THE DYNAMICS OF EVOLUTIONARY CHANGE 



has shown us that the problem of the nature and origin of 

 species is by no means solved and that divergent opinions ex- 

 ist both as to what species are and how they came into being. 

 Nevertheless, a large array of facts obtained from a great variety 

 of experiments tells us that species are distinct from each other 

 chiefly because they are separated by barriers which restrict or 

 prevent gene exchange between populations. 



The basic importance of reproductive isolation in maintain- 

 ing adaptive diversity within a community of organisms is evi- 

 dent from the widespread occurrence of two phenomena which 

 have been much studied in recent years, hybridization and char- 

 acter displacement. If two related populations which have 

 been isolated from each other in different habitats are per- 

 mitted by environmental changes to come together, then their 

 subsequent evolution will depend upon the degree of reproduc- 

 tive isolation which has developed between them. If they are 

 still able to cross and produce fertile hybrids, the result of their 

 contact v ill be the production of an intermediate, hybrid swarm, 

 in which the identity of the original populations will become 

 lost to a greater or lesser degree. Numerous examples of this 

 can be cited, particularly in higher plants (Stebbins, 1950, 1959a). 

 On the other hand, if their ability to intercross and produce 

 fertile hybrids is so much reduced that each population can 

 maintain intact its own adaptive properties, then the popula- 

 tions will tend to compete with each other. Since direct com- 

 petition tends to destroy reproductive capacity, natural selec- 

 tion will favor genotypes of each population which are as dif- 

 ferently adapted as possible from the norm of the competing 

 population and so will cause the two populations to diverge 

 from each other. Lack (1947) has shown that in the finches of 

 the Galapagos Islands, a race of a particular species, if it is the 

 sole occupant of a particular island, will have a rather wide 

 and generalized range of variation. Another race of the same 

 species, which on a different island is sharing its habitat with 

 a distinct but related species, will have a narrower and more 

 specialized range of variation. This phenomenon, known as 

 character displacement, is also described in ants by Brown and 

 Wilson (1956), who review additional examples in birds. 



The diversity and genetic complexity of reproductive isolat- 



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