Data from: Mating system and historical climate conditions affect population mean seed mass: evidence for adaptation and a new component of the selfing syndrome in Clarkia

1. The evolution of seed size may be influenced by intrinsic attributes of
populations, such as mating system, and extrinsic factors, such as
climate. Several hypotheses propose that the evolution of
self-fertilization from an outcrossing progenitor will be accompanied by a
reduction in seed size, but this prediction has not been rigorously
tested. Many studies report that the mean seed size of populations or taxa
is associated with long-term climate conditions. Here, we examined the
effects on seed size of both mating system and climate within a single
genus. 2. In the California wildflower genus, Clarkia (Onagraceae), we
sampled seeds from 58 populations representing three pairs of sister taxa;
each pair included a predominantly outcrossing and a facultatively selfing
taxon. We then examined the independent effects on population mean seed
size of mating system, elevation, long-term (30-year) climate conditions,
and climate anomalies (the deviation between conditions in the year of
collection and the long-term mean), focusing on maximum monthly
temperature (Tmax), cumulative moisture deficit, and cumulative
precipitation (PPT) during Clarkia’s growing season (fall, winter, and
spring). 3. In each taxon pair, the selfing taxon had smaller seeds than
the outcrosser. Local, long-term (1921-1980 and 1981-2000) mean Tmax, PPT,
and elevation were independently and negatively associated with seed size.
Long-term means for Tmax and PPT explain geographic variation in seed size
better than climate anomalies in the year of collection. 4. Synthesis: We
corroborated two key hypotheses concerning the drivers of geographic
variation in mean seed size. Small seeds in Clarkia co-evolve with selfing
(although the mechanism remains elusive) and in response to chronically
warm and wet conditions. The effect of long-term mean precipitation on
seed size differs qualitatively from the effect of precipitation
anomalies; relatively large seeds are produced in populations experiencing
wetter-than-normal years. Ongoing climate change may therefore generate
conflicting selection on seed size in Clarkia: intensifying drought is
likely to lead to an evolutionary increase in seed size due to its effects
on seedling survivorship, while climate-driven declines in pollinators or
selection favoring more rapid reproduction may promote the evolution of
self-pollination, facilitating the evolution of smaller seeds. 

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