TRENDS IN SYSTEMATIC BOTANY
71
ploids and autotetraploids differ as follows: morphologically the tetraploids exhibit considerable gigantism and physiologically they are more
vigorous and possess greater ecological amplitude; the diploids are restricted in range, whereas the tetraploids have an extended range. The
diploids occupy a geologically old area and are considered a relict
population; the autotetraploids occupy a geologically younger area and
are considered derived. Taxonomically these races are not distinguished.
The taxonomist may require the services of a cytologist to aid him in
the determination of chromosome numbers in the polyploid races of a
species, but there is often a correlation between cell size and chromosome number that may be useful. As a rule tetraploid races have larger
stomata and pollen grains than diploid races of the same species, and in
addition they have larger cells in general and fewer stomata (Miintzing,
1936; Sax and Sax, 1937). If this is once established by actual checks
with chromosome numbers, one needs only a good microscope to survey
the distribution of diploid and tetraploid races without counting chromosomes.
Amphiphidy.
The abrupt emergence of a new species from unlike
parents through polyploidy could not long escape notice. Consequently,
the first method of speciation to become clearly understood was by
amphiploidy. In this, interspecific hybridization is followed by chromosome doubling in the hybrid to establish a cytologically stable form
genetically reconstituted from both parental types and independent
from either of them. When a natural species was discovered that appeared to recombine the characters of two other species near by, it was
suspected that the process of amphiploidy might be involved in its
origin. Cytological examination of it and its presumed parents often
lent weight to the presumption.
A long list of cases of naturally occurring amphiploids has now been
recorded. The authenticity for many of these rests on three kinds of
data: (1) the amphiploid is more or less intermediate between its
parents in its morphological and physiological characteristics; (2) it may
occupy a somewhat different environmental niche than either parent, but
one commonly intermediate to theirs; and (3) its chromosome number
is the sum of the parental numbers, and the parental chromosomes are
sometimes morphologically identifiable in the amphiploid. So long as
the amphiploid was not under a botanist's observation at the time of its
creation, the evidence remains to some extent circumstantial. On the
other hand, if the natural amphiploid can be artificially recreated, the
evidence for its evolutionary history would seem to be irrefutable.
Clausen et al. (1945) have reviewed a number of instances of natural
71
ploids and autotetraploids differ as follows: morphologically the tetraploids exhibit considerable gigantism and physiologically they are more
vigorous and possess greater ecological amplitude; the diploids are restricted in range, whereas the tetraploids have an extended range. The
diploids occupy a geologically old area and are considered a relict
population; the autotetraploids occupy a geologically younger area and
are considered derived. Taxonomically these races are not distinguished.
The taxonomist may require the services of a cytologist to aid him in
the determination of chromosome numbers in the polyploid races of a
species, but there is often a correlation between cell size and chromosome number that may be useful. As a rule tetraploid races have larger
stomata and pollen grains than diploid races of the same species, and in
addition they have larger cells in general and fewer stomata (Miintzing,
1936; Sax and Sax, 1937). If this is once established by actual checks
with chromosome numbers, one needs only a good microscope to survey
the distribution of diploid and tetraploid races without counting chromosomes.
Amphiphidy.
The abrupt emergence of a new species from unlike
parents through polyploidy could not long escape notice. Consequently,
the first method of speciation to become clearly understood was by
amphiploidy. In this, interspecific hybridization is followed by chromosome doubling in the hybrid to establish a cytologically stable form
genetically reconstituted from both parental types and independent
from either of them. When a natural species was discovered that appeared to recombine the characters of two other species near by, it was
suspected that the process of amphiploidy might be involved in its
origin. Cytological examination of it and its presumed parents often
lent weight to the presumption.
A long list of cases of naturally occurring amphiploids has now been
recorded. The authenticity for many of these rests on three kinds of
data: (1) the amphiploid is more or less intermediate between its
parents in its morphological and physiological characteristics; (2) it may
occupy a somewhat different environmental niche than either parent, but
one commonly intermediate to theirs; and (3) its chromosome number
is the sum of the parental numbers, and the parental chromosomes are
sometimes morphologically identifiable in the amphiploid. So long as
the amphiploid was not under a botanist's observation at the time of its
creation, the evidence remains to some extent circumstantial. On the
other hand, if the natural amphiploid can be artificially recreated, the
evidence for its evolutionary history would seem to be irrefutable.
Clausen et al. (1945) have reviewed a number of instances of natural