Summary:  When parthenogenetic populations of terrestrial animals have different geographic distributions than closely related bisexual populations, the sexual forms tend to occupy regions of higher biotically imposed stress, generally being found at lower latitudes and altitudes, in mesic rather than xeric areas, on the mainland as opposed to islands, and in undisturbed as opposed to disturbed habitats.  If sexual reproduction is favored by unpredictably changed conditions, then it would appear that regions of greater biotic stress are more unpredictable than those where abiotic factors are a major source of stress.  We suspect that this greater unpredictability stems from interspecific interactions (competition, predation).  We further suggest that the changing genotypes of the organisms with which an individual interacts are the major source of this biotic uncertainty; that, once evolved, sex in one population may lead to the contagious spread and persistence of sex in a community of highly interacting individuals.
 
 

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