Increase in Number of Species 193 



woodpeckers of the genus Dendrocoptis, Miller (1955) suggested 

 that the southeastern D. borealis is a hybrid species between the 

 widespread D. villosus and the southwestern D. scalaris. In all four 

 of these examples it is practically impossible to know whether the 

 probable hybrid species arose by an unusual occurrence of parallel 

 mutations, by hybridization which simply changed one of the parent 

 species, or by the establishment of a hybrid population which 

 eventually became stabilized as an additional species. 



Platysamia, Abies, and DrosophiJa, however, demonstrate the 

 possibility of new phylogenetic lines arising from hybridization. 

 The earliest theoretical stage in such a process would be a single 

 local hybrid swarm. Examples of these are abundant, especially in 

 the plants. In certain Illinois sand areas the two oaks Quercus 

 marilandica and Q. palustris form extensive hybrid populations. 

 Stebbins (1950) and Clausen (1951) cited many more. In the 

 animals smaller numbers of examples have been recorded. Rudd 

 (1955) reported a hybrid population of shrews and other authors 

 have reported local hybrid populations of smaller mammals 

 (McCabe and Cowan, 1945). Hubbs (1955) reported hybrid 

 swarms in several species of fish. Miller (1939) recorded small 

 hybrid populations of juncos from the mountains of southwestern 

 United States. 



An unusual aggregation of local hybrid swarms showing different 

 proportions of parental characters occurs between two red-eyed 

 towhees of Mexico (Sibley, 1950, 1954; Sibley and West, 1958). 

 The northern parent, Pipilo erythrophthalmus, extends as a fairly 

 uniform population into central Mexico and occurs as smaller, 

 isolated populations east and southeast of Mexico City. The other 

 parent is the Mexican species P. ocai which is known in pure form 

 only in areas south of Mexico City. In two of these southern areas 

 both species occur sympatrically with little or no hybridization. 

 Hybrid swarms exist in many other areas between those occupied 

 by typical members of either species (Fig. 85). In this figure the 

 number zero refers to pure P. ocai, 24 to pure P. erythrophthalmiis, 

 and numbers in between to hybrid indexes indicating the average 

 resemblance of each population to the two parental types. Sibley 

 (1950) suggested that Pleistocene events may have caused the 

 ranges of the two parent species to make contact and separate, thus 

 bringing about opportunities for hybridization. 



The mode of origin of these hybrid swarms presents theoretical 

 problems. Normally species in nature either do not hybridize or, 

 if they do, the hybrids backcross with the parents, and no discrete 



