xxxir. 6 SLOW RATES OF EVOLUTION 773 



It is most interesting to consider possible reasons for these very 

 slowly evolving (bradytelic) populations (Simpson, 1953). (1) It 

 might be low mutation rate or low variability, but there is no evidence 

 that opossums (say) are less variable than other mammals and indeed 

 they have undergone much speciation. (2) It is often implied that 

 survival is assisted by some special habit, such as being nocturnal or 

 abyssal, but others with the same habits evolve fast. (3) Survival some- 

 times seems to be assisted by isolation (e.g. lemurs) but there is no 

 evidence that this is necessarily a factor (Latimeria). (4) Low rate of 

 evolutionary change is not a function of 'primitive' organization as 

 such, indeed as we have seen the reverse is true. In any case Sphenodon 

 and Crocodylm were not especially 'primitive' when they stopped evol- 

 ving in the Triassic and Cretaceous. (5) Long survival must depend 

 upon some special relationship between the genetical and information- 

 carrying powers of the species, the risks imposed by the environment, 

 and the stability of the latter. (6) If the adaptive zone is a narrow one 

 it must be stable and persist. This would seem to be unlikely in very 

 'difficult' habitats such as deserts or impermanent ones (salt lakes) or 

 variable ones, such as Alps. (7) Long survival is perhaps more to be 

 expected in a broad adaptive zone such as the ocean or shore, lowland 

 rivers or forest belts, especially in the tropics. Such environments 

 present, however, many niches that can be considered as 'corridors', 

 leading to diversification, and it is surprising that forms nevertheless 

 remain stable in them. Thus opossum-like creatures gave rise to 

 various offshoots in South America but themselves changed little. (8) 

 Bradytelic populations must be genetically so integrated that any devia- 

 tion is subject to counter-selection (though in that case it is hard to see 

 how the offshoots have arisen). (9) Simpson concludes that these brady- 

 telic organisms 'have run the whole repertory of baffles and . . . 

 persist indefinitely'. Most organisms are turned off into one or other 

 of the corridors presented by the environment; when a group has met 

 and passed them all it persists. 



The discussion of organisms that have evolved only very slowly is 

 thus a stimulus to considering the whole balance of factors by which 

 a population of organisms maintains its homeostasis. Evidently there 

 are some circumstances in which it can do this with little genetic 

 change. In the great majority, however, change of the genes and hence 

 of the structure and physiology is a part of the very mechanism by 

 which the living system continues to survive in spite of changes 

 around it. 



