much longer time. Some living mammal species are 

 1,000,000 years old. and some lower vertebrate and 

 invertebrate species have persisted relatively un- 

 changed for 30 million years (Simpson 1949, 1953). 

 Tlie rate of change, divergence, or evolution of 

 populations into new species depends partly on the 

 rate at which new mutations are occurring in the 

 species' gene pool, and partly on the rate and extent 

 to which the environment is changing. In a long- 

 continued uniform environment, a species becomes 

 stabilized in a favorable relationship with the habitat 

 and community. The various ecological niches are 

 effectively occupied, and little evolution occurs. \'ew 

 mutations can add little to perfected adaptation. If 

 the habitat changes, however, established adaptations 

 may no longer be appropriate, and variations hitherto 

 rejected might now prove beneficial. A mutation se- 

 lected for in one or more species may initiate a chain 

 of events that alters the internal balance of the whole 

 community, with resultant rapid evolution (Olson 

 1952). For instance, there has been considerable dif- 

 ferentiation of animals into subspecies during and 

 since the Pleistocene glaciation, but probably most of 

 our present-day species originated in pre-Pleistocene 



SUMMARY 



Speciation is tiie process of evolutionary 

 differentiation often leading to species formation, a 

 ])rocess usually also involving separation or diver- 

 gence of populations into different ecological niches. 

 Sympatric species do not interbreed because of eco- 

 logical, ethological. mechanical, or genetic isolating 

 mechanisms. 



Geographic isolation of two populations of the 

 same parental species appears prerequisite to com- 

 plete speciation. As long as a significant amount of 

 gene flow occurs between populations, they diverge 

 no further than subspecies. 



Populations may show different characteristics 

 as nonheritable variations, heritable polymorphism, 

 genetic drift, mutations, and hybridization. 



With natural selection, those individuals possess- 

 ing adaptive variations obtain a greater chance for 

 survival and reproduction, and consequently contrib- 

 ute more to the gene pool of the population. This 

 leads to a change in the characteristics of the popula- 

 tion and possible speciation. Exposure to new habi- 

 tats or niches may thus bring adaptive radiation into 

 a variety of new species. 



Speciation 267 



