66
DAVID D. KECK
where morphological markers, other than the chromosome number itself,
are virtually absent (Böcher et al. 1953).
The most intensive experimental studies within a limited group of
organisms may yield data more complex than can be utilized within a
practical taxonomic classification. The chromosomal repatternings in
the fruit fly, Drosophila, resulting in structural hybridity and the erection of genetic species within groups that are morphologically indistinguishable, afford an example. Dobzhansky (1946) has described
Drosophih
equinoxialis which will not cross with D. willistoni despite
a "virtual lack of morphological differences." Average individuals cannot be sorted into one species or the other by any morphological character, yet because of the essentially complete reproductive isolation Dobzhansky argues that there can be "no reasonable doubt that these forms
are full-fledged species rather than races of the same species." Similar
examples may be found among the higher plants, where the study
of reciprocal translocations in the chromosomes of North American
Euoenotheras has yielded a rather elaborate classification of chromosomal
complexes based on phylogeny and geographic distribution, but one
that is not reflected in the taxonomy of the group (Cleland, 1954).
The extreme view, that a sterility barrier between groups is sufficient
evidence of specific difference, tends to vitiate a fundamental premise of
taxonomy, that the system of classification should be utilitarian. A number of cases are known of single genes producing intersterility between
forms that are phenotypically indistinguishable. One is thus led to
agree with the statement of Ownbey and Weber (1943) that "there are
just enough exceptions to make the axiomatic application of hybrid
sterility or inviability to the problem of species delimination as unsatisfactory as the axiomatic application of the traditional criterion, morphological difference."
Taxonomic decisions that have leaned heavily upon chromosome number have been discussed many times, the following recent papers being
merely representative: Anderson, 1937; Burbanck, 1941, 1944; Camp,
1945; Cave and Constance, 1942, 1944, 1947, 1950; Church, 1949; R. T.
Clausen, 1942; Covas, 1949; Duncan, 1954; Ehrendorfer, 1953a; Fernandes, 1951; Gaiser, 1954; Garber, 1950; Gould, 1953; W. F. Grant,
1953, 1955; Gregory, 1941; Johnson, 1945; Keck, 1945, 1946; Lewis and
Lewis, 1955; Lewis and Snow, 1951; A. Löve, 1954a, b; D. Löve, 1953;
Löve and Löve, 1954; Mantón, 1950; Menzel, 1950; Montgomery, 1955;
Neves, 1952; Μ. Ownbey, 1940; Stern, 1949; Taylor, 1945; and Warburg,
1938.
Chromosome morphology is likewise a criterion often of value in
DAVID D. KECK
where morphological markers, other than the chromosome number itself,
are virtually absent (Böcher et al. 1953).
The most intensive experimental studies within a limited group of
organisms may yield data more complex than can be utilized within a
practical taxonomic classification. The chromosomal repatternings in
the fruit fly, Drosophila, resulting in structural hybridity and the erection of genetic species within groups that are morphologically indistinguishable, afford an example. Dobzhansky (1946) has described
Drosophih
equinoxialis which will not cross with D. willistoni despite
a "virtual lack of morphological differences." Average individuals cannot be sorted into one species or the other by any morphological character, yet because of the essentially complete reproductive isolation Dobzhansky argues that there can be "no reasonable doubt that these forms
are full-fledged species rather than races of the same species." Similar
examples may be found among the higher plants, where the study
of reciprocal translocations in the chromosomes of North American
Euoenotheras has yielded a rather elaborate classification of chromosomal
complexes based on phylogeny and geographic distribution, but one
that is not reflected in the taxonomy of the group (Cleland, 1954).
The extreme view, that a sterility barrier between groups is sufficient
evidence of specific difference, tends to vitiate a fundamental premise of
taxonomy, that the system of classification should be utilitarian. A number of cases are known of single genes producing intersterility between
forms that are phenotypically indistinguishable. One is thus led to
agree with the statement of Ownbey and Weber (1943) that "there are
just enough exceptions to make the axiomatic application of hybrid
sterility or inviability to the problem of species delimination as unsatisfactory as the axiomatic application of the traditional criterion, morphological difference."
Taxonomic decisions that have leaned heavily upon chromosome number have been discussed many times, the following recent papers being
merely representative: Anderson, 1937; Burbanck, 1941, 1944; Camp,
1945; Cave and Constance, 1942, 1944, 1947, 1950; Church, 1949; R. T.
Clausen, 1942; Covas, 1949; Duncan, 1954; Ehrendorfer, 1953a; Fernandes, 1951; Gaiser, 1954; Garber, 1950; Gould, 1953; W. F. Grant,
1953, 1955; Gregory, 1941; Johnson, 1945; Keck, 1945, 1946; Lewis and
Lewis, 1955; Lewis and Snow, 1951; A. Löve, 1954a, b; D. Löve, 1953;
Löve and Löve, 1954; Mantón, 1950; Menzel, 1950; Montgomery, 1955;
Neves, 1952; Μ. Ownbey, 1940; Stern, 1949; Taylor, 1945; and Warburg,
1938.
Chromosome morphology is likewise a criterion often of value in