Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 468

Warwick,Thompson & Black (1987) to predict rapid evolution of a race locally
adapted to Canadian conditions.
Few studies have attempted to measure the effects of genetic diversity of
the founders on colony success. In an unusual study, Martins & Jain (1979)
sowed seeds of a newly introduced Eurasian forage legume, Trifolium hirtum ,
along roadsides in northern California. The seed sources for these colonies
had low,medium,or high genetic polymorphism for eight morphological and
allozyme markers.In the first year following sowing,14 of 135 colonies estab-
lished and success was uncorrelated with degree of polymorphism. The fol-
lowing year, however, seven additional colonies established from dormant
seed, all from sowings with medium and high polymorphism. Combined, the
two years data showed a weakly significant relation between polymorphism
and establishment success ( p

0.1). In addition, however, Martins & Jain
(1979) noted that the genotypes emerging the second year were different from
those emerging the first year, further indicating the importance of genetic
diversity for colonizing success. An implication of this study is that even if
preventive measures cannot eliminate dispersal of all weed seeds, reducing
the number of individuals reaching new locations may limit the frequency
of successful establishment by lowering the genetic diversity of new
populations.

Somatic variation
Somatic variation includes both somatic polymorphism and plastic
developmental response to environmental conditions (Dekker, 1997). An
example of the former is the non-genetic seed polymorphism that governs the
variable germination requirements of some species (e.g., Chenopodium album -
Williams & Harper, 1965; Xanthium strumarium - Weaver & Lechowicz, 1983).
Examples of developmental plasticity include the great range in size and seed
production of many annual weeds when subjected to a range of light, compe-
tition, or soil fertility conditions (Table 10.5) (Moran, Marshall & Müller,
1981; Rice & Mack, 1991; Sultan & Bazzaz, 1993 a ; Hermanutz & Weaver,
1996). In essence, somatic variation increases the size of a species' potential
niche.
The degree of plasticity in a population is itself an evolved trait that some-
times responds to the selective forces of agriculture (Bradshaw, 1965). For
example, Camelina sativa var. sativa is a widely distributed weed whose highly
plastic morphology makes it an effective competitor in a variety of crops. In
contrast, C. sativa var. linicola is a flax mimic; it grows tall, with a narrow
crown, like the crop to which it is specialized, and has lost the flexibility of its
more generally adapted ancestor (Stebbins, 1950; Bradshaw, 1965). Knowing