Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 466

Figure 10.3 Simulation ofevolution in outcrossing and inbreeding populations.
Even a low rate ofoutcrossing allows rapid and sustained adaptation to selection
pressure.(1) Random mating and moderate directional selection without
heterozygote advantage.(2) 95% selfing and moderate directional selection with
heterozygote advantage.(3) 95% selfing and moderate directional selection
without heterozygote advantage.(Redrawn from Allard & Hansche,1964.)
self-compatible species may evolve to changing environmental conditions in
ways that make control more difficult. This hypothesis requires further
investigation.
Similar adjustments in variability occur even more easily in perennial
weeds that spread vegetatively.The process is illustrated by Solidago altissima in
abandoned agricultural fields of various ages in central New York (Table 10.4).
Variability was low in the youngest field due to the small number of early col-
onizers. Variability in a somewhat older field was higher, probably because
additional individuals had time to arrive. Finally, variability was low in older
populations as the competitively superior genotypes replaced others (Table
10.4). Probably a similar sequence occurs in many perennial weed species as
populations establish and develop. In cases where ongoing dispersal into a
field is limited, variability would still be expected to increase in the short run
as recombination mixed the genetic material of the original founders. Then,