540 - Heredity and Evolution 



DAYS 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 



Fig. 28-2. Multiplication rates in Drosophila. 



similarly, as is shown tor Drosophila in Fig- 

 ure 28-2 and for man in Figure 28-3. In each 

 case the population reaches a constant maxi- 

 mum, under a given set of conditions; and 

 this maximum is attained in a relatively short 

 time, that is, within a relatively small num- 

 ber of generations. The level of this maxi- 



P0PULATI0N 

 MILLIONS 



200 



200 



mum is determined largely by environmental 

 conditions, as may be seen in Figure 28-2. 

 After the maximum is reached, a population 

 may fluctuate from year to year or — within 

 shorter periods, in rapidly reproducing or- 

 ganisms — in accordance with the food supply 

 and other environmental conditions. But by 



AD 1600 



1700 



1800 



1900 



2100 



Fig. 28-3. Human population of the earth: abscissas, years; ordinates, popula- 

 tion in millions. The solid portion of the curve is based on actual data; the 

 dotted portions are purely theoretical, calculated from the equation of the 

 curve. The increase in population during this period, from its previous stationary 

 level, is the result of the great change in environmental conditions caused by 

 the Industrial Revolution. The upper, dotted portion of the curve predicts the 

 future population on the assumption that there will be no further great environ- 

 mental changes — a very doubtful assumption. (Adapted from Pearl.) 



