Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 456

adapted to human-dominated ecosystems, (ii) by reversion of cultivated
species to weedy forms, (iii) by hybridization, (iv) by evolution of new forms
within existing weed species, and (v) by speciation of geographically isolated
populations. These pathways are not always distinct, and complex genetic
interchanges have been hypothesized within crop-weed-wild assemblages of
closely related species (Zohary, 1965). Nevertheless, these pathways provide a
structure for organizing an understanding of weed origins. Unless a species
can reproduce in agricultural fields, selection for traits that further adapt the
species as an agricultural weed is unlikely.Thus, a species must be preadapted
to agricultural conditions to some extent if it is to become an agricultural
weed.
Preadaptation for weediness
For most weed species, truly wild populations are known from the
taxon's center of origin. Typically, these inhabit naturally open or disturbed
habitats: stream margins, marshlands, beaches, dunes, cliffs, scree, exposed or
high-elevation sites,and animal-disturbed areas (Godwin, 1960; Baker,1974).
Such habitats provide the limited competition required for persistence of
these species. Which types of habitats are most likely to contribute agricultu-
ral weeds has not been assessed for any agricultural weed flora. In related
work, Marks (1983) found that most of the plants invading abandoned agri-
cultural fields in the northeastern USA originated in wetlands and on cliffs.
Characteristics that adapt plant species to fertile disturbed habitats,
namely rapid growth, early maturity, high allocation to reproduction, resis-
tance to trampling, and resilience following shoot burial and damage to the
root system, preadapt species to thrive in sites disturbed by agriculture. From
the perspective of a potential weed, bare fields created by plowing represent a
bonanza of resources, and species that are preadapted to exploit such condi-
tions experience a great increase in fitness when agriculture is introduced to a
region.
Because potential weeds existed in the landscape at the time humans first
began deliberate cultivation of plants, even the earliest crops were probably
infested with weeds. Thus, for example, Garfinkel, Kislev & Zohary (1988)
reported that a store of carbonized lentils from Israel radiocarbon-dated at
8800 bp contained seeds of Galium tricornutum .This date is close to the earliest
records for Middle Eastern agriculture. Galium tricornutum still infests lentil
fields of non-industrial Middle Eastern farmers today. Similarly, strata depos-
ited from 6300 bp to 4800 bp in old oxbows of the Vistula River near Krakow,
Poland showed marked correlation between the abundance of cereal fossils
and the abundance of weed pollen, particularly Plantago major (or P. paucifolia)