Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 479

sure for a resistance trait controlled by multiple genes from a high to a moder-
ate level tends to increase rather than decrease the rate of adaptation (Cousens
& Mortimer, 1995, p. 280). With characters controlled by multiple genes,
lower selection pressure allows more escapes and thus more possibilities for
resistant forms to arise by recombination. Hard selection that results in low
population size is thus a defense against resistance controlled by multiple
genes.
Cropping system diversity and the management of weed
adaptation
Regardless of whether the response to a practice is via polygenes or a
single major gene, alternation of methods between years provides an impor-
tant approach for limiting adaptation to control measures.This is particularly
the case if fitness of genotypes adapted to some practice, A, is less than that of
unadapted genotypes in years when some other practice, B, is used. In such
cases, the frequency of A-adapted types will decline in years when practice B is
used. On the other hand, if practice B is neutral with respect to A-adapted
types then the rate of adaptation to practice Ais proportional to how often it is
used (Gressel & Segel, 1990; Jasieniuk, Brûlé-Babel & Morrison, 1996).
Alternation between the practices then slows adaptation to practice A, but
does not prevent evolution of a high level of resistance eventually.
Continuous monocultures appear to favor special adaptations like crop
mimicry and herbicide resistance (Gressel, 1991). In contrast, diverse crop
rotations may favor genetic variability since variation in selection pressures
between years would tend to slow fixation of particular alleles. This hypothe-
sis requires testing. To the extent that the several crops and corresponding
cultural practices of a rotation put different selection pressures on a weed
population, diverse crop rotations might even disrupt multigene adaptations
to particular control practices that may be used in several crops (e.g., cultiva-
tion, mulch).
Fine-grained spatial variation in the cropping system probably also tends
to slow weed adaptation.Growing different crops repeatedly in separate fields
creates a selective regime potentially favoring genetic divergence in the
several subpopulations. In contrast, when different crops are grown together
in polycultures, then differences in the competitive and cultural environ-
ments of the several crops act as disruptive selection on many potentially
adaptive traits of the weeds.
In actual practice,diverse crop rotations and intercropping systems usually
do not repeat the exact sequence and arrangement of crops, but rather
respond flexibly to changes in market conditions, weather, weeds, and other