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associated with the latter (Clausen
1954
; Nygren
1966
; DeWet and Stalker
1974
;
Asker
1979
; Catling
1981, 1982
). When compared with autogamy, agamospermy
requires less extreme morphological adaptation (Ornduff
1969
) and produces no
inbreeding depression (Charlesworth and Charlesworth
1979
). At the same time,
fertility is assured; the potential for rapid colonization is retained; heterozygosity is
maintained; and the fixation of particular patterns of variability (new adaptive gene
combinations) may be facilitated (Baker
1955
; Antonovics
1968
; Jain
1976
; Solbrig
1976
; Marshall and Weir
1979
; Catling
1980b, 1982
; Lloyd
1988
). Short-term fit-
ness may, thus, be combined with the advantages of genetic recombination (Catling
1982
). The latter includes, for example, the ability to recombine new, advantageous
mutants, permitting evolutionary responses to changing environmental conditions;
the ability of the best-adapted genotypes to escape the accumulation of linked, dis-
advantageous but nonlethal mutants (“Muller's Ratchet”); and the ability to fill the
maximum number of environmental niches through a high level of genetic variation
among individuals.
Agamospermy is also thought to confer advantages over sexual reproduction in
certain areas, where it may reflect adaptation to isolation or to a reduction in the
availability or activity of pollinators (Lloyd
1980, 1988
; Manning
1981
; Catling
1982
; Schmidt and Antlfinger
1992
). Such sites include wide expanses of recently
glaciated territory, edges of a species range, recently established vegetation zones,
or more or less isolated bioclimatic zones (e.g., Bayer and Stebbins
1980
; Catling
1982
; Schmidt and Antlfinger
1992
). For example, the three species,
S. magnicamp-
orum, S. ochroleuca
, and
S. odorata
, which appear to be sexual over most of their
distributions, reveal an apparent association between agamospermy and range lim-
its, particularly northeastern range limits (Catling
1982
).
S. casei
is distributed
within formerly glaciated territory (Prest
1970
; Ives et al.
1975
; Catling
1990
), and
S. casei
var.
novascotiae
lies in the relatively isolated bioclimatic zone of southern
Nova Scotia (Fernald
1921
; Roland and Smith
1962
-1969; Catling
1981, 1982,
1990
). The association of agamospermy with northeastern range limits, glaciated
territory, and isolated bioclimatic zones, its augmentation of other isolating mecha-
nisms, and its presence in a complex of closely related taxa suggest that it has been
a significant factor in the production of new species (Catling
1982, 1983a
).
Sexual Species or Populations
This section examines sexual reproduction in uniformly sexual species of
Spiranthes
and the facultatively agamospermous populations of the
S. cernua
complex whose
asexual reproduction was discussed above. Uniformly sexual species include:
S. lacera
(Rafinesque) Rafinesque var.
lacera
(northern slender ladies'-tresses),
S. lacera
var.
gracilis
(Bigelow) Luer (southern slender ladies'-tresses),
S. tuberosa
Rafinesque (lit-
tle ladies'-tresses),
S. vernalis
Engelmann and Gray (spring ladies'-tresses),
S. lacini-
ata
(Small) Ames (lace-lip ladies'-tresses),
S. romanzoffiana
Chamisso (hooded
ladies'-tresses),
S. diluvialis
Sheviak (Ute ladies'-tresses),
S. lucida
(H. H. Eaton)
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