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indicates that even among annuals only, selfers have smaller seeds than outcrossers
(Figure 2c). Future studies are required to test whether smaller-seeded selfing an-
nuals are more likely than their outcrossing annual relatives to disperse further or
colonize new habitats and thereby incur potential reproductive assurance benefits
of selfing.
An alternative explanation, however, is offered by an extension of the time-
limitation hypothesis: strong r-selection favors an acceleration of all stages of the
life cycle (Figure 1), including not only earlier reproductive maturity (Figure 1a)
and a shorter pollination time (facilitated through selfing) (Figure 1b), but also a
shorter seed and fruit maturation time, which, on a per-seed basis, is facilitated
in turn through the production of smaller seeds. Andersson [18] found that self-
fertilizing individuals of Crepis tectorum took an average of 16 days for fruit
maturation, whereas outcrossing individuals of the same species required 43.3
days. Small seed size may also be simply a trade-off of selection for high fecundity,
also favored by strong r-selection [11].
Habitat Selection and Time-Limitation
While most selfers are annuals, it is not the case that most annuals are selfers. An
unbiased literature survey [27] suggests that roughly half of all annual species are
selfers and half are outcrossers. If, however, selfing annuals evolved in habitats
with a short window of time for completing the life cycle (Figure 1), then self-
ing annuals should be significantly more common than expected (i.e. comprising
greater than 50% of resident annuals) within habitats associated with historically
regular, early-season disturbances (e.g. cultivated fields, gardens), or in habitats
where severe droughts follow quickly after a wet season (i.e. deserts, Mediterra-
nean climates, vernal pools). Hence, we should expect to find more selfers than
outcrossers among annual weeds of cultivated habitats and among desert annuals
in particular. Similarly, for annuals with both selfing and outcrossing ecotypes or
races, we should expect selfers (or a higher selfing rate) to be more commonly as-
sociated with these severely time-limited habitats [11].
While rigorous tests of these predictions have yet to be explored, some prelim-
inary support is available from published surveys. From a representative sample of
Mediterranean annuals [28], we find a much greater representation of selfers: i.e.
34 selfers versus 11 outcrossers. Selfing and outcrossing desert annuals have been
shown to be distributed along a moisture gradient. Outcrossing annuals are found
generally in the wetter areas and selfers in the more arid zones, as seen in Clarkia
xantiana [29] and between outcrossing populations of Limnanthes alba and its
selfing relative L. floccosa [24]. Since the length of the growing season is limited
by the amount of moisture in the soil, selfers have a much narrower window of
