Species and Species Change 117 



in which they appear to be species on the basis of recognizable 

 characteristics but are not genetically isolated from each other. 

 This situation, which is explained in more detail in the next chapter, 

 leads to much confusion in defining species. What these in between 

 categories are called is not important as long as they are recognized 

 as evidence of a dynamic phase of evolution. 



A further point of confusion about the definition of species is 

 encountered in the geological record. If fossil representatives were 

 available for every few generations in a phylogenetic line, the 

 samples would grade into each other and form a gradually chang- 

 ing series, as from Species 1 to Species 2 in Fig. 45. When it is 

 desirable to divide such a series into species categories, this must 

 be done on a purely arbitrary scale. Usually the fossil record of any 

 one phylogenetic line represents forms far apart in time and hence 

 moderately different from each other. In these cases, however, new 

 fossil finds may unearth populations tending to bridge the previously 

 known morphological gaps, necessitating the use of arbitrary 

 measures in defining the species (Simpson, 1951). 



Individuals in species which reproduce asexually do not form 

 an interbreeding population and cannot be tested regarding genetic 

 similarity with individuals of other species thought to be different. 

 Asexual lines sometimes possess marked physiological differences 

 but no other diagnostic criteria. Many of the lines breed true in 

 remarkable fashion and in this respect may form unusually stable 

 phylogenetic lines. These asexual species can be defined only ar- 

 bitrarily on the basis of designated standards of difference and 

 similarity. These similarities may include both morphological and 

 physiological traits (Hoare, 1957). 



Asexual lines appear to have arisen as side branches from bi- 

 sexual lineages at various times in evolutionary history. The great 

 bulk of evidence indicates that most of the continuing lineages in 

 evolutionary history comprise bisexually reproducing organisms, a 

 conclusion greatly strengthened by discoveries concerning sex in 

 microorganisms (Wenrick, Lewis, and Raper, 1954). The bisexual 

 species, therefore, are of special evolutionary importance and 

 are the type discussed in this chapter. 



Most of the species found living together (that is, sympatric 

 species) are genetically isolated from each other in rigid fashion, 

 with the result that each species forms a distinctive interbreeding 

 population. The remarks in this chapter apply especially to these 

 species well defined in this genetic sense. 



