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DAVID D. KECK
types are found to differ from each other by many characters and many
genes, but in general they have been largely overlooked by those biologists, including the taxonomists, who have searched for more visible
characters than the ecotypes usually have to display.
The Ecospecies.
As genetic barriers come into being between ecotypes, so that a free exchange of hereditary materials between them is
interrupted, an ecospecies may be formed. At this next evolutionary
level, crossings between ecotypes may continue but on a more restricted
and less successful basis. Reduction in vigor of the offspring of the F 2
hybrid generation may occur, but so long as such ecotypes can be linked
together through other ecotypes with which they are interfertile, they
remain in one ecospecies. When an ecotype, or group of ecotypes,
however, becomes so set off from other ecotypes that a free exchange of
genes is no longer possible or is accomplished only with a loss of fertility
or vigor in the offspring, it belongs to a distinct ecospecies. This means
that the ecospecies is composed of all those ecotypes that are able to exchange genes freely without loss of fertility or vigor in the offspring. One
ecospecies is separated from the next by genetic barriers.
In all species complexes that cover climatically different areas, the
fundamental principle, regional differentiation, may be expressed either
as a series of ecotypes belonging to one species, or as a series of closely
related ecospecies, or, more commonly, by both. Complexes differ in
their capacity to occupy an area.
Perhaps most of the species of the taxonomist are the equivalent of
ecospecies. If they occupy more than a very limited territory, they are
very apt to be composed of ecotypes or subgroups that are physiologically best adapted to survive in their particular environmental niches,
but that are still capable of interbreeding freely without a reduction of
vigor in succeeding generations.
The Cenospecies.
Experiments demonstrate that ecospecies may group
into larger entities, called cenospecies. A cenospecies
contains all those
ecospecies so related that they are able to exchange genes among themselves to a limited extent through hybridization. Species entirely unable
to exchange genes belong to different cenospecies, for gene exchange
between cenospecies is impossible. A cenospecies may consist of only
one ecospecies and even of only one ecotype, as in the case of the Dawn
Redwood, Metasequoia
glyptostroboides.
It is sometimes possible, however, for members of different cenospecies to hybridize, although such
offspring are sterile.
The Comparium.
Cenospecies linked together, either directly or
through intermediaries, by this ability to hybridize are considered to be-
DAVID D. KECK
types are found to differ from each other by many characters and many
genes, but in general they have been largely overlooked by those biologists, including the taxonomists, who have searched for more visible
characters than the ecotypes usually have to display.
The Ecospecies.
As genetic barriers come into being between ecotypes, so that a free exchange of hereditary materials between them is
interrupted, an ecospecies may be formed. At this next evolutionary
level, crossings between ecotypes may continue but on a more restricted
and less successful basis. Reduction in vigor of the offspring of the F 2
hybrid generation may occur, but so long as such ecotypes can be linked
together through other ecotypes with which they are interfertile, they
remain in one ecospecies. When an ecotype, or group of ecotypes,
however, becomes so set off from other ecotypes that a free exchange of
genes is no longer possible or is accomplished only with a loss of fertility
or vigor in the offspring, it belongs to a distinct ecospecies. This means
that the ecospecies is composed of all those ecotypes that are able to exchange genes freely without loss of fertility or vigor in the offspring. One
ecospecies is separated from the next by genetic barriers.
In all species complexes that cover climatically different areas, the
fundamental principle, regional differentiation, may be expressed either
as a series of ecotypes belonging to one species, or as a series of closely
related ecospecies, or, more commonly, by both. Complexes differ in
their capacity to occupy an area.
Perhaps most of the species of the taxonomist are the equivalent of
ecospecies. If they occupy more than a very limited territory, they are
very apt to be composed of ecotypes or subgroups that are physiologically best adapted to survive in their particular environmental niches,
but that are still capable of interbreeding freely without a reduction of
vigor in succeeding generations.
The Cenospecies.
Experiments demonstrate that ecospecies may group
into larger entities, called cenospecies. A cenospecies
contains all those
ecospecies so related that they are able to exchange genes among themselves to a limited extent through hybridization. Species entirely unable
to exchange genes belong to different cenospecies, for gene exchange
between cenospecies is impossible. A cenospecies may consist of only
one ecospecies and even of only one ecotype, as in the case of the Dawn
Redwood, Metasequoia
glyptostroboides.
It is sometimes possible, however, for members of different cenospecies to hybridize, although such
offspring are sterile.
The Comparium.
Cenospecies linked together, either directly or
through intermediaries, by this ability to hybridize are considered to be-