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nation of several such mechanisms. In short, a species is the most inclusive Mendelian population.” Moreover, he again asserts: “Species is not only a category of classification, but also a form of supraindividual biological integration” (1970).

In Systematics and the Origin of Species, Mayr (1942) commended Dobzhansky for identifying interbreeding and reproductive isolation as the distinguishing features of the species concept and proposed a short definition: “Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” He has used, identically or with some word differences, this definition in later writings. Indeed, Mayr is generally perceived as the leading exponent of the biological species concept and the most successful investigator of the application of this concept to a great variety of species and species groups throughout the animal world, as several papers in this collection bear witness.

Mayr repeatedly wrote that species are real and not merely human constructs that are convenient for organizing biological diversity, as some taxonomists, as well as nominalist philosophers, would claim. He supported the claim by Ghiselin (1974), Hull (1977), and others that species are metaphysical individuals, once this language was introduced in the evolutionary literature (Mayr, 1976, 1987). The integration of its gene pool provides the necessary cohesion for any particular species taxon to be considered an “individual.” The integration of gene pools, in turn, derives from the two dimensions incorporated in the definition of species, the ability of its members to interbreed, and their reproductive isolation from other species.

MODELS OF CHROMOSOMAL SPECIATION

Changes in chromosome number or structure may contribute to speciation. Polyploidy, the multiplication of the chromosome complement, may yield a new species in a single generation, reproductively isolated from its ancestral species. For example, a tetraploid plant crossed with a diploid ancestor produces sterile hybrid progeny. Polyploidy is more common among angiosperms than among gymnosperms. Nearly 50% of all existing angiosperm species are estimated to have arisen by ancient polyploidy, more of them by allopolyploidy (doubling of the chromosome complement in a hybrid between two previously existing species) than by autopolyploidy (multiplication of the chromosome complement of a single species). Polyploidy is also common among ferns. Some important cultivated plants are polyploids, such as wheat, oat, tobacco, potato, banana, strawberry, sugar cane, and coffee. Polyploidy is less common in animals; polyploidy species occur among hermaphrodites, such as earth-



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