GENERAL ZOOLOGY 



heritable variations and the reproductive capacities of organisms, a struggle 

 for existence and natural selection inevitablv follow. 



Among inherited variations of many sorts, some will be of value to the in- 

 dividual in its struggle for existence; that is, some will have survival value. 

 According to Darwin, if the members of a species of plant varied in their 

 ability to resist frost, those that were sufficiently resistant would survive 

 temperatures that would be fatal to the great majority. Inheritance of the 

 variation by the next generation would follow, and such a process of selection, 

 repeated through many generations, would produce a population better fitted 

 to meet this particular condition of existence. Thus, evolution might occur 

 by modification of this feature of the organism in a manner to suit a changing 

 environment or to enable the species to extend its range northward. In a 

 similar manner, heritable differences in resistance to a disease would produce 

 a more resistant race; if wits were more important than strength, selection 

 would develop a more cunning type. If concealment were of survival value, 

 coloration and other features that tend to make the individual resemble its 

 surroundings would be at a premium and therefore selected. Darwin called 

 the process by which useful variations were sorted out natural selection, be- 

 cause it resembles the artificial selection practiced by breeders of animals 

 and plants in picking individuals that please the fancies or necessities of 

 man. Herbert Spencer called the process survival of the fittest, because the 

 individuals best fitted to the conditions of existence were the survivors in the 

 struggle for existence. In terms of genetics, the rate and extent of any evolu- 

 tion thus directed by selection depend on the occurrence of heritable vari- 

 ations that can be acted on by selection. Minor fluctuations in the expression 

 of a characteristic are of no importance, since they are not inherited. 



In our ecological discussions we considered, for the most part, short-term 

 environmental changes and their effects on the populations of particular 

 habitats. Yet great changes also occur over very long periods of time, as 

 when continents are made and unmade by geologic evolution, or when pro- 

 found climatic changes occur, such as the advent of an Ice Age or the change 

 from forest to desert conditions. These are less important in the day-to-day 

 activities of animals in a community than environmental changes which may 

 seem insignificant in comparison. The introduction or destruction of a plant 

 upon which various animals feed may produce far-reaching changes in the 

 environmental conditions of a given species. New enemies entering a district 

 may bring new standards of selection; new parasites or pathogenic micro- 

 organisms may put a premium on qualities that have not hitherto been 

 selected. In the interplay of forces it is possible that conditions, and 

 therefore selection, may remain stable for long periods, or that selection 

 may suddenly take new directions. Changes of many sorts are conceivable 

 within the limits of the selection pressure and the heritable characters avail- 

 able for selection. In terms of natural selection, the environment may be 

 compared to a sieve that selects individuals presented to it but does not 

 determine their nature. As long as the sieve remains unchanged, it allows 



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