Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 470

reduction in population size of established populations by disturbance and
weed control efforts create periods of extreme inbreeding and genetic drift.As
discussed previously,selfing,apomixis,and vegetative reproduction then rep-
licate the surviving genotypes as the population subsequently expands. More
importantly, divergent selection pressure exerted by spatial variation in soil
and climate conditions, cropping systems, and weed control methods act to
differentiate weed populations. Great variation in the ecology of weed species
can thus be generated rapidly and sometimes over short distances.
Ecotypic variation across the geographic range of a weed species can have
important consequences for management. For example, Hypericum perforatum
grows in both natural and disturbed habitats in Europe, but it is rarely a pest
there (Pritchard,1960).In contrast,it has been an aggressive and economically
serious weed of grazing lands in Australia, California, and South Africa.
Common garden studies have shown that individuals from Australia and
California are substantially taller than individuals from Britain and other
places where the species is not a pest (Pritchard, 1960). To some extent, its
status as a weed in recently colonized portions of its range has been due to
release from herbivory (Huffaker, 1957). Blossey & Nötzold (1995) hypothe-
sized that release from herbivore pressure in invasive species like H.perforatum
selects for reallocation of resources from defense to vegetative growth and
seed production.
Weeds also differentiate in the process of range extension into new climatic
zones. Between 1926 and the early 1970s Sorghum halepense spread northward
in eastern North America from 38 latitude (Warwick, Phillips &

Andrews, 1986), a range extension into an unfavorable climatic zone of about
550 km. In most of its range, this tropical grass is a perennial. However, most
northern populations are annual. Probably these populations acquired the
annual habit and associated characters by hybridization with the domesti-
cated S. bicolor (Warwick, Thompson & Black, 1984). The shift to an annual
habit in S. halepense has been accompanied by changes in several quantitative
characters, including increased seed weight, increased percentage seedling
emergence,faster seedling growth,earlier flowering,and greater allocation to
reproduction (Warwick,Thompson & Black,1984).Although the annual habit
and associated characters are apparently adaptive in the northern popula-
tions,nothing obviously precludes the same characters from being adaptive in
more southerly climates as well. Will the annual race spread south, or will
gene flow from the more common perennial form prevent this? Ecotypic diffe-
rentiation associated with range extension has also been documented for
Datura stramonium and Abutilon theophrasti (Weaver, Dirks & Warwick, 1985;
Warwick & Black, 1986; Warwick, 1990).

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