76
DAVID D. KECK
from this the conclusion that no speciation has thus occurred on the
diploid level. The diploid and tetraploid strains are isolated from each
other, as proved by the absence of triploids and other intermediates, and
the hexaploids constitute a third isolated group. She concludes that the
New World Polygonatum forms cannot be classified into species either
on the diploid level, or between diploids and tetraploids, or between
tetraploids and hexaploids. In this complex then, as in the now wellknown American Tradescantias (Anderson and Sax, 1936), a cytological
knowledge is prerequisite for an adequate taxonomic treatment.
In contrast to polyploidy, the more usual method of speciation is
through a gradual accumulation of genie and chromosomal changes.
Geneticists and cytologists alike have given much attention to the mechanisms through which evolution operates in these ways. Accordingly, in
the following chapter there will continue to be frequent references to
cytology.
XIV. GENETICS
It is remarkable that the true extent of natural hybridization took so
long to be widely appreciatd among biologists. Even field botanists were
reluctant to accept this as more than a rare phenomenon. Those wide
crosses that produced obviously sterile F x hybrids were recognized, but
the more subtile situation, in which the hybrids retained some fertility
and an ability to backcross interminably with the parental types, was
often incorrectly interpreted. A considerable body of data on natural
hybridization, hybrid swarms, and the intergradation of species through
hybridization was at hand when Anderson (1949) succinctly brought
together this story in an easily read little book called
Introgressive
Hybridization,
which has had much influence upon evolutionary thinking. Certainly the title of his book calls attention to one of the very potent forces in evolution.
An infiltration of genes from one taxon to another (introgression) may
affect various characters of the plant. The extent of the effects can only
be determined through a study of the parental types. A considerable
body of literature has now been built up to show the presence, extent,
and often the taxonomic significance of introgression in many groups of
seed plants from conifers to Composites. Representative contributions
have been made by Anderson and Gage (1952), Anderson and Hubricht
(1938), Baker (1951), Dansereau (1941), Dansereau and Desmarais
(1947), Dansereau and Lafond (1941), Ehrendorfer (1955), Erickson
(1945), Fassett and Calhoun (1952), V. Grant (1950a), Hall (1952),
Heiser (1949a, b), G. B. Ownbey (1951), Rollins (1949, 1954), and Wet-
DAVID D. KECK
from this the conclusion that no speciation has thus occurred on the
diploid level. The diploid and tetraploid strains are isolated from each
other, as proved by the absence of triploids and other intermediates, and
the hexaploids constitute a third isolated group. She concludes that the
New World Polygonatum forms cannot be classified into species either
on the diploid level, or between diploids and tetraploids, or between
tetraploids and hexaploids. In this complex then, as in the now wellknown American Tradescantias (Anderson and Sax, 1936), a cytological
knowledge is prerequisite for an adequate taxonomic treatment.
In contrast to polyploidy, the more usual method of speciation is
through a gradual accumulation of genie and chromosomal changes.
Geneticists and cytologists alike have given much attention to the mechanisms through which evolution operates in these ways. Accordingly, in
the following chapter there will continue to be frequent references to
cytology.
XIV. GENETICS
It is remarkable that the true extent of natural hybridization took so
long to be widely appreciatd among biologists. Even field botanists were
reluctant to accept this as more than a rare phenomenon. Those wide
crosses that produced obviously sterile F x hybrids were recognized, but
the more subtile situation, in which the hybrids retained some fertility
and an ability to backcross interminably with the parental types, was
often incorrectly interpreted. A considerable body of data on natural
hybridization, hybrid swarms, and the intergradation of species through
hybridization was at hand when Anderson (1949) succinctly brought
together this story in an easily read little book called
Introgressive
Hybridization,
which has had much influence upon evolutionary thinking. Certainly the title of his book calls attention to one of the very potent forces in evolution.
An infiltration of genes from one taxon to another (introgression) may
affect various characters of the plant. The extent of the effects can only
be determined through a study of the parental types. A considerable
body of literature has now been built up to show the presence, extent,
and often the taxonomic significance of introgression in many groups of
seed plants from conifers to Composites. Representative contributions
have been made by Anderson and Gage (1952), Anderson and Hubricht
(1938), Baker (1951), Dansereau (1941), Dansereau and Desmarais
(1947), Dansereau and Lafond (1941), Ehrendorfer (1955), Erickson
(1945), Fassett and Calhoun (1952), V. Grant (1950a), Hall (1952),
Heiser (1949a, b), G. B. Ownbey (1951), Rollins (1949, 1954), and Wet-