Direct Evidence that the Ilyodon Morphs are a Single Biological Species

• Published in: Evolution Vol. 38; no. 2; pp. 402 - 407
• Main Authors: Grudzien, Thaddeus A., Turner, Bruce J.
• Format: Journal Article
• Language: English
• Published: Malden, MA Society for the Study of Evolution 01-03-1984; Blackwell
• Subjects: Animal morphology; Animals; Biological and medical sciences; Direct evidence; Evolution; Female animals; Fish; Freshwater; Fundamental and applied biological sciences. Psychology; Genetics of eukaryotes. Biological and molecular evolution; Mating behavior; Mouth; Natural mating; Population genetics, reproduction patterns; Reproduction; Species; Vertebrata; Population genetics; Pisces; Polymorphism
• ISSN: 0014-3820; 1558-5646
• DOI: 10.1111/j.1558-5646.1984.tb00298.x

Certain populations of the goodeid fish Ilyodon occur as bimodal (dichotomous) assemblages of broad and narrow mouthed individuals. These have previously been given species status (I. xantusi and I. furcidens, respectively). Progeny of females inseminated in the field and from laboratory crosses show that each morph (broad and narrow) occasionally produces offspring of the opposite type. These morphs are therefore considered components of a morphological (presumably trophic) polymorphism in mouth width within a single biological species. The genetic basis of the polymorphism is not clear; both disruptive selection and ecophenotypic factors may be responsible for the development of young fish into dichotomous adult populations. A greater proportion of intermediate individuals was observed in broods reared in laboratory conditions than was detected in the field sample. Unknown environmental factors may determine the unimodal or bimodal morphology distributions seen in populations of Ilyodon throughout their range.