118 



CLASSIFICATION, PHYLOGENY, AND EVOLUTION: 



The consequences of isolation can be much more 

 subtle. The primary example of this is the geographic 

 race. Geographic races are groups of populations that 

 replace one another geographically, each race having 

 one or more identifying features. These characteris- 

 tics may be no more than the greater prevalence of 

 certain traits or they may be unique; however, unique 

 features are unlikely without geographic isolation. 



Subtle isolation is found where individual popula- 

 tions of a species are not strictly isolated from one 

 another. Rather, adjacent populations do contact 

 and interbreed with one another; however, in the 

 linear replacement of one race by another there is a 

 subtle factor of isolation between the remote popula- 

 tions. In such cases it is not uncommon to discover 

 that two spatially remote races, although linked by 

 races that freely interbreed with their neighboring 

 races, cannot produce offspring. Moreover, this isola- 

 tion is not due to some unknown mechanism; normal 

 isolation mechanisms that accumulate over distance 

 can be shown to be the cause. Therefore, the type of 

 isolation that distinguishes species can exist between 

 populations that are somewhat distant from one 

 another but are connected by a complete chain of 

 interbreeding races. This suggests a like means of 

 origin of both races and species. 



A special case of subtle isolation of geographic 

 races can be especially confusing locally. If races of 

 the above type form a circular rather than a linear 

 geographic distribution pattern, it is possible for two 

 races to be overlapping, quite difTerent in appearance, 

 and unable to produce offspring. In the area of over- 

 lap, the two races behave as good species; however, 

 one can find step-by-step interbreeding of races 

 around the circle of races. 



In some cases isolation mechanisms that cause ab- 

 solute barriers between species and between portions 

 of the same species break down between diHerent 

 species. In such hybridization between species, the 

 offspring may be either unable to develop, less viable 

 and unable to reach reproductive age, more vigorous 

 than either parent but sterile, or viable and able to 

 reproduce. Hybrids that reproduce clearly represent 

 the breaking down of isolation mechanisms. Most of 

 these hybrids ])roduce only a lew offspring — so few 

 that both the hybrids and their offspring are ex- 

 tremely rare. However, in plants more than in ani- 

 mals there is a tendency for some interspecific hy- 

 brids to contribute to the evolution of one or both 



of the species that hybridize. A prime example of this 

 is called inlwgressive hyhndizalion, a phenomenon in 

 which hybrid ofTspring cross back to one or both 

 parents and cause the genes of one parental species 

 to become a part of the gene complement of a por- 

 tion of the other parental species. Another possible 

 consequence of plant hybridization is the production 

 o{ new polyploid (more than two sets of chromosomes) 

 species. The multiple chromosome sets of the hybrids 

 cause low genetic compatibility for crosses with 

 either parent; however, the polyploids often can cross 

 among themselves (hence act as and are a single 

 species). This is not as remarkable as it might seem; 

 genetic mechanisms cause greater potential reproduc- 

 tive success when hybrids cross among their own kind 

 than with either parent type. 



SPECIATION 



The means of species formation could be of two 

 major types, nongeographic and geographic, .\ori- 

 geographic specialion must occur within the area of the 

 parent species. In animals, it is extremely unlikely; at 

 best it must be a rare phenomenon. The only animal 

 possibilities appear to be in self-fertilizing hermaph- 

 roditic or in parthenogenetic forms; however, firm evi- 

 dence for such speciation does not exist. In plants, a 

 common type of nongeographic speciation was said to 

 come from interspecific hybridization creating poly- 

 ploid species. Although other types of nongeographic 

 plant speciation are possible, they are probably of 

 minor significance. Therefore, the only kind of non- 

 geograpiic speciation that is of significance happens 

 when plant species hybridize and form polyploids, 

 and the most significant type of speciation is geo- 

 graphic. (Geographic speciation is the simple case of ge- 

 netic change and spatial separation allowing the 

 origin of isolation mechanisms. 



HIGHER-CATEGORY FORMATION 



The origin of new genera, families, orders, classes, 

 phyla, and kingdoms may appear (at least for the 

 kingdoms) as steps so great that some unique mecha- 

 nism is required. However, there really is no absolute 

 need or proof of such evolution. The fragmentary 

 fossil record does not imply such "big steps" in evolu- 

 tion. The time was available for the necessary de- 

 velopments. The simple process of geographic isola- 

 tion and mutation during the time represented by a 



