E

Fig. 5.3. Several imaginary phyletic lineages illustrate "species" limits under the cladistic concept (clad.) and the paleontological concept (pal.). Schematic representation. Groups of populations representing the various lineages at particular time levels (t1-t4) are different biological species (oval circles). Vertical scale-geological time; horizontal scale-morphological and other biological changes. A-L represent paleontological "species," except C-F, which together are one paleontological "species" but represent 2 cladistic "species." The current time level is t4

Stresemann) during the 1920s, even though many of such allopatric forms actually were too distinct to be treated as subspecies.

The units of a regional fauna are zoogeographical species (Bock 1956, Mayr and Short 1970f) composed of two elements, (a) taxonomically isolated species (isospecies; that are not members of superspecies) and (b) superspecies (Ama-don 1966; Bock and Farrand 1980; Amadon and Short 1992). Much earlier Mayr had already emphasized that one must count zoogeographical species rather than individual species in a particular region of the world, in order to avoid misleading implications. In his letter to Stresemann dated 29 September 1936 he wrote: "A renewed count of the birds of New Guinea resulted in 513 breeding zoogeographical species (reckoning [the superspecies] Astrapia and Parotia as one species each)." However, it was not until 1970 that Mayr (l.c.) introduced the term "zoo-geographical species" formally (actually already used in Bock 1956). He himself credited it later to B. Rensch (Mayr 1989k: 156) without a specific reference. However, Rensch (1934) had used the term species geographicum only in the sense of polytypic species (not in the sense of superspecies plus taxonomically isolated species).

In their analysis of zoogeographic species Mayr and Short (1970f) studied the avian species taxa of a well-defined region (North America) to answer the question of how often the application of the biological species concept leads to difficulties, controversies, or ambiguities presenting in full their data on which their conclusions were based. Most of the tabulation and accompanying text was written by L. Short and evaluated in discussions with Mayr. They showed that invariably the application of the biological species concept helped to clarify difficult situations. In a number of peripheral isolates one could not be sure whether or not they had already reached species level, but this created as much difficulty for a morphological as for the biological species concept. Only in Mexican populations of towhees, Pipilo, the same two forms behave in one area as two biospecies and in another area as one because of extensive hybridization. Several other cases of "interspecific hybridization between largely allopatric members of the same superspecies" are not clear-cut either, especially those where there are large zones of geographical range overlap and extensive hybridization.

The theoretical concept of biological species is nondimensional and refers most clearly to genetic-reproductively isolated populations at a particular locality (Mayr 1941i, 1942e, 1946l, 1953b, 1963b). Because a fully differentiated biospecies represents a genetic unit, a reproductive unit and an ecological unit, Mayr (1951l: 92, 1982d: 273) specified as one aspect of biological species a specific niche in nature, i.e., ecological isolation permitting sympatry with competitors. This theoretical notion of biological species must be distinguished from the multidimensional species taxon. If several differentiated groups of populations are in contact and intergrade, they belong to the same species taxon (Fig. 5.4). Allopatric, i.e., geographically separated, representative taxa are assigned subspecies or species status on the basis of inference (Mayr 1948c, Mayr et al. 1953a: 104, 1969b: 197, Mayr and Ashlock 1991i: 104-105). Auxiliary criteria used for that purpose include:

species 1 species 2 species 3 a b c d e f species 1 species 2 species 3 a b c d e f

Fig. 5.4. Two sets of species and subspecies taxa each of which forms one large continental unit (left) and several geographically isolated populations (right) with their hypothetical cladograms. Schematic representation. H hybrid zone between subspecies, P parapatric contact zone between species (geographic exclusion without or nearly without hybridization). In the cladograms hatching indicates known intergradation (hybridization), dashes indicate presumed hybridization. In all areas the respective sympatric populations (taxa) of these two sets are specifically distinct with respect to each other (biological species). In the upper set, forms a and b hybridize and together represent species 1 which does not hybridize with parapatric species 2 (consisting of subspecies c and d). The status of the island populations d-f as species or subspecies (i.e., their hybridization or non-hybridization if they were in contact) is judged on the basis of inference (see text). Because forms alpha andbeta of the lower assemblage hybridize freely where they meet, forms gamma and delta are also assumed to hybridize if they would establish contact. All four taxa are considered subspecies of one polytypic species. Species 1 to 4 are all monophyletic

Fig. 5.4. Two sets of species and subspecies taxa each of which forms one large continental unit (left) and several geographically isolated populations (right) with their hypothetical cladograms. Schematic representation. H hybrid zone between subspecies, P parapatric contact zone between species (geographic exclusion without or nearly without hybridization). In the cladograms hatching indicates known intergradation (hybridization), dashes indicate presumed hybridization. In all areas the respective sympatric populations (taxa) of these two sets are specifically distinct with respect to each other (biological species). In the upper set, forms a and b hybridize and together represent species 1 which does not hybridize with parapatric species 2 (consisting of subspecies c and d). The status of the island populations d-f as species or subspecies (i.e., their hybridization or non-hybridization if they were in contact) is judged on the basis of inference (see text). Because forms alpha andbeta of the lower assemblage hybridize freely where they meet, forms gamma and delta are also assumed to hybridize if they would establish contact. All four taxa are considered subspecies of one polytypic species. Species 1 to 4 are all monophyletic

(1) degree of difference between sympatric species, (2) degree of difference between intergrading subspecies within widespread species, (3) degree of difference between hybridizing populations in related species. Occasional criticisms of the biological species concept refer mostly to its practical application in delimiting species taxa rather than to the theoretical notion of biological species itself. The highlights of Mayr's views on the Biological Species Concept, as expressed again in several recent articles (1988h, 1996e, 2000e), may be summarized as follows: A biological species is a reproductively (genetically) cohesive assemblage of populations, an assemblage of well-balanced harmonious genotypes. The devices which maintain the integrity of species, protecting the harmonious gene pools, are isolating mechanisms, a term proposed by Dobzhansky (1935, 1937: 228-258) who, unfortunately, did not distinguish between external geographical/ecological barriers operating in the allopatric phase of speciation and intrinsic mechanisms. Mayr (1963b: 89-109) stressed the enormous diversity of these devices, e.g., sterility genes, chromosomal incompatibilities, ecological exclusion, behavioral properties especially in higher animals. Species usually occupy ecological niches that are sufficiently different to permit the sympatric occurrence of these species. However, member species of a superspecies exclude one another allo- or parapatrically because of ecological competition. The Biological Species Concept is not applicable to asexual organisms (agamospecies) which form clones, not interbreeding populations, as defined by the biologist. Their gene pools do not require protection by isolating mechanisms, because they usually are not subject to genetic recombination. A list of Mayr's publications dealing with the species problem totals 94.

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