250 INTRODUCTION TO EVOLUTION 



cerning the australopithecines. They are customarily separated from Homo 

 sapiens by placing them not only in a different species, but also in a dif- 

 ferent genus and even in a different subfamily (Australopithecinae). 

 This subfamily is usually divided into two or more genera (p. 235). On the 

 other hand, Mayr ( 1950) suggested that all of them be included in our own 

 genus and in one species, and called Homo transvaalensis. Grouping them 

 in one species suggests the unlikelihood that they would have been repro- 

 ductively isolated from each other. 



It will be noted that according to the suggested classification, all men of 

 whatever geologic age would be placed in genus Homo (Mayr, 1950; 

 Brown, 1958). Variation within the genus would be recognized by separat- 

 ing the various forms, especially those living at different times, into dif- 

 ferent species. Men living at any one time would usually be considered 

 to belong to one species, though there would be exceptions. Thus, as we 

 have noted, australopithecines and Pithecanthropus may have lived as 

 contemporaries during portions of their respective existences. Probably 

 they were so unhke that they would not have intermarried if they had 

 come into contact with each other, and hence the placing of them in sepa- 

 rate species (//. transvaalensis and H. erectus) would be justified. 



Races of Homo sapiens 



We have noted that modern men are usually considered to belong to 

 one species, despite racial differences. 



What is a race and how does it differ from a species? This question will 

 receive further attention in our discussion of classification (pp. 320-323), 

 where the point is made that race is equivalent to the subdivision of a 

 species known as a subspecies. Here we may note that races or subspecies 

 differ from species by the absence of that reproductive isolation forming 

 an important hallmark of species. In addition to reproductive isolation, 

 one species differs from another in some of its genes. The difference in 

 genes usually manifests itself in differences in visible structure and char- 

 acteristics, although not always. Similarly, one race differs from another 

 in some of the genes present, but in this case the differences are usually 

 less than they are between species. Indeed, the differences between races 

 are more likely to take the form of variations in frequencies of occurrence 

 of certain genes than they are to manifest themselves in the form of pos- 

 session of certain genes by one race, with absence of those genes in an- 

 other. 



Thus races usually differ from species in two ways: (1) absence of re- 



