432 EVOLUTION, GENETICS, AND EUGENICS 



to changed environmental requirements took place, such an isolated 

 group would already have a good start toward becoming a separate 

 race or species because of the effects of inbreeding. 



Isolation and selection. — Divergence may also be promoted by 

 selection. When an isolated group of a species finds itself in a different 

 environment from that of the parent stock, selection may tend to pre- 

 serve individuals with either the dominant or the recessive of any unit 

 character or any combination of these that may happen to be more 

 advantageous than others under the changed conditions of life. In this 

 way some genes may be entirely eliminated from a stock, thus increas- 

 ing the genetic differences between the parent species and their isolated 

 derivatives. If, for example, a small section of a species were to become 

 isolated in a region where the available food was quite different from 

 that in the main range of the species, some of the individuals, because 

 of favorable combinations of unit characters, might immediately find 

 themselves better equipped to cope with the new food conditions 

 than others. Those adapted to the new food conditions would live 

 and breed together and would tend to become homozygous for the 

 various characters that favored their survival under the new condi- 

 tions. 



THE VARIOUS CATEGORIES OF ISOLATION 



a) Geographic isolation. — Taxonomists who have studied and 

 plotted the distribution of species of animals and plants are most 

 familiar with the effects of geographic isolation. They consider that 

 one of the most certain facts of nature is that isolation is always ac- 

 companied by marked differences between the isolated branches of a 

 species. The late David Starr Jordan, 1 a leading student of geographic 

 isolation in America, discusses the subject in a very illuminating 

 fashion : 



"It is now nearly forty years since Moritz Wagner (1868) first 

 made it clear that geographic isolation (raumlicke Sonderung) was a 

 factor or condition in the formation of every species, race, or tribe of 

 animal or plant we know on the face of the earth. This conclusion 

 is accepted as almost self-evident by every competent student of 

 species or of the geographical distribution of species. But to those 

 who approach the subject of evolution from some other side the 

 principles set forth by Wagner seem less clear. They have never been 

 confuted, scarcely ever attacked, so far as the present writer remem- 



1 Science, N.S., Vol. XXII (1905) 



