THE GROUPS AND ANCESTRY OF ORGANISMS 



117 



tinuous variations in heredity and called these muta- 

 lions. Later, the work of a great many men studying 

 genetics tied the visible change of traits in mutants to 

 changes in the chromosomes themselves. Therefore, 

 in place of Darwin's infinite variations, biologists now 

 speak of mutations as the basic material of evolution. 



This idea of sudden, spontaneous change in hered- 

 ity emphasizes the importance of an aspect of geo- 

 graphic distribution in evolution. This aspect is the 

 factor of isolation. Two groups of the same species 

 may be isolated by a mountain or other barrier. 

 Within each group there is the possibility of different 

 sets of chance mutations arising, being incorporated 

 into the populations, and leading to speciation (Fig- 

 ure 7.9). But final isolation between species is not 

 only geographic; it may be physiological, such as ge- 

 netic factors limiting cross-fertility or normal de- 

 velopment of an embryo; it may be anatomical, such 

 as structure preventing breedings; it may be psycho- 

 logical, such as behavior patterns preventing breed- 

 ing; ecological, such as environmental barriers 

 preventing different populations' coming together; or 

 seasonal, such as diflTerent populations' living in the 

 same space but breeding at different times of the year. 

 However, in all of these isolation mechanisms the 

 basic causes of species formation are genetic change 

 (mutation) and natural selection that first occurs dur- 

 ing geographic isolation. 



NATURE OF ISOLATION 



The different types of isolation work in one or more 

 of three ways. They may prevent mates from meet- 

 ing, prevent mates from breeding when they do meet, 

 and/or prevent offspring from developing normally. 

 Geographic isolation bars the contact of mates. Phys- 

 iological isolation keeps offspring from developing 

 normally. Anatomical isolation may physically ob- 

 struct breeding. Psychological isolation prevents the 

 fulfillment of courtship activities. Seasonal isolation 

 causes organisms to reach the breeding state at differ- 

 ent times. Ecological, or habitat, isolation is a special 

 aspect of geographic isolation. 



Isolation mechanisms rarely work independently. 

 Consider the possibilities in two geographically iso- 

 lated populations of a single species. Independent ge- 

 netic mutations and selection of mutant genes can 

 take place. Some of these mutations start the devel- 

 opment of any to all of the nongeographic isolating 



CONTINUOUS DISTRIBUTION 



DISCONTINUOUS DISTRIBUTION 



gene mutation and selection occur 

 Independently 



BARRIER BREAKDOWN 



intermixing reveals 

 biological isolation^ hence 

 two distinct species 



Figure 7.9 Geographic speciation, showing how genetic change dur- 

 ing isolation normally is necessary for the origin of new species. 



mechanisms between the populations. Therefore, if 

 the organisms once again come together because the 

 barrier between them is destroyed, the two groups 

 may interbreed but lack fertility; the offspring may be 

 stillborn, or the young may be less vigorous than 

 normal and die shortly after birth. In other words, 

 some phase of physiological isolation may be present. 

 In addition, some ramification of anatomical, behav- 

 ioral, and/or seasonal isolation may exist. If one or 

 mo.'e of these isolation mechanisms prevents the de- 

 velopment of offspring from crosses between the two 

 populations, there is no doubt that at least one new 

 species has evolved; the two old populations of a 

 single species are now two distinct species. 



