The Differentiation of Populations I 217 



Definition of Symbols 



A, Head width/head length 0.92 or 



greater 



B, 50'^'r or more with tibia bars 



C, Average number of tibia bars 



(when present) 1.4 or greater 



D, 50''/r or more without femur bar 



E, 50% or more without tympanic 



spot 



F, 50% or more with Hght reticulum 



G, Number of dorsal spots less than 



13 

 H, Number of lateral spots 12 or more 



I, More lateral than dorsal spots 



|, 50% or more without lateral 

 reticulum 



K, 50% or more of males with ovi- 

 ducts 



L, 50% or more of males with no, or 

 poorly developed, external vocal 

 sacs 



a, Head width/head length less than 



0.92 



b, Less than 50% with tibia bars 



c, Average number of tibia bars ( when 



present) less than 1.4 



d, Less than 50% without femur bar 



e, Less than 50% without tympanic 



spot 



f, Less than 50% with light reticulum 



g, Number of dorsal spots 13 or more 



h. Number of lateral spots less than 

 12 



i, Lateral spot number equal to, or 

 less than, dorsal spot number 



], Less than 50% without lateral 

 reticulum 



k, Less than 50% of males with ovi- 

 ducts 



1, Less than 50% of males with no, 

 or poorly developed, external 

 vocal sacs 



living from southern Canada to Mexico, has a range of 15 to 32 °C; 

 where these two overhip, R. sylvatica breeds in the early spring, R. 

 catesbiana in midsummer. ) 



Laboratory crosses of individuals from difiFerent populations of 

 R. pipiens produce normal offspring when the parents are drawn 

 from populations that are geographically adjacent (e.g., central and 

 southern Florida) or lie at roughly the same latitude (e.g., Texas 

 and central Florida). However, the greater the north-south gap 

 separating the home populations of the parents, the greater also is 

 the proportion of defective (inviable) oflFspring. Eggs from Ver- 

 mont females fertilized by New Jersey or Wisconsin males do not 

 differ in development from those fertilized by Vermont males. If 

 the spermatozoa that fertilize the eggs come from a Louisiana male, 

 some abnormal development occurs, but there is no significant in- 

 crease in mortality. Texas-Vermont hybrids have many develop- 

 mental difficulties, and mortality may reach 100 percent. In a 

 single cross between a male from an eastern Mexican population 

 and a Vermont female most of the hvbrid embrvos died in the 

 gastrula or neurula stage. Thus, the R. pipiens situation might be 

 considered analogous to that of Ensatina discussed below. In the 



