220 I The Process of Evolution 



Closely Related Isolates 



Species Swarms in Fishes. The east African lakes, Victoria, 

 Tanganyika, and Nyassa, support a large number of closely related 

 fish species of the family Cichlidae. For example, in Lake Victoria 

 are found some 70 endemic and 6 nonendemic species of the genus 

 Haplochromis living in three different ecological zones. One group 

 consists of deep-bodied forms with short snouts, horizontal mouths, 

 equal jaws, and bicuspid outer teeth (Fig. 10.4). These fish are 

 found inshore and are bottom feeders. The cichlids of a second 

 group have more slender bodies and longer snouts, their mouths 

 are slightly oblique, their lower jaws prognathous, and their outer 

 teeth conical and caniniform (Fig. 10.4). The members of this group 

 are fish-eating predators, hunting the middle depths of open and 

 inshore waters. A third group of Haplochromis are slender and long- 

 snouted. They have very oblique mouths (in two forms almost 

 vertical), extreme prognathism of the lower jaw, and caniniform 

 outer teeth (Fig. 10.4). These are predaceous surface feeders, eat- 

 ing principally other fish and insects. 



There is only a moderate amount of diversity in this large com- 

 plex of closely related distinct clusters. Although some of the forms 

 are virtually indistinguishable morphologically, they have been 

 found to be ecologically differentiated and to have distinctive breed- 

 ing coloration. The greatest morphological variation is in the teeth 

 and structures of the head, which is hardly surprising in view of 

 the diverse feeding habits within the group. 



Sibling Species of Alpine Butterflies. In some cases, superficial 

 similarity may disguise a rather large amount of diversity. Lorkovic 

 has shown that the holarctic butterflies of the Erebia tyndarus group, 

 although very much alike in outward appearance, have wide di- 

 vergence in chromosome number (n = 8, 10, 11, 15, 24, 25, 51, and 

 perhaps 52) and (to a lesser extent) in the morphology of the male 

 and female genital structures. Such outwardly similar forms are 

 often called sibling species. In the western Alps (Fig. 10.5) two 

 forms, Erebia cassioides (n = 10) and E. nivalis (n = 11), occupy 

 two barely overlapping ecological zones, the former in the subalpine 

 (1,400 to 2,400 meters) and the latter in the alpine (2,300 to 2,700+ 

 meters). Although E. cassioides and E. nivalis share a narrow border 

 strip, there is little evidence of gene flow between them. Only 2 of 

 400 specimens examined were not unequivocally assignable to one 

 species or the other. The two forms have quite distinct life cycles, 

 E. cassioides completing its development in one year, E. nivalis 



