span are debatable.

In Drosophila melanogaster daily fecundity has been recorded throughout life of flies kept at various gravity levels (1-5 g). Hypergravity (HG) did not decrease total fecundity but modified its expression during life. The 1 g group could be opposed to the various HG groups, i.e. a clear regression of the various fecundity items on the HG level could not be disclosed. Results are discussed in relation with Pearl's rate of living theory.

A rereading of The Rate of Living [Pearl, University of London Press, London 1928] shows that Pearl’s thoughts about the meaning of inheritance and heredity, about the relations of between and within populations measurements and about the ways in which life span should be measured are unclear. New ways in which the theory should, eventually, be tested are suggested and the genetic implications of these suggested experimental ways are given. A review is made, mainly in insects, accessorily in rodents, of the different manners in which the theory has been essayed, namely by relating life span with temperature, rate of energy expenditure, growth rate and activity. It is concluded that the theory rests on a weak theoretical basis and even on a series of highly nonplausible assumptions and that, furthermore, the evidence experimentally accumulated is, with a few odd exceptions, not in favor of the theory.

Oregon-R, a wild type laboratory stock of Drosophila melanogaster, was divided into 3 subpopulations which were submitted to different environmental temperatures.During 6 years, duration of development, thorax size and male wet weight were measured several times in the 3 subpopulations.A genetic divergence between subpopulations was already observed 36 weeks after initiation.That series of experiments confirms the results obtained with Vetukhiv's subpopulations of Drosophila pseudoobscura.Furthermore it shows that a genetic differentiation between subpo- pulations may arise much faster than had been suspected, even in subpopulations initiated, contrary to the Vetukhiv's subpopulations, from a population with a narrow genetic base.Different hypotheses, which may explain the origin of the genetic variability present in subpopulations derived from a laboratory stock maintained in a constant environment during more than 15 years, are discussed.

Alternating developmental temperature within the viable range 13-33 degrees C, with increasing amplitude around a mean value of 23 degrees C but constant period (2 days, a 1 day/1 day rhythm), resulted in increased duration of development, decreased body weight and decreased growth rate of both male and female flies. Life span of female flies also decreased with increasing amplitude of temperature of oscillation but that of males increased (compared to that at the mean temperature) at moderate temperature amplitude. Variation of the period of oscillation (from a rhythm of 6 h/6 h up to 2 days/2 days) with constant amplitude (23 +/- 5 degrees C), on the other hand, did not affect either duration of development or body weight of the flies. However, life span of the females only was unaffected but that of males was increased (above that at mean constant temperature 23 degrees C) at the fast oscillation patterns (6 h/6 h and 1 day/1 day). Finally, life span of both sexes was positively correlated with growth rate in the constant period/varying amplitude case, whereas in previous studies variation of growth rate in the same range by other means showed a biphasic or an invariant pattern of life span dependence on growth rate. These results are in agreement with the hypothesis of epigenetic influence on life span by alternating developmental temperature and corroborate the previous conclusion that growth rate per se does not determine life span.

A large increase in the total phenotypic variance of thorax size was observed in a cage population of Drosophila melanogaster, maintained at 28 °C, a few months after it had been the victim of a naturally occuring population crash, the number of individuals in the population having, at a given moment, been reduced to half a dozen.In order to ascertain whether that increase in total phenotypic variance was due to an increase in environmental or in genetic variance that population was submitted, together with five other normally developing cage populations, to a selection programme for high and low bristle number.The additive genetic variance of these various populations was thereafter estimated.The additive genetic variance of the 28 °C cage population, victim of a population crash, was found to be highly significantly larger than all the other ones.The consequences of that unexpected observation on the theories of evolution are discussed.It is argued that that result confirms some of the predictions of the genetic revolution (genetic transilience) hypothesis of speciation.

The relationship between growth rate and life span was studied in Drosophila by varying the amount of yeast available to each developing larva at constant temperature, 25°C. With one approach the larvae developed in a standard medium at constant larval density and a varying amount of yeast added on the medium. Across the entire growth rate range covered in this way (10–100 μg/day, male flies) imaginal life span depended on growth rate in a biphasic way, the relationship having a parabolic form with a maximum at about 55 to 60 μg/day. Similar covariation of growth rate and life span was obtained by varying larval density at a constant amount of added yeast. With both these approaches growth rate variation was due to opposite variations of both components of growth rate, i.e. duration of development and body size. However, development in a medium without nutrients but with a varying amount of added yeast at constant larval density led to a similar biphasic relationship between growth rate and life span although duration of development did not vary. Therefore, the present results are not compatible with the hypothesis that there is a single causal negative relationship between growth rate and life span and demonstrate that duration of development is not a causative factor of the biphasic relationship between growth rate and life span established here.

The previous finding of a biphasic relationship between life span and growth rate of Drosophila, developed at 25°C, was confirmed at other temperatures in the usual range (19–28°C) and for development in either a standard medium or one deprived of nutrients but with varying amounts of yeast added on the medium. The role of body size in this relationship was studied by developing flies in a nutrient-less medium with a constant, submaximal amount of added yeast and varying temperature. It was found that under these conditions, which abolished the usual inverse relationship between body size and developmental temperature, body size variations did not account for the observed variations in life span. Thus, corroborating previous studies from this laboratory, body size was ruled out as a causative factor in the life span—growth rate relationship.

A survey of all the papers relating to life span in Drosophila melanogaster published in Experimental Gerontology from its origin in 1964 to 1981 shows that, contrary to a common belief, the mean life span of females exceeds that of males in only approximately 50% of the cases. It is shown that mean life span, as it is measured in most experiments, is a poor estimate of the potential life span of a Drosophila strain. However, the analysis of four extensive studies of Drosophila melanogaster life span strongly suggests that the potential (or maximal) life span is consistently higher for females than it is for males. That analysis also shows that, vis-à-vis the controlled or uncontrolled variations of the environment, the males have a broader norm of reaction or, in other words, a smaller homeostasis than the females. A model, mainly based on these two results, allows us to explain how the mean life span of males is so often higher than that of females.