Natural Selection 99 



pterodactyls represented lineages which changed rapidly but all 

 of which were extinct by the end of Mesozoic time. 



These examples demonstrate that some of the continuing flow 

 of genetic changes become established in the population; others do 

 not. Further, over long periods of time much-changed and little- 

 changed lineages alike may persist or become extinct. Thus there is 

 no necessary corollary between amount of change and persistence. 



In broad outline, current explanations of natural selection follow 

 the pattern of reasoning laid down in 1858 by Darwin and Wallace 

 (Darwin, 1859). Genetic information has increased greatly since 

 that time, as has knowledge concerning other facets of evolution, 

 and these changes have modified a number of evolutionary concepts. 



The action of natural selection is simple in theory but tremen- 

 dously complex in operation. The root of the selection starts in the 

 innate ability of living things to reproduce. There are such tremen- 

 dous environmental hazards between the time of being born and 

 that of reproducing, that of the many young produced by an 

 individual, only one or a few survive in the long run. Theoretically, 

 these survivors will be those with genetic constitutions producing 

 individuals best fitted to cope with the environments; those which 

 perish will have genetic constitutions producing individuals inferior 

 with respect to critical environmental factors. The complexity arises 

 from the fact that the collective progeny of one individual or pair 

 have many genetic variables, as does the environment have many 

 variables. The complexity of natural selection lies in the many 

 combinations of circumstances arising from the interplay of these 

 two sets of variables. 



The empirical action of natural selection may be explained by 

 a hypothetical example. If a breeding population of snakes consists 

 of 1,000 specimens, 500 of them spotted and 500 banded, what will 

 be the proportion of spotted to banded snakes in the thousand 

 specimens which will survive to form the next generation? We 

 will assume that predators see the banded snakes more easily than 

 the spotted ones and catch more of them. In this case the second 

 generation might contain 750 spotted and only 250 banded. Thus 

 the selective action of the predators on the phenotypes has changed 

 the genetic composition of the snake population. More snakes bear- 

 ing the genetic makeup for "spotted" survived to breed than did 

 those bearing the genetic makeup for "banded." 



If "banded" were new in the population, arising as a spontaneous 

 mutation, then this selection would rapidly weed it out. If, in con- 

 trast, "spotted" were a new mutant character, the selection would 



