700 GENETICS AND EVOLUTION 



5. The surviving individuals will give rise to the next generation, 

 and in this way tlie "successful" variations are transmitted to the suc- 

 ceeding generations. The less fit will tend to be eliminated before they 

 have reproduced. 



Successive generations in this way tend to become better adapted to 

 their environment; as the environment changes, further adaptations 

 occur. The operation of natural selection over many generations may 

 produce descendants which are quite different from their ancestors, 

 different enough to be separate species. Furthermore, certain members 

 of a population with one group of variations may become adapted to 

 the environment in one way, while others, with a different set of varia- 

 tions, become adapted in a different way, or become adapted to a differ- 

 ent environment. In this way two or more species may arise from a single 

 ancestral stock. 



Animals and plants exhibit many variations which are neither a 

 help nor a hindrance to them in their struggle for survival. These are 

 not affected directly by natural selection but are transmitted to suc- 

 ceeding generations. 



Darwin's theory of natural selection was so reasonable and well 

 documented that most biologists soon accepted it. One of the early, seri- 

 ous objections to the theory was that it did not explain the appearance 

 of many apparently useless structures in an organism. We now know 

 that many of the visible differences between species are not important 

 for survival, but are simply incidental effects of genes that have other 

 physiologic effects of great survival value. Other nonadaptive differences 

 may be controlled by genes that are closely linked in the chromosomes 

 to genes for traits which are important for survival. 



Another of the early objections to the theory was that new variations 

 would be lost by "dilution" as the individuals possessing them bred with 

 others without them. We now know that although the phenotypic ex- 

 pression of a gene may be altered when the gene exists in combination 

 with certain other genes, the gene itself is not altered and is transmitted 

 unchanged to succeeding generations. 



299. Modern Changes in the Theory of Natural Selection 



The rediscovery of Mendel's laws in 1900 made necessary two major 

 corrections to the theory of natural selection: (1) only inherited varia- 

 tions can provide the raw material for natural selection, and (2) in- 

 cipient species must be separated by some sort of geographic or ecologic 

 isolation to prevent interbreeding. 



Modifications and Mutations. Darwin did not clearly distinguish 

 between variations resulting from some chemical or physical action of 

 the environment on the developing individual, and variations resulting 

 from some alteration of the hereditary materials, the genes and chromo- 

 somes. The first type of variations, called modifications, are not in- 

 heritable and play no role in evolution, but variations arising from 

 changes in the genes or chromosomes, called mutations, are the raw 

 materials for evolution by natural selection. Evolution, clearly, cannot 



