188 



Increase in Number of Species 



SPECIES COMPLEX 



Vit is and 

 Parthenocissus 



Fig. 81. Phylogenetic tree of the comes group of the leafhopper genus 

 Enjthroneiira. Species names of the leafhoppers have been omitted but each 

 apical Hne represents one species. 



several other eastern American insect groups having a lower vagility 

 (conducive to species fission) but nevertheless having only eight 

 to 16 species each. Both circumstances are suggestive of host isola- 

 tion as the factor producing the unusually large number of species 

 in these three species flocks of Erythroneura. 



The mating behavior in this genus is such that it appears to 

 confer almost perfect genetic isolation by host. All observations to 

 date indicate that in both sexes the stimulus for mating usually 

 occurs after the species is on its breeding host. After the local 

 populations become thus stratified, mating and egg-laying ensue. 

 Thus if a group of individuals became established on and habituated 

 to a new host, they would be fairly well isolated genetically as a 

 breeding population from the parent species in the same locality. 

 How the transfer is effected and the habituation becomes estab- 

 lished is not known. 



With both the fleas and the Erythroneura leafhoppers, circum- 

 stances of host, phylogeny, and life history indicate sympatric 

 species fission by a host transfer mechanism. 



Reviewing the evidence in bisexually reproducing organisms, it 

 appears that, in species fission, ecological isolation does not have 

 the same widespread importance as geographic isolation. In con- 

 trast, in organisms having either rigid host specificity or peculiari- 

 ties of biological timetable, ecological isolation may be an important 

 factor in the increase in number of species. 



