78
DAVID D. KECK
rise to geographic gradients in the differentiating characters. These have
been given the name "clines" by Huxley (1938). Clines are usually
somewhat irregular, as would be anticipated, and they may arise through
another method, namely, through race formation in species composed of
self-incompatible individuals. An abundance of intermediates would be
anticipated in such a case, as contrasted with the situation in species with
a high degree of self-compatibility, in which the new races stand out
distinctly as ecotypes (Baker, 1953).
From the genetical viewpoint sound species, in contrast to races, are
closed systems in which an exchange of genes with other species is prevented by reproductive isolation (Stebbins, 1945). Or, putting it another
way, the presence of isolating mechanisms is a fundamental common
property of species. It is the basis of their objective reality. From this
it can be argued that "the mechanisms isolating species from each other
must be considered the only true specific characters, if the expression
'specific character' is to have any real meaning" (Dobzhansky, 1937).
Taxonomic objections to accepting this literally have been mentioned
above and several of them have been summarized by Gates (1951).
When two species are found growing side by side without frequent
formation of hybrids, this may be taken as evidence of the presence of
reproductive isolation between them. Such species usually stand out
more clearly than those which occupy different territories, but whose
ranges may overlap, with a modest amount of hybridization occurring in
this area. The strength of the isolation barriers in such instances may
not be obvious, and there may be difficulty in deciding whether one is
dealing with distinct ecospecies or merely ecotypes of one ecospecies.
Lacking experimental evidence, one is forced to draw his conclusions
about allopatric species by determining whether their morphological
distinctness is comparable with that of sympatric species, and then hopefully treating them as species, too.
The above concept of species for cross-fertilizing forms does not apply
to organisms that are obligate self-fertilizing, or that reproduce by
apomixis, or other asexual means. In such organisms, classification into
species must be arbitrary—based solely on observed discontinuities in
structure and function.
Isolation, whether genetic or ecological, plays a basic role in species
formation. Geographic discontinuities offer some of the most interesting
and at the same time most unpredictable cases. The striking affinity between the floras of eastern America and eastern Asia has been discussed many times. Stebbins (1942a) cites several cases of species well
scattered among the angiosperm families that have a distribution in each
DAVID D. KECK
rise to geographic gradients in the differentiating characters. These have
been given the name "clines" by Huxley (1938). Clines are usually
somewhat irregular, as would be anticipated, and they may arise through
another method, namely, through race formation in species composed of
self-incompatible individuals. An abundance of intermediates would be
anticipated in such a case, as contrasted with the situation in species with
a high degree of self-compatibility, in which the new races stand out
distinctly as ecotypes (Baker, 1953).
From the genetical viewpoint sound species, in contrast to races, are
closed systems in which an exchange of genes with other species is prevented by reproductive isolation (Stebbins, 1945). Or, putting it another
way, the presence of isolating mechanisms is a fundamental common
property of species. It is the basis of their objective reality. From this
it can be argued that "the mechanisms isolating species from each other
must be considered the only true specific characters, if the expression
'specific character' is to have any real meaning" (Dobzhansky, 1937).
Taxonomic objections to accepting this literally have been mentioned
above and several of them have been summarized by Gates (1951).
When two species are found growing side by side without frequent
formation of hybrids, this may be taken as evidence of the presence of
reproductive isolation between them. Such species usually stand out
more clearly than those which occupy different territories, but whose
ranges may overlap, with a modest amount of hybridization occurring in
this area. The strength of the isolation barriers in such instances may
not be obvious, and there may be difficulty in deciding whether one is
dealing with distinct ecospecies or merely ecotypes of one ecospecies.
Lacking experimental evidence, one is forced to draw his conclusions
about allopatric species by determining whether their morphological
distinctness is comparable with that of sympatric species, and then hopefully treating them as species, too.
The above concept of species for cross-fertilizing forms does not apply
to organisms that are obligate self-fertilizing, or that reproduce by
apomixis, or other asexual means. In such organisms, classification into
species must be arbitrary—based solely on observed discontinuities in
structure and function.
Isolation, whether genetic or ecological, plays a basic role in species
formation. Geographic discontinuities offer some of the most interesting
and at the same time most unpredictable cases. The striking affinity between the floras of eastern America and eastern Asia has been discussed many times. Stebbins (1942a) cites several cases of species well
scattered among the angiosperm families that have a distribution in each