116 



CLASSIFICATION, PHYIOGENY, AND EVOLUTION: 



formation certain principles are formulated that ex- 

 pand our ideas about the mechanisms and character- 

 istics of evolution. 



The present distribution patterns of plants and 

 animals are roughly approximated by six biogeo- 

 graphical realms. Each of these realms is definable in 

 terms of its assemblage of organisms, but there are 

 species in one realm that also live in others. Such 

 biogeographical realms are explainable only in terms 

 of evolutionary history and are best understood in 

 terms of the place where a species originated, its center 

 of origin. The center of origin is the range of the origi- 

 nal population of the species. From this center, mem- 

 bers of the species move out, expanding their range, 

 until some aspect of an environment prevents further 

 spread. The environmental factor that stops the 

 spread of a species is a barrier, including such things 

 as oceans and large mountains. The ocean is a pri- 

 mary barrier between biogeographical realms. Bar- 

 riers within realms are instrumental in the evolution- 

 ary independence among its species. 



Barriers can be biological as well as physical. The 

 general eflPect of biological barriers is to separate or- 

 ganisms having similar life requirements. Naturally, 

 different species having near identical requirements 

 are going to compete for the same things, and in their 

 struggle for existence be separated geographically, 

 each species occupying the environment in which it 

 does best. Such separation of biologically similar or- 

 ganisms usually involves closely related taxa; hence, 

 there are few known instances of closely related 

 species inhabiting the same geographic area. 



GENETICS AND CYTOLOGY 



The entire fields of plant and animal breeding, 

 areas of applied genetics, are examples of man- 

 controlled evolution at a low level. It cannot be de- 

 nied that man took wildlife and by selection devel- 

 oped a variety of domestic animals and cultivated 

 plants. In the case of the horse, a single species of 

 wild animal became a great variety of breeds, each 

 specialized along particular lines for specific purposes. 



Cytology, the study of cells, presents a variety of 

 evidence for evolution. One of the most important 

 phases of cytological study is that of chromosome 

 structure. The number, size, and form of chromo- 

 somes are used both to investigate and to verify lines 

 of evolution. 



TAXONOMY 



Taxonomy is a synthesis of evidences for evolution. 

 Where the evidence is detailed, there is often less diffi- 

 culty in determining relationships and constructing a 

 classification; the various types of information fit to- 

 gether. However, when information is fragmentary, 

 phylogeny is more obscure, and assumptions and con- 

 clusions about past history might be subject to great 

 error. Where information is fragmentary, additional 

 data may change beliefs about the phylogeny of a 

 particular group and become the basis for scientific 

 changes in names. 



EVOLUTION MECHANISMS 



The development of genetic principles offered an 

 explanation of organism variation. During the last 

 half of the nineteenth century, several people, es- 

 pecially Weismann and Naegeli, called attention to 

 the lack of explanation of variation in Darwin's 

 theory. Naegeli, in the earliest days of knowledge 

 about chromosomes, postulated a linear series of 

 determiners (genes) in each chromosome, because 

 of the careful provision in cell division for longi- 

 tudinal duplication of chromosomes. Weismann, 

 while still comparatively young, was afflicted with 

 near-blindness and had to turn from experimental 

 work to purely imaginative speculation. The results 

 were brilliant. From his work in refuting Lamarck- 

 ism, Weismann theorized a continuity of the germ 

 plasm. He said that germ cells, as determiners of 

 heredity, are apart from and practically a parasite on 

 the organism. The germ cells, he said, are potentially 

 immortal, perpetuating themselves in each genera- 

 tion, and are unaffected by any changes in the re- 

 mainder of the body, except death. Moreover, he 

 recognized the constancy of chromosome number in a 

 species. Fertilization was known, but not the cell di- 

 vision that directly or indirectly produces gametes 

 (meiosis); he therefore predicted that there must be, 

 in the formation of the germ cells (sperms and eggs), 

 a regular halving of the chromosome number! 



Back in 1866, Gregor Mendel (1822-1884), an 

 Austrian monk, had discovered the basic laws of he- 

 redity. However, Mendel's work remained unnoticed 

 until 1900, when a number of individuals "redis- 

 covered" these basic laws. One of the first discov- 

 erers pointed out the sudden, spontaneous, discon- 



