Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 461

Europe as well. Hybridization of table beet with the sweet form of leaf beet in
Europe followed by selection led to the modern sugar beet. Thus, the recent
evolution of a new weed race continues the long history of genetic interaction
between wild, domesticated, and weedy races of B.vulgaris.
Hybridization can also create new weeds without involvement of a culti-
vated species. A well-studied case is the British hexaploid species Senecio cam-
brensis which arose by chromosome doubling in hybrids between diploid S.
vulgaris and tetraploid S. squallidus (Ashton & Abbott, 1992; Harris & Ingram,
1992). Additional examples of the generation of new weeds by hybridization
are discussed in Stebbins (1965) and Sun & Corke (1992).
Evolution of new biotypes within existing weed species
Among weed scientists, the term biotype refers to a group of individu-
als that share some distinctive heritable trait or suite of traits. The concept is
not completely distinct from that of the ecotype , which refers to a race that is
adapted to particular ecological conditions (Lincoln, Boxshall & Clark, 1998).
An ecotype, however, consists of one or more populations, whereas multiple
biotypes are frequently present within a single population. Because most
weeds reproduce regularly by selfing, apomixis, or vegetative propagation,
well-adapted genotypes are easily preserved across generations as relatively
stable biotypes.
New weed biotypes may act as distinct problems if they respond to man-
agement differently than the parental type. For example, many herbicide-
resistant biotypes have evolved in response to herbicide application (Holt &
LeBaron, 1990; Shaner, 1995). Usually these resistant biotypes require
changes in management strategy, and may complicate weed management
considerably.
Other sorts of biotypes with apparently recent origin and significant conse-
quences for management have been identified.For example,Abbas,Pantone &
Paul (1999) reported that a biotype of Xanthium strumarium with up to 25 seeds
per bur has appeared recently in central Texas. Additional examples are dis-
cussed in the section “Ecotype formation in weeds”below.
Speciation of geographically isolated populations
In addition to the forces acting to create new weed biotypes,races,and
species discussed above, speciation by geographic isolation will likely add
weed species to the world flora over the coming millennia. Many agricultural
weed species have now been introduced onto dozens of different landmasses
(Table 10.2). To some extent populations on separate continents have already
diverged genetically (see section “Ecotype formation in weeds”below) and can