BIOLOGICAL BACKGROUNDS FOR POPULATION STUDIES 



275 



out, it is approximated more by some spe- 

 cies-populations than by others. 



Probably the most quoted illustration of 

 physiological longevity is the work of Pearl 

 and Parker (1924) and Pearl (1928) on 

 Drosophila melanogaster. Their experiments 

 were conducted under highly artificial con- 

 ditions, but the argument at least is perti- 

 nent. Pearl reasoned that if inbred flies 

 were starved and kept under controlled 

 conditions with their longevity recorded, 

 the survivorship curve represented only 

 the organisms' potentialities for life ("in- 



then die were it not for "exogenous" or 

 environmental eflFects of one sort or another. 

 This idea has been criticized (see Wright, 

 1928),** but the data do provide an example 

 of physiological longevity. 



Another illustration, based on the work 

 of Noyes (1922) and the computations of 

 Pearl and Doering (1923), is aflForded by 

 the survivorship of the rotifer, Proales de- 

 cipiens. The mortality in populations of this 

 organism, which reproduces parthenogeneti- 

 cally, is an excellent example of the physio 

 logical longevity curve, as shown in Figure 



10 20 30 40 50 60 70 80 90 100 



GENTILES OF LIFE SPAN 

 Fig. 74. Survivorship curves for the rotifer, Proales decipiens, man, and Drosophila melano- 

 gaster. Ordinate is a logarithmic scale. ( From Pearl and Doering. ) 



herent vitality"), since all external sources 

 of energy and nonliving matter were 

 excluded. In the actual experiments 

 such survivorship curves were obtained 

 for 1000 wild-type fed flies; 1000 wild- 

 type starved flies; 1000 vestigial-winged, 

 fed flies; and 1000 vestigial, starved 

 flies. The data are summarized in Figure 73 

 and graphed on a relative abscissa so that 

 the curves for starved flies, which, of course, 

 lived a much shorter time, can be com- 

 pared directly with those for fed flies. The 

 point is clearly made that the graphs for the 

 starved approach closely the right angle 

 characteristic of physiological longevity, 

 while the graphs for the fed are character- 

 istic of ecological longevity. One gathers 

 that Pearl would conclude that all Droso- 

 phila imagoes would live a long time and 



74. The point is further strengthened b) 

 contrasting the median life durations of 

 Proales with man and fed Drosophila. 

 When half of the total rotifer life span has 



" Part of Wright's criticism is that this 

 analysis does not take into account "exogenous" 

 events of the pre-imaginal period; particularly 

 of the maggot, a feeding stage. It is certainly 

 true that there may be extreme mortality for 

 pre-imaginal stages in Drosophila caused by 

 environmental factors. Winsor (1937) speaks 

 pertinently to this point: "... The data on 

 fecundity and fertility relative to density of 

 population in Drosophila indicate a pre-im- 

 aginal mortality varying markedly with density. 

 This mortality may reach 90 to 99 per cent at 

 high densities such as correspond to saturation 

 levels of population. It seems highly probable 

 that this mortality is a factor of major im- 

 portance in the regulation of numbers." 



