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response to competition for pollinators [7]. The evolution of self-fertilizing spe-
cies from outcrossing progenitors has occurred repeatedly and independently in
several lineages [1,3,14], each of which may have been associated with different
contexts of natural selection vis-à-vis the fitness benefits of selfing.
Flower Longevity
The families and genera in which selfers had shorter bud development times also
had significantly shorter flower longevities (Figure 4). In fact, all of the selfers had
flowers that remained open for less than four days (except in Trifolium hirtum;
Fabaceae), with a large proportion of flowers open for only one day, which is com-
mon amongst self-fertilizing species [17]. The present data again indicate that this
generalization apparently holds true even within annuals exclusively. By having
flowers that remain open longer, outcrossers increase the probability of visitation
by pollinators and successful cross-pollination [17]. This fitness benefit is real-
ized, however, only if there is sufficient time remaining after cross-pollination
to complete seed and fruit development before the maternal plant succumbs to
density-independent mortality in strongly r-selecting habitats [11]. If time is lim-
iting in this context, selection should favor selfing (Figure 1b) with no advantage
in having long-lived flowers.
It is important to note that our data measure maximum flower longevity, since
there were no pollinators in the greenhouse, nor was hand pollination conduct-
ed. Pollination has been shown to induce floral senescence in numerous species
[26]. This effect was not tested on any of the study species, which means that
our observed flower longevities in outcrossing species may be longer than would
normally be seen in the wild. Nevertheless, since selfing may have evolved as a
method of shortening pollination time, and flower longevity was used as a mea-
sure of pollination time, the maximum floral longevity gives an indication of
how long outcrossers can delay flower abscission or self-pollination (i.e. through
delayed selfing).
Seed Size
Strong r-selection associated with the annual life form presumably favors wide
dispersal mechanisms (for colonizing new and distant sites) which may be con-
ferred by small seed sizes [19]. The reproductive assurance hypothesis would pre-
dict, therefore, that most selfers are annuals because annuals are more likely than
perennials to disperse further, or colonize new habitats where conditions are un-
suitable for successful outcrossing (because of a shortage of mates or pollinators)
and where selfing, therefore, provides reproductive assurance. The present study
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