356 INTRODUCTION TO EVOLUTION 



1000 young individuals. Of group A, 800 individuals survive to maturity; of 

 group B, only 500 individuals survive. But in group A the reproductive 

 rate is such that each individual is replaced by one descendant, whereas 

 in group B each individual is replaced by two descendants. Which is the 

 more successful group? Obviously group B is, since in the next generation 

 it will number 1000 individuals, while group A will number only 800. 



The most successful individuals or groups are those which contribute 

 their genes in greatest number to the building of the next generation. 



"Individuals having most offspring are the fittest ones" (Lerner, 1959). 

 It is well to remember that this is what "fittest" means in natural selection 

 theory, and all that it means. Much mistaken thinking to the contrary not- 

 withstanding, "fittest" does not mean "strongest" or "fastest" or "health- 

 iest" or "most intelligent." Of course individuals or societies lacking in all 

 such traits may not be likely to leave the most offspring. But the measure 

 of their fitness is not possession of the attributes listed; it is the leaving of 

 offspring. 



Nature of "the Struggle for Existence" 



The phrase "struggle for existence" is unfortunate. It carries too many 

 overtones of "Nature red in tooth and claw." True, predatory animals do 

 play a part in reducing the number of surviving members of a population, 

 and hence in determining which members shall contribute most to the 

 next generation. But competition for available food supply is also a factor, 

 operating principally in times of exceptional stringency, as, for example, 

 during droughts, floods, exceptionally severe or prolonged winters, or as a 

 result of extreme overpopulation of a given territory. 



So far we have stressed competition between individuals in the same 

 species (for food, territory, etc.): intraspecific competition. We should 

 also note that interspecific competition occurs and may at times be impor- 

 tant in evolution. Two closely related species (recently arisen from a com- 

 mon ancestral species, perhaps) may compete for the same food supply. 

 If this competition is keen it may lead to changes in the two species so 

 that competition will be lessened. Thus two species of ground finch living 

 on the same Galapagos island may come to differ from each other in beak 

 size by virtue of the fact that it is advantageous for one to specialize on 

 large seeds, the other on small seeds. Or alternatively, one species may 

 be so much more efficient than the other in utilizing the food supply that 

 the less efficient species becomes extinct (at least in the territory originally 

 shared in common). (See Hardin, 1960.) 



Many important characteristics are less obviously related to struggle 



