Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 463

the short planning horizons of modern societies preclude significant attempts
to block most of the routes for development of new weeds discussed above,the
first step is recognition that the problem exists. Perhaps future cultures will
learn how to prevent further increase in the weed flora, even as they cope with
the weeds that are currently developing in farm fields today. A tentative,
partial approach to the problem is proposed in the section “Controlling the
spread of new weeds”below.
Weed genecology
This section discusses several aspects of weed genecology (genetical
ecology) that are useful in understanding the evolutionary response of weeds
to management. Evolutionary changes in weeds are driven by a variety of
factors.The two most important are (i) selection exerted by control efforts and
other aspects of cropping systems, and (ii) genetic bottlenecks associated with
colonization of new locations. Response to selection is a function of the
genetic variability in the population and the intensity of the selection pres-
sure. To a large extent, genetic variability is determined by the weed's breed-
ing system, and by losses of variability that occur during colonization events.
Selection acting on this genetic variability commonly leads to differentiation
of ecotypes specialized for particular physical environments and cropping
systems. However, most weeds have a high degree of phenotypic plasticity
that allows them to survive and reproduce in a wide range of environmental
conditions even without adaptive change in the genome. Each of these issues
is discussed further below.
Genetic variability and breeding systems
During the establishment of new populations, weeds are faced with
the need to reproduce even though potential mates may be few and scattered,
or absent altogether. Consequently, most weeds are either self-compatible,
apomictic (able to set seeds asexually), or propagate vegetatively.
The correlation between life history and breeding system is strong. Most
annual and stationary perennial weeds are self-compatible or apomictic
whereas most wandering perennials regularly outcross. For example, in a
study of 64 Canadian weeds, all 33 annuals and 21 of 23 stationary perennials
set seed when inflorescences were bagged to prevent cross-pollination.In con-
trast, none of the eight wandering perennials set seeds when bagged
(Mulligan & Findlay, 1970).
Outbreeding species tend to have high heterozygosity and rapid genera-
tion of new gene combinations (Table 10.3) (Clegg & Brown, 1983) due to