Biology Reference
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growth and development rates between selfing and outcrossing populations of
only a single species. Since their effective sample size is only one, this makes it dif-
ficult to extrapolate the predominant selection pressures that may have promoted
the general association of selfing with the annual life cycle.
Figure 1.
Two components of the 'time-limitation' hypothesis for the evolution selfing in annuals.
In (a),
selfing is a trade-off of selection favoring a shorter time to reproductive maturity (fully developed flowers)
under strong r-selection. As a tradeoff (dashed arrows), flowers become smaller with greater overlap in location
and timing of anther dehiscence and stigma receptivity, thus increasing the rate of selfing as an incidental
consequence. In (b) strong r-selection favors a shorter pollination time directly; i.e., selfing is selected for directly
because it shortens the amount of time between flower maturation and ovule fertilization, thus leaving sufficient
remaining time for seed and fruit maturation before the inevitable early mortality of the maternal plant under
strong r-selection. In this case, smaller flower size and shorter flower development time are favored by selection
because they facilitate selfing (see text).
The objective of the present study was to compare, for annuals exclusively, life
history traits associated with selfing versus outcrossing using several species from
a wide range of plant families. Phylogenetically-independent contrasts (PIC) were
used to control for confounding effects due to common ancestry among species
[16]. Using a database of 118 species involving 14 families, plant size, flower
size, and seed size were compared between selfing and outcrossing annuals. The
time-limitation hypothesis predicts that all of these traits should be smaller in
selfing annuals because the severely time-limited growing season that promotes
selfing also imposes an upper limit on the maximum sizes that can be attained for
plant traits [11] (Figure 1). The trend for outcrossers to be taller, and have larger
flowers and larger seeds has often been noted [1,17-19]. We used a multi-species,
