662 



ECOLOGY AND EVOLUTION 



often found on islands and peninsulas, and 

 suggest this is owing to the relatively slight 

 temperature fluctuations characteristic of 

 maritime climates. SpeciaUzation of course, 

 may be correlated with survival under 

 rather extreme physical conditions or ex- 

 treme biotic conditions, or both. Likewise, 

 primitive types might survive under both 

 mild physical or biotic conditions. In many 

 instances biotic factors through competition 

 probably play an important role in the sur- 

 vival of relict forms. 



The phenomenon of rapid or "explosive" 

 (tachytelic) evolution of major groups 

 (megaevolution) during a geological epoch, 

 followed by less rapid (horotelic) subse- 

 quent evolution, has been questionably ex- 

 plained as the result of peaks of genetic 

 mutations (p. 600). Higher rates of muta- 

 tion than are commonly observed would 

 not necessarily produce any evolutionary 

 change (Simpson, 1944, p. 47). If muta- 

 tion rate were the limiting factor, the 

 length of generations might be expected to 

 show a strong negative correlation with rate 

 of evolution, but no such correlation is ap- 

 parent. Explosive evolution would seem bet- 

 ter understood as an eflFect associated with 

 reduced competition or lack of competition 

 within available niches. Under favorable 

 conditions of partial isolation of small local 

 inbreeding populations with occasional cross 

 breeding between populations with unique 

 balanced genetic patterns, an enormous po- 

 tential variability exists, and selection of 

 competing populations as contrasted wdth 

 competing individuals may occur. Under 

 exceptionally favorable ecological condi- 

 tions, rapid evolution of new higher sys- 

 tematic categories is possible (Cain, 1944, 

 p. 325; Wright, 1945, p. 416; Amadon, 

 1947). Such rapid evolution might ini- 

 tially exhibit a change toward general 

 rather than special adaptation. 



During the Paleocene and Eocene, pla- 

 cental mammals evolved into most of the 

 modern orders from a ferungalate stock. 

 The habitats had been largely occupied by 

 Mesozoic reptiles with similar adaptations 

 such as wings, streamlined shapes, carniv- 

 orous feeding adjustments, and cursorial 

 legs. The causes of the widespread extinc- 

 tion of many reptile groups toward the end 

 of the Mesozoic are not understood. What- 

 ever the causes, the reptiles left vacant 

 niches that in due time were occupied by 



ecologically equivalent placental mammais. 

 The major orders of placental mammals 

 had appeared by Eocene times, and no new 

 orders evolved in the much longer period 

 from the beginning of the Oligocene to 

 the present. This may be explained by the 

 fact that a niche, once occupied by an 

 adapted form, could not be invaded by an- 

 other form initially less well adapted. If 

 unoccupied, however, the niche might be 

 exploited by an initially poorly adapted 

 form that in time would become adjusted 

 through natural selection. Under reduced 

 competition, organisms vary more widely 

 and tend to occupy vacant niches, while 

 an increase in competition with its more 

 rigorous selection results in less surviving 

 variation, more specialized adaptation, and 

 a lessened capacity to radiate adaptively. 



Before the junction of North and South 

 America (p. 723) in the late Pliocene 

 period, twenty-seven families of land mam- 

 mals occurred in North America and 

 twenty-nine families in South America. No 

 families occurred in both (Didelphidae 

 and Procyonidae are possible exceptions). 

 After the Pliocene faunal interchange, 

 twenty-two families were common to the 

 two regions. South America now has 

 twenty-nine families, and North America 

 has twenty-three. It would appear that the 

 advance of one group usually means the 

 recession or extinction of a competing 

 group, the total number of groups in a 

 given area remaining fairly constant in re- 

 cent geological time (Mayr, 1946; Simpson, 

 1940; Darlington, 1948). 



The early radiation of the orders of in- 

 sects is even more remarkable than that of 

 the orders of mammals (Carpenter, 1930). 

 Fossil primitive winged insects, including 

 the Paleodictyoptera and the blattoids 

 (roachlike insects), are found in rocks of 

 the Pennsylvanian period and undoubtedly 

 occurred somewhat earlier. A large number 

 of modem orders first appear in the Per- 

 mian with forerunners of still other orders 

 known first from the Triassic and Jurassic. 

 With the exception of the Lepidoptera, 

 which may have evolved with the rise of 

 the flowering plants in Cretaceous times, 

 there was no further increase in the num- 

 ber of main insect orders after the Permian. 

 In the rapid evolution of the placental 

 mammals, the niches had probably been 

 vacated. In the case of the insects, the 



