674 



ECOLOGY AND EVOLUTION 



in two diffeient organisms may be analo- 

 gous in some cases and homologous in 

 others. In the instance of analogous ab- 

 sence of a character, the common ancestor 

 may be presumed to have had the charac- 

 ter in question. In the case of homologous 

 absence of a character, the common ances- 

 tor also lacked the character. The loss of 

 eyes in various species of cave isopods 

 (Asellidae) exemplifies the point. These 

 cave crustaceans were fomierly classified 

 under the generic name of Caecidotea on 

 the assumption that the regressive charac- 

 ters were homologous. Evidence now indi- 

 cates the separate convergent evolution of 

 eyeless cave forms within the genus Asel- 

 liis, and the polyphyletic group Caecidotea 

 consequently is placed in synonymy with 

 Asellus (Miller, 1933; Van Name, 1936, p. 

 465). 



In laboratory animals, eye reduction 

 often results from simple genetic mutations 

 (Chase, 1944, 1945). It seems prob- 

 able that reduction of eyes in cave or sub- 

 terranean forms is genetically complex. 

 Wide variation in the degree of regression 

 of different parts of the eye and associated 

 structures is found among the various blind 

 species. 



An ecocline within an interbreeding 

 population of characinid fishes from eyed 

 river forms {Astyanax mexicanus. Fig. 247), 

 which have a widespread variation in 

 eye size, to blind cave forms {Anoptich- 

 thys jordani; Fig. 247), has been described 

 from La Cueva Chica in the state of San 

 Luis Potosi in Mexico (Breder, 1942, 1943, 

 1943a). The gradation, from "normal" eyes 

 of various sizes through uncovered sunken 

 eyes and covered sunken eyes to blind 

 forms with little eye structure, is correlated 

 with a gradation in loss of pigmentation. 

 These regressive characters are more pro- 

 nounced the farther the fishes are from the 

 light and the mouth of the cave. Five gen- 

 erations of blind forms raised in the light 

 retained the blind condition. Mating of 

 eyed, pigmented fishes with blind, light- 

 colored fishes produced all eyed and pig- 

 mented forms. Specimens with degenerate 

 eyes on only one side, however, indicate 

 some possibility of physiological degenera- 

 tion even with the same heredity. 



These data suggest that a number of 

 alleles or multiple genes control the expres- 

 sion of the characters under diflFerential se- 



lection, migration, and physiological condi- 

 tions of the river population compared with 

 the cave population. The interbreeding 

 population would indicate either a hybridi- 

 zation between the river and cave forms, 

 or that the cave form is an ecotype (ecolog- 

 ical subspecies) of the river form, rather 

 than a separate genus, as originally de- 

 scribed (Pavan, 1946; see p. 612). The two 

 populations may have been isolated at one 

 time, and this isolation may have broken 

 down from subterranean connections or 

 possible flooding. Then again, the two 

 populations may be under different selec- 

 tion pressures at the two ends of the cline, 

 a partial isolation at the mouth of the cave 

 giving rise to the observed stepped cline. 



Another cave fish showing greater eye 

 reduction and absence of a connecting optic 

 nerve has been found in the neighboring 

 cave, Cueva de los Sabinos (Breder, 1944). 

 It shows further regressive evolution and 

 modification of the skull. The direction of 

 this modification is indicated by Breder 

 through the comparison of polar coordinates 

 (Fig. 247) of the normal and two blind 

 forms, as well as the reactions of the fishes 

 to light. The sensory apparatus shows re- 

 duction in the evolutionary series except 

 in the organs of taste, and possibly in the 

 olfactory mechanisms (Breder and Rasquin, 

 1943). The eyed fishes use dark retreats 

 only when escaping or when the dark 

 water has a higher temperature. 



The blind Cueva Chica fishes avoid fight, 

 while the bhnd Cueva de los Sabinos forms 

 are indifferent to it. The river fishes school 

 while the bhnd ones do not, and those 

 from the river school with their own type 

 on the basis of sight, thus tending to avoid 

 cave forms. Such behavior differences may 

 well produce the partial isolation between 

 the river and cave types (Breder and Gres- 

 ser, 1941). The tendency of eyed fishes to 

 enter caves is the result of negative photo- 

 taxis, positive rheotaxis, and positive ther- 

 motaxis. The blind Cueva Chica forms tend 

 to stay in the cave because they do not 

 school, are negatively phototactic, positively 

 rheotactic, and move toward warmer water. 



It is assumed that blind individuals 

 would not survive long in the river with 

 normal predators. In an experimental pool, 

 half simulating cave conditions and half 

 open pond, out of an initial population 

 of ten bhnd types and nine river types, one 



