23 



between clones from different states were rejected, however. This im- 

 plies that the species has been derived de novo several times, or that a 

 single clone has "speciated" in different parts of its range. This work 

 supports current thought in genetics that environmental and genetic uni- 

 formity are correlated. 



62. — . 1977b. Animal parthenogenesis: a new evolutionary-ecologi- 

 cal model is needed. SCIENCE 197(^306): 837-8't3. 



It is suggested that the weed hypothesis concerning parthenogene- 

 sis in Cnemidophorus is correct, but not the claim for distinct habi- 

 tats for each species within the broad geographic "weed" area, as each 

 such habitat is a local climax formation and not weedy. It is suggest- 

 ed that those parthenospecies for which specific habitats are proposed 

 exhibit distinct riparian-dwelling affinities. Theories and models of 

 obligatory parthenogenesis are reviewed. There are three compelling 

 reasons for believing that parthenogenetic species can only evolve in 

 isolation from the generating bisexuals: hybridization by males of con- 

 generic species would impede clone establishment, competition would im- 

 pede clone expansion, and present distributions show largely distinct 

 habitats between congeneric uinsexual and bisexual species. Hence, it 

 is reasonable to assume that parthenogenesis evolves either at the per- 

 iphery of the range, or if within the range, in areas periodically de- 

 void of the generating species. It is suggested that parthenogenetic 

 species rely on novel habitats, and that the availability of habitat is 

 the key to success, not the meiotic ability to produce unreduced eggs. 

 The salient feature of the distribution of several species of unisexual 

 Cnemidophorus ( exsanguis, neomexicanus, tesselatus, uniparens and ye- 

 lox ) is the tendency to be floodplain dwellers. Range maps of neomexi- 

 canus and tesselatus in relation to drainage patterns are presented. 

 These 5 species also occupy climax communities, but usually where bi- 

 sexual species are absent. _C. inornatus, a bisexual species, is an ex- 

 ception; it occurs sympatrically with several unisexuals in disturbed 

 areas. Conversely, the bisexual C_. tigris is almost exclusively re- 

 stricted to adjacent climax communities characterized by sandy soils. 

 In certain localities of southern New Mexico (in the vicinity of Ele- 

 phant Butte Reservoir and the Rio Grande) it occurs abundantly in mix- 

 ed mesquite-creosote associations, but is virtually absent from adja- 

 cent pure stands of creosote growing in gravelly soils. Edaphic con- 

 ditions appear to be important. 



The significance of disturbed habitats to parthenogenesis in other 

 kinds of animals is reviewed, along with the displacement of bisexual 

 species by unisexuals. Cytogenetic factors are important in the evolu- 

 tion of parthenogenesis. It is suggested that parthenogenesis is more 

 advantageous in non-territorial rather than territorial animals because 

 the latter would expand too slowly to take advantage of disclimax situ- 

 ations. This explains the absence of unisexual species of birds and 

 mammals; they are so vagile that sexual species would recolonize dis- 

 turbed areas too rapidly for unisexuals to establish themselves. Clone 

 succession is proposed as better-adapted clones replace less-adapted 



