90
DAVID D. KECK
and Rubus (Gustafsson, 1943). Clausen et dl. (1948b) observed that
the sexual interspecific hybrids in Poa again gave rise to apomictic lines
in later generations. This interlude in the histories of species that normally reproduce by the asexual process of apomixis, in which opportunity
is afforded for a thorough reshuffling of the genes through sexual reproduction and then, as it were, sealing up the new combinations in reestablished apomictic lines, is manifestly of profound significance in the
process of speciation. Apomixis, therefore, is a process through which
plants are enabled to store potential interspecific variability for later
release (Clausen, 1954).
From the evolutionary point of view the apomict, and in fact any
asexual organism, has an advantage in that its survival is dependent
only upon adaptation to its external environment, rather than to both
environmental and hereditary conditions. On the other hand, it is at a
disadvantage in that it cannot increase its variability through crossfertilization. If its mutational rate is very low it will have almost no
flexibility to meet environmental change.
XVII. HYBRID COMPLEXES
A useful summary of the known types of hybrid complex is supplied
by V. Grant (1953), who furnishes the definition, "Groups of species
in which hybridization has obscured the morphological discontinuities
between the basic diploid types may be termed collectively hybrid
complexes''
In increasing order of taxonomic complexity these are as
follows:
The homogamic complex, in which the hybrid derivatives customarily
are sexual diploids with normal meiosis. Structural hybridity, if present
in the Fi, is eliminated in later generations through natural selection.
In addition to the examples listed by Grant there may be mentioned
Hemizonia § Centromadia
and § Euhemizonia,
Madia sativa Mol. sens,
lat. (Clausen, 1951), and Physalts (Menzel, 1951).
The clonal complex, in which the hybrids reproduce mainly or entirely by clonal divisions. Grant cites Opuntia as an example.
The heterogamic complex, in which the hybrid derivatives are permanent structural heterozygotes, as in Oenothera §
Euoenothera.
The polyploid complex (Babcock and Stebbins, 1938), in which the
hybrid derivatives are sexual polyploids. This is a common situation
in nature and is exemplified by Artemisia
(Keck, 1946; Ward, 1953);
Bromus (Stebbins and Tobgy, 1944; Stebbins, 1949);
Chrysanthemum
(Shimotomai, 1933); Clarkia (Lewis and Lewis, 1955);
Eriogonum
(Stebbins, 1942b); Eupatorium
(W. F. Grant, 1953); Geum (Gajewski,
DAVID D. KECK
and Rubus (Gustafsson, 1943). Clausen et dl. (1948b) observed that
the sexual interspecific hybrids in Poa again gave rise to apomictic lines
in later generations. This interlude in the histories of species that normally reproduce by the asexual process of apomixis, in which opportunity
is afforded for a thorough reshuffling of the genes through sexual reproduction and then, as it were, sealing up the new combinations in reestablished apomictic lines, is manifestly of profound significance in the
process of speciation. Apomixis, therefore, is a process through which
plants are enabled to store potential interspecific variability for later
release (Clausen, 1954).
From the evolutionary point of view the apomict, and in fact any
asexual organism, has an advantage in that its survival is dependent
only upon adaptation to its external environment, rather than to both
environmental and hereditary conditions. On the other hand, it is at a
disadvantage in that it cannot increase its variability through crossfertilization. If its mutational rate is very low it will have almost no
flexibility to meet environmental change.
XVII. HYBRID COMPLEXES
A useful summary of the known types of hybrid complex is supplied
by V. Grant (1953), who furnishes the definition, "Groups of species
in which hybridization has obscured the morphological discontinuities
between the basic diploid types may be termed collectively hybrid
complexes''
In increasing order of taxonomic complexity these are as
follows:
The homogamic complex, in which the hybrid derivatives customarily
are sexual diploids with normal meiosis. Structural hybridity, if present
in the Fi, is eliminated in later generations through natural selection.
In addition to the examples listed by Grant there may be mentioned
Hemizonia § Centromadia
and § Euhemizonia,
Madia sativa Mol. sens,
lat. (Clausen, 1951), and Physalts (Menzel, 1951).
The clonal complex, in which the hybrids reproduce mainly or entirely by clonal divisions. Grant cites Opuntia as an example.
The heterogamic complex, in which the hybrid derivatives are permanent structural heterozygotes, as in Oenothera §
Euoenothera.
The polyploid complex (Babcock and Stebbins, 1938), in which the
hybrid derivatives are sexual polyploids. This is a common situation
in nature and is exemplified by Artemisia
(Keck, 1946; Ward, 1953);
Bromus (Stebbins and Tobgy, 1944; Stebbins, 1949);
Chrysanthemum
(Shimotomai, 1933); Clarkia (Lewis and Lewis, 1955);
Eriogonum
(Stebbins, 1942b); Eupatorium
(W. F. Grant, 1953); Geum (Gajewski,