180 Increase in Number of Species 



the American composite genus Layia, the six species having 7 

 gametic chromosomes can interbreed freely and successfully. In 

 the related genus Holocarpha, in contrast, although the species are 

 all closely related, each has a distinctive chromosome morphology 

 (Fig. 78), and potential interbreeding is greatly reduced or elimi- 

 nated. Clausen pointed out that these populations tend to be 

 isolated in valleys. The occurrence of mutations in chromosome 

 structure (see Fig. 78) apparently produces certain biotypes which 

 are better fitted for the local environment than is the parent type 

 with the result that a relatively homozygous local population soon 

 evolves which can no longer form harmonious zygotes with gametes 

 from other areas. Each such population is, as Clausen clearly rec- 

 ognizes, a new phylogenetic hne thoroughly independent in an 

 evolutionary sense. The decision of the systematist not to name 

 these phylogenetic lines as species does not detract from their 

 intrinsic evolutionary importance. 



In Californian populations of certain species of Clarkia, Lewis 

 (1953a) demonstrated conditions comparable in many respects to 

 the Holocarpha populations. In Clarkia the species also occur in 

 dense local colonies, have a low vagility, and also have a tendency 

 to produce chromosomal mutations better adapted to more xeric 

 conditions than are the parental types (Lewis, 1953Z?). Test popula- 

 tions of Clarkia deflexa (Fig. 79) indicate that many populations 

 are genetically isolated from certain others and that one (G) may 

 already be an independent phylogenetic line. Because several 

 colonies are genetically compatible only with distant colonies with 

 which they seldom if ever cross, a large number of colonies may 

 be evolutionarily independent from a practical standpoint. In com- 

 paratively recent time the distinctive species Clarkia lingulata ap- 

 parently evolved from such a local colony of C. biloba by a rapid 

 process involving the addition of a chromosome to the genome plus 

 chromosomal mutations involving a translocation and an inversion 

 (Lewis and Roberts, 1956). 



THE ISOLATION MACHINE 



The foregoing pages attest the prevalence and importance of 

 geographic isolation as a factor in species fission. On this basis, 

 geographic regions having either a ring or string of islands or a series 

 of island-like habitats in any ecological setting would constitute 

 a veritable species machine. The combination of alternating range 

 disruptions and conjunctions coupled with the possibilities of colo- 



