Fondren Science Series


There are several reasons for using chromosome form and number (the karyotype) in phylogenetic studies. Since the material of the genotype forms part of the chromosomes, the karyotype is less influenced by external factors than are other morphological and physiological characteristics (John and Lewis, 1966). There are, however, two obvious ways natural selection can act on changes in the karyotype. The first involves the ability of the karyotype to proceed through the mechanics of mitosis or meiosis. If mitosis is blocked the cell line would die. If the rearrangement interferes with proper functioning of meiosis the result is reduced or there is no gamete production. In either case a new karyotype must pass this test, regardless of the advantage it confers to the animal's phenotype. Further, this aspect of natural selection would act before the new karyotype could be inherited. The second way in which natural selection could favor one karyotype over another would be the degree of phenotypic fitness as the result of different karyotypes. That karyotypic variation can affect the phenotype is well documented (see John & Lewis, 1966, for a review). Thus, from a genetic standpoint the karyotype offers a unique morphological level for study.

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