682 



ECOLOGY AND EVOLUTION 



that gene frequencies would fluctuate 

 through chance alone, and in time would 

 drift some distance from the incidence that 

 was once characteristic of the population. 

 The larger the breeding population, the 

 more likely is it that the gene frequencies 

 would conform to Hardy's theory; and the 

 smaller the population, the sooner would 

 fixation of chance variations occur. The 

 eflFective size of the population depends in 

 part upon the sexual behavior and the num- 

 bers of breeding males and females, and 

 may be closer to the smaller of these two 

 numbers, particularly to the number of 

 females. 



The various forms of reproductive isola- 

 tion between populations result in the 

 divergence of species (Chap. 32, p. 606). A 

 lack of reproductive isolation would pre- 

 vent the branching of the phylogenetic 

 tree, and all evolution would be linear. In 

 the infraspecies populations, a lack of par- 

 tial isolation in large randomly breeding 

 groups would result in the swamping of 

 each favorable combination as soon as it 

 arose (pp. 602, 646), and no selection be- 

 tween competing races could occur (pp. 

 603, 616, 649)." Partial and complete re- 

 productive isolation tends to speed evolu- 

 tion. Lack of isolation would have a retard- 

 ing effect. 



If an organism has been able to adjust 

 to an environment in which competition 

 does not develop to any great extent, it 

 may survive with a primitive organization 

 through ages without becoming extinct or 

 evolving adaptations to fit a more rigorous 

 habitat (see pp. 655, 662). It is character- 

 istic of primitive relicts to be either 

 geographically or ecologically protected. 

 Svheriodon was able to survive in New 

 Zealand, where mammalian predators were 

 absent before the advent of white man. 

 Several primitive termites, notably the 

 Termopsinae, survive in temperate regions 

 out of competition with the more highly 

 specialized tropical termites. The ginkgo 

 tree would probably have become extinct 

 in recent centuries were it not for its pro- 

 tection by man in the temple gardens of 

 the Orient. The monotremes of Australia 

 and New Guinea would probably not have 

 survived competition of placental mammals. 

 It is likely that they have survived marsu- 

 pial competition through specialized aquatic 



life in the case of the duckbill, and spiny 

 protection in Echidna (Gregory, 1947). 



It is not by chance that Australia has the 

 most abundant fauna of primitive types— 

 notably primitive mammals, primitive ants, 

 and primitive termites. All these Australian 

 animals evolved during Mesozoic times and 

 were cut off during the Cretaceous from 

 the rest of the world. The primitive groups 

 of Australia survived to the present time 

 with little change in the absence of com- 

 petition from the more highly evolved Ter- 

 tiary relatives arising on other continents. 



Many primitive mammals of South Amer- 

 ica became extinct with the Pliocene in- 

 vasion by superior northern groups. The 

 survivors either had good protection (por- 

 cupines, armadillos), were nocturnal (opos- 

 sums), or had evolved specialized adapta- 

 tions to little occupied niches (sloths, ant- 

 eaters). Of course, such survival of primi- 

 tive forms results from many subtle anrl 

 complicated factors that can no longer be 

 fully analyzed, but the facts indicate the 

 importance of lack of competition in the 

 survival of otherwise primitive organisms 

 that have evolved slowly, compared with 

 their contemporaries in more competitive 

 environments. Absence of competition al- 

 lows slowly evolving forms to survive. 



Specialization results in limitations to- 

 ward further evolution. A species with its 

 current organization may reach an evolu- 

 tionary ctd de sac because its possible field 

 of adaptive values has already been ex- 

 ploited, and change giving advantage over 

 other organisms becomes improbable (see 

 pp. 632, 643). The more complete the 

 adaptation to a given set of stable ecologic 

 factors, the less chance there is for further 

 evolution (Fig. 229). 



Simpson (1944, p. 149) says: "Organic 

 change is so nearly universal that a state 

 of 'evolutionary motion' is inherent in phy- 

 letic survival. It is probable that the con- 

 tinuous application of some sort of force, 

 such as selection pressure, is necessary to 

 maintain a state of rest and that the mere 

 removal of restraint may be followed by 

 acceleration" (see pp. 662, 666). 



Selection tends to stabilize the species 

 by the elimination of deleterious genes over 

 long periods of time (Haldane, 1936). 

 The stronger the selection pressure, partic- 

 ularly on small populations, the more the 

 field of variability is limited. Selection acts 



