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At the same time, these results are not inconsistent with the predictions of
selection resulting from pollinator/mate-limitation associated with the traditional
reproductive assurance hypothesis. Just as with many situations where two differ-
ent mechanisms can potentially produce the same outcome/pattern, it is not easy
here to clearly distinguish between the roles of “pollinator/mate-limitation” and
“time-limitation”. Nevertheless there are two important contributions from our
study: First, in reporting significant life history differences between selfers and
outcrossers, our multi-species study is unique in its comparison of monocarpic
annual species exclusively. All previous multi-species studies of trait comparisons
between selfers and outcrossers have involved variable mixes of monocarpic and
longer-lived polycarpic species. Second, by comparing annuals exclusively, our
results provide indirect support for the time-limitation hypothesis, not by reject-
ing the role of pollinator/mate-limitation, but rather by representing a system in
which it is more plausible to argue for the role of time-limitation; i.e., compared
with pollinator/mate-limitation, time-limitation as a selection factor favoring self-
ing is likely to have been much stronger, more persistent and more widespread.
The strength of this argument lies in the fact that the annual life history is un-
equivocally a product of some type of time-limitation favoring an abbreviated life
cycle, which is promoted by (among other things) selfing (as opposed to outcross-
ing) (Fig. 1). It is much less plausible to suspect that selection associated with
pollinator/mate-limitation has been sufficiently strong and persistent to favor
selfing in such a wide range of annual taxa across the many genera and families
considered here. We emphasize, therefore, that for annuals the time-limitation
hypothesis provides a more parsimonious explanation for the differences in traits
between selfers and outcrossers. We consider each of these traits in turn below.
Plant Height and Time to Anthesis
Taller plants may attract more pollinators and, hence, experience greater out-
crossing rates [20,21]. The pollination benefit of being relatively tall, therefore,
is presumably experienced only by outcrossers. If, however, selfers have evolved
from outcrossers [3], then why should selfers be shorter than their outcrossing
ancestors? The relatively small size, including short height of selfers can be pre-
dicted as an indirect consequence of selection, from time-limitation, favoring
precocious maturation time [22,23] (Figure 1a). In the present study, however,
selfers and outcrossers did not differ significantly in age at first flower. Andersson
[18] found similar results between selfing and outcrossing populations of Crepis
tectorum. Arroyo [24], however, reported that selfing individuals of Limnanthes
floccosa flowered earlier than the outcrossing L. alba, as predicted by the time-
limitation hypothesis. The results for flowering times in the present study may be
confounded by the controlled greenhouse environment of constant day-length,
