Biology Reference
In-Depth Information
time to complete their life cycle before desiccation. During a severe drought, seed
production in L. alba was reduced by one sixth, whereas the seed set of L. floccosa
found in the same area was virtually unaffected by the identical drought [24].
The association between 'weediness' and self-fertilization has also been noted
[2,30]. An extensive survey of colonizing herbaceous plants of Canada showed
that agricultural weeds of row crops and grain fields are almost exclusively an-
nuals, and most of these are self-compatible [31]. A published list of the world's
worst weeds of agricultural crops [32] includes 76 species, 41 of which are annu-
als. Based on previous literature, we were able to identify the breeding system for
24 of these annuals, and, as predicted, the majority (20 out of 24) are selfers.
Conclusions
Botanists have long known that selfing is particularly associated with the annual
life cycle in flowering plants [2]. The present study shows further that, among
annuals exclusively, selfing is particularly associated with shorter plant heights,
smaller flowers, shorter bud development time, shorter flower longevity and
smaller seed sizes compared with annuals that are outcrossing. Also, in spite of the
null prediction that selfing and outcrossing annuals should be equally represented
if there is no bias associated with time-limitation, we found instead that two of
the most time-limited habitats on earth that support flowering plants have a sig-
nificantly higher percentage of selfers among the resident species that are annuals.
Because we focused on annual species only, all of these results are explained more
parsimoniously by selection associated with time-limitation than by selection as-
sociated with pollinator/mate limitation. The role of pollinator/mate-limitation
(as traditionally associated with the reproductive assurance hypothesis for the evo-
lution of selfing) is likely to be of greater importance in longer-lived polycarpic
species (not considered here), simply because by comparison, there is no convinc-
ing basis to argue that selection associated with time-limitation is likely to have
been important in species with longer life cycles. We suggest therefore, that most
selfers, because most of them are annuals, are likely to have evolved not because
of fitness benefits through reproductive assurance associated with selection from
pollinator/mate limitation, but rather because of fitness benefits associated with
selection from time limitation.
The effect of time-limitation under strong r-selection is to minimize the dura-
tion of the life cycle, with selfing favored directly (Figure 1b) and/or indirectly
(Figure 1a). There is no basis for predicting that either mechanism is more prob-
able than the other; both are likely to operate simultaneously and perhaps in-
distinguishably. Indeed, the predicted effects under direct and indirect selection
involve the same phenotypic outcome for the same suite of traits (Figure 1). It
