Agriculture Reference
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and one on a shingle beach differed in most of the characters analyzed even
though they were separated by just 50 m and an embankment (Table 10.6b).
This implies that the strength of the contrasting selective pressures in the two
populations was sufficient to maintain differences despite evidence of gene
flow.
Jordan (1989
a
) compared populations of
Diodia teres
from a coastal dune
and a soybean field. The weed population emerged two days earlier after
planting and rapidly produced more leaves and meristems than the nonweed
population. In the absence of competition, the weed grew 38% larger than the
nonweed; in the presence of a soybean crop,the weed grew twice as large as the
nonweed. This difference in final size was due solely to rapid early growth,
since the weed grew slower than the nonweed after the crop became competi-
tive.Thus,the weed race had apparently evolved to avoid competition by com-
pleting more growth early in the season.Unlike the previously cited studies in
which the selective regime favored precocious flowering,the weed population
of
Diodia teres
had plenty of time to complete development,hence selection for
early growth led to larger, not smaller, mature plants. In further analysis,
Jordan (1989
b
) showed that the genetic variability of the nonweed population
was sufficient to allow rapid evolution of growth characteristics in response to
selection by presence of the crop. Moreover, selection by the crop on the
nonweed population was predicted to select for the phenological shift
observed in the weed.
A troublesome form of weed adaptation involves mimicry of crop charac-
teristics.Mimicry of vegetative characteristics occurs primarily in agricultural
systems where hand-weeding is practiced regularly. It is most common in
grass weeds, probably because of the superficial resemblance of many grass
species to grain crops, especially in the seedling stage when hand-weeding is
most likely (Barrett, 1983).Weed taxa closely related to the crop pose a special
problem as mimics because they can acquire characteristics from the crop by
introgressive hybridization. Apparently due to a combination of introgres-
sion and selection, weedy varieties of
Sorghum
and
Zea mexicana
mimic varia-
tion in the appearance of the related crop across regions of Africa and
Mesoamerica, respectively (de Wet, Harlan & Price, 1976; Wilkes, 1977).
Langevin, Clay & Grace (1990) found F
1
hybrids between weedy
Oryza sativa
and all six cultivars of domesticated rice they tested. However, because all of
the F
1
hybrids flowered late,F
1
plants resulting from crosses with early-season
cultivars would be destroyed during the harvest before they could shed seed.
This would prevent backcrossing of hybrids with the weed.Thus,use of short-
season varieties in this case offers a potential means for limiting convergence
of the weed with the crop.
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