TRENDS IN SYSTEMATIC BOTANY
91
1954); 7m (Anderson, 1936); Ranunculus
(Landolt, 1954); Rubus
(Brown, 1943); Tradescantia
(Anderson and Sax, 1936); Triticum (Sears,
1948); and Viola (Valentine, 1950).
The agamic complex (Babcock and Stebbins, 1938), in which the hybrids or hybrid derivatives reproduce partially or wholly by unfertilized
seeds or bulbils, which may attain a wide dispersal. Examples are provided by Crepis § Psilochaenia (Babcock and Stebbins, 1938), Parthenium (Rollins, 1945), Cafomagrostis
(Nygren, 1946), Arabis (Böcher, 1951,
1954), and Dentaria
(Montgomery, 1955). Such complexes are also
found in AlchemiUa, Antennaria, Crataegus,
Festuca, Hieracium,
Poa,
Potentilla, Rosa, Senecio, Taraxacum, Vaccinium, and many other genera.
It is at once noticeable that these are among the most difficult genera
from the point of view of the taxonomist.
The term heteroploid complex was proposed by Babcock and Stebbins
(1938) to cover a species group in which polyploidy and hybridization
with or without apomixis produce a large amount of recombination of
the characteristics of species originally quite distinct from one another.
This term applies equally to the sexual complex and to the agamic complex. All of these terms have come into, frequent use, as they describe
genetic groups that have played a fundamental role in the evolution
of the higher plants.
A few examples are chosen to show some of the groupings and evolutionary patterns, as disclosed by cytogenetic investigations, which should
be taken into consideration by the taxonomist.
The variation patterns in the leafy-stemmed Gilias are about as complex as in some of the taxonomically most puzzling complexes in that the
species are not well separated morphologically (V. Grant, 1953). The
variation in most of the other difficult complexes is accounted for by
polyploidy, apomixis, or permanent structural heterozygosity, none of
which apply to these diploid, sexual species of Gilia. The evolutionary
factor all have in common, however, is hybridization, which may blur
the distinctions between species with or without producing observable
numerical or structural changes in the chromosomes. Grant also points
out that complexes in which sexual recombination is inhibited in one way
or another can expand only so far, and then they must gradually die
out. Homogamic complexes, however, need not suffer any decline, for
the derived forms are as fully capable of producing new gene recombinations and variations as the original species.
Zauschneria is an excellent example of the evolution of the polyploid
complex, for the genetic barriers separating its four ecospecies are still
very definite. This example suggests how a genus might become re-
91
1954); 7m (Anderson, 1936); Ranunculus
(Landolt, 1954); Rubus
(Brown, 1943); Tradescantia
(Anderson and Sax, 1936); Triticum (Sears,
1948); and Viola (Valentine, 1950).
The agamic complex (Babcock and Stebbins, 1938), in which the hybrids or hybrid derivatives reproduce partially or wholly by unfertilized
seeds or bulbils, which may attain a wide dispersal. Examples are provided by Crepis § Psilochaenia (Babcock and Stebbins, 1938), Parthenium (Rollins, 1945), Cafomagrostis
(Nygren, 1946), Arabis (Böcher, 1951,
1954), and Dentaria
(Montgomery, 1955). Such complexes are also
found in AlchemiUa, Antennaria, Crataegus,
Festuca, Hieracium,
Poa,
Potentilla, Rosa, Senecio, Taraxacum, Vaccinium, and many other genera.
It is at once noticeable that these are among the most difficult genera
from the point of view of the taxonomist.
The term heteroploid complex was proposed by Babcock and Stebbins
(1938) to cover a species group in which polyploidy and hybridization
with or without apomixis produce a large amount of recombination of
the characteristics of species originally quite distinct from one another.
This term applies equally to the sexual complex and to the agamic complex. All of these terms have come into, frequent use, as they describe
genetic groups that have played a fundamental role in the evolution
of the higher plants.
A few examples are chosen to show some of the groupings and evolutionary patterns, as disclosed by cytogenetic investigations, which should
be taken into consideration by the taxonomist.
The variation patterns in the leafy-stemmed Gilias are about as complex as in some of the taxonomically most puzzling complexes in that the
species are not well separated morphologically (V. Grant, 1953). The
variation in most of the other difficult complexes is accounted for by
polyploidy, apomixis, or permanent structural heterozygosity, none of
which apply to these diploid, sexual species of Gilia. The evolutionary
factor all have in common, however, is hybridization, which may blur
the distinctions between species with or without producing observable
numerical or structural changes in the chromosomes. Grant also points
out that complexes in which sexual recombination is inhibited in one way
or another can expand only so far, and then they must gradually die
out. Homogamic complexes, however, need not suffer any decline, for
the derived forms are as fully capable of producing new gene recombinations and variations as the original species.
Zauschneria is an excellent example of the evolution of the polyploid
complex, for the genetic barriers separating its four ecospecies are still
very definite. This example suggests how a genus might become re-