Ways of Speciation in Fishes

Abstract

recombinations, becomes more and more different from the parent stock. As a byproduct of genic differentiation, the isolated form will finally develop characters by which fertility with the original species becomes impossible; speciation is the result of a prolonged geographic isolation. Just the inverse type of speciation would consist of perhaps one single mutation by which individuals bearing the mutated gene, e.g., in a homozygous state, are preferentially or absolutely restricted to mate inter se. In such instances of homogamy, the parent species would be split into two new species, morphologically alike but sexually isolated from one another. As a result of homogamy and of the lack of genic exchange between the two species, morphological and physiological differentiation will follow; our knowledge about this second manner of speciation still is quite limited. The problem of intralacustrine speciation has been discussed for some time by many authors. Some vigorously support the idea of speciation only by isolation in space even within a single lake basin, i.e., by microgeographical isolation. In some instances the probability of correctness of such a view is great. Especially shallow lakes may dry up except for some springs around the border of the lake. In Anatolia, Lake Aci gol is a good example of this type. During the summer months, the shallow lake is transformed