320 Comparative Evolution of Biomes 



come about only in one area even though apparently equally propi- 

 tious circumstances occur in other areas also. 



Some investigators have suggested that the number of species 

 increases in direct proportion to the size and age of the area (Willis, 

 1922). The example of Enjthroneura above indicates the fallacy 

 of this "age and area" theory, because in this group of Enjthroneura 

 the older fauna on the larger Eurasian area has only about a fourth 

 the number of species found in the younger fauna in the single, 

 smaller North American segment. 



Regional differentiation on a relatively small scale of evolutionary 

 divergence occurs in two groups of grass-feeding leaf hoppers. In 

 one, the compact Stirellus group, four genera including StireUiis 

 and Gillettiella occur in North America, and about a dozen genera 

 including Acontira and Paivanana occur in Eurasia (Oman, 1949). 

 In the other, the Doratura group, Athijsanella and Driotura occur 

 in North America, whereas Doratura and possibly others occur in 

 Eurasia. Evidence from phylogeny and distribution suggests that 

 early Cenozoic members of each group dispersed between Eurasia 

 and North America and since then have evolved on each continent 

 into a number of readily identifiable but otherwise similar genera 

 inhabiting comparable grassland communities. All of these genera 

 are relatively southern in distribution, which may explain the ap- 

 parent lack of intercontinental dispersal since the evolution of the 

 present-day genera. 



An important mechanism contributing to the increased diversity 

 of isolated regions of a biome concerns colonists from other biomes. 

 A biome is continually enriched by species from adjoining biomes 

 which become adapted to live in it. Isolated sections of a biome 

 have different neighbors, and the act of colonization is undoubtedly 

 highly random and unpredictable. The unequal result is demon- 

 strated by the desert biome. The African region was colonized by 

 a branch of the spurge family Euphorbiaceae, which has since 

 evolved into many cactus-like species. The American regions of the 

 desert biome were colonized by the cactus family Cactaceae which 

 has also evolved into many prickly species. 



A less striking example occurs in the widespread grass-feeding 

 leafhopper subfamily Hecalinae. Several American lines became 

 xeric-adapted and now, in the form of the genera Memnonia, Hecul- 

 liis, and Dicyphonia, are distinctive elements of the arid prairie 

 biomes of central and southwestern North America. From mesic 

 grassland Icafhoppers allied to Deltoceplialus-likc lines evol\ ed dis- 

 tinctive xeric American genera such as Lonatura and Unoka (De- 



