EVIDENCE FOR SELECTION AGAINST HYBRIDS IN THE FAMILY CYPRINIDAE (GENUS NOTROPIS)

Notropis cornutus and N. chrysocephalus are two cyprinid fishes which hybridize extensively in the midwest. In this paper, statistical analysis was used to determine if the deficiency of Notropis cornutus × N. chrysocephalus hybrids can be attributed partly to selection against hybrids. Principal component analysis was used to construct a folded hybrid index (FHI) using morphological and electrophoretic characters which distinguish N. cornutus, N. chrysocephalus, and their hybrids. A Wilcoxon signed‐rank test comparing mean FHI between successive age classes within cohorts and regression analysis of FHI on age supported the conclusion that hybrids are less fit than parentals in nature; however, this analysis did not include a test of selection at zygotic and larval life history stages or partial infertility of hybrids. Selection eliminated hybrids at a rate of 9.2% per year. Considering the intensity of selection and the age of hybridization, it is surprising that premating reproductive isolating mechanisms have not been perfected by selection against gametic wastage as predicted by classical speciation theory. It appears that the reinforcement of premating isolating mechanisms is an extremely slow process, if it occurs at all, despite apparently intense selection against hybrids.

[1]  W. Moore,et al.  STABILITY OF THE NORTHERN FLICKER HYBRID ZONE IN HISTORICAL TIMES: IMPLICATIONS FOR ADAPTIVE SPECIATION THEORY , 1985, Evolution; international journal of organic evolution.

[2]  T. Dowling,et al.  Level of Reproductive Isolation between Two Cyprinid Fishes, Notropis cornutus and N. chrysocephalus , 1984 .

[3]  Gerald R. Smith,et al.  Origin and Geography of the Fish Fauna of the Laurentian Great Lakes Basin , 1981 .

[4]  E. Nevo,et al.  ORIGIN AND EVOLUTION OF ETHOLOGICAL ISOLATION IN SUBTERRANEAN MOLE RATS , 1981, Evolution; international journal of organic evolution.

[5]  K. Kaneshiro.  SEXUAL ISOLATION, SPECIATION AND THE DIRECTION OF EVOLUTION , 1980, Evolution; international journal of organic evolution.

[6]  W. Moore.  A single locus mass-action model of assortative mating, with comments on the process of speciation , 1979, Heredity.

[7]  D. Buth.  Biochemical systematics of the cyprinid genus Notropis—I. The subgenus Luxilus , 1979 .

[8]  H. Paterson.  More evidence against speciation by reinforcement , 1978 .

[9]  W. Moore.  An Evaluation of Narrow Hybrid Zones in Vertebrates , 1977, The Quarterly Review of Biology.

[10]  B. W. Menzel.  Biochemical systematics and evolutionary genetics of the common shiner species group , 1976 .

[11]  C. Gilbert.  Hybridization versus Integradation: An Inquiry into the Relationship of Two Cyprinid Fishes , 1961 .

[12]  Carl L. Hubbs,et al.  Hybridization between Fish Species in Nature , 1955 .

[13]  T. Dobzhansky.  Speciation as a Stage in Evolutionary Divergence , 1940, The American Naturalist.

[14]  N. Marshall.  Annulus Formation in Scales of the Common Shiner, Notropis cornutus chrysocephalus (Rafinesque) , 1939 .