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CHAPTER 18 



Speciation Involving One Species 



A species of cross-fertilizing organisms usu- 

 ally consists of a Dumber of races adapted 



to the different environments of the terri- 

 tories thej occupy. All these races are kept 

 in genetic continuity by interracial breeding 

 and hybrid race types, so that the species, as 

 a whole, has a single gene pool containing 

 no portion completely isolated from any 

 other. On the other hand, different cross- 

 fertilizing species are genetically discontinu- 

 ous from each other. Thus, the gene pool 

 of one species is so isolated from the gene 

 pools of all other species that none can lose 

 its identity via crossbreeding, or hackcross- 

 ing subsequent to crossbreeding. Moreover, 

 the gene pools of different species are iso- 

 lated from each other for genetic — not 

 merely environmental — reasons. 



The formation of new species, speciation, 

 has occurred frequently in past evolution; 

 since evolution is continuing, new species 

 are still being formed. The speciation mech- 

 anism considered most common for cross- 

 fertilizing individuals involves the production 

 of two or more species from a single one. 

 How can this come about? 



Hypothetically, one can start with a single 

 panmictic, genetically-polymorphic species. 

 Since environments vary we will assume that 

 different populations occupy different por- 

 tions of a territory and, although enough 

 interpopulation breeding takes place to form 

 one gene pool, most of the breeding is intra- 

 population. If, in the course of time, two 

 (or more) of these populations diverge ge- 

 netically — each one uniquely adapted to its 

 own territory — these populations become 

 different races of the same species. The dif- 

 ferences in the gene pools of these two races 

 may increase more and more because of mu- 

 tation, natural selection, and genetic drift. 

 As this differentiation process continues, the 

 genes which make each of the races adaptive 

 in their own territories may, by their mani- 



fold phenotypic effects, make matings be- 

 tween the two races still less likely to occur 

 or may cause the hybrids of such matings 

 to be less adaptive than the members of 

 either parent race. Accordingly, partial re- 

 productive isolation may be initially an acci- 

 dental or an incidental byproduct of the 

 adaptability of genotypes to a given en- 

 vironment. The greater this effect, however, 

 the greater we would expect the selective 

 advantage to be of genes which increase the 

 reproductive isolation between two diverging 

 races further still. If races continued to di- 

 verge genetically in this way, they would 

 eventually form separate and different gene 

 pools, and instead of being two races of the 

 same species would become two different 

 species. Note that speciation is an irrevers- 

 ible process; once a gene pool has reached 

 the species level, it can never lose its identity 

 via cross breeding with another species. 



In this generalized account of how specia- 

 tion usually occurs races have acted as in- 

 cipient species. But remember that under 

 other circumstances two races can also cross- 

 breed to become a single race. For exam- 

 ple, although several thousand years ago dif- 

 ferent allopatric populations of human be- 

 ings were definitely different races which 

 might have formed different species had the 

 same conditions of life continued, some of 

 these races subsequently merged into one 

 race because civilization and migration facili- 

 tated crossbreeding. 



Gene exchange between races can be hin- 

 dered in several ways. Those barriers lead- 

 ing to complete reproductive isolation in- 

 clude the following: 



1. Geographical. Water, ice, mountains, 

 wind, earthquakes, and volcanic activity 

 may separate races. 



2. Ecological. Changes in temperature, hu- 

 midity, sunlight, food, predators, and 

 parasites may alter or completely change 

 a race's habitat. 



