|Context-dependent concordance between physiological divergence and phenotypic selection in sister taxa with contrasting phenology and mating systems
|Year of Publication
|Mazer, Susan J., David J. Hunter, Alisa A. Hove, and Leah S. Dudley
|American Journal of Botany
|adaptation, Clarkia, climate change; herbarium; herbarium specimens; pheno‐climatic models; phenological index; phenology, divergence, gas exchange, mating system evolution, phenology, phenotypic selection, water use efficiency
Premise: The study of phenotypic divergence of, and selection on, functional traits in
closely related taxa provides the opportunity to detect the role of natural selection in
driving diversification. If the strength or direction of selection in field populations
differs between taxa in a pattern that is consistent with the phenotypic difference
between them, then natural selection reinforces the divergence. Few studies have
sought evidence for such concordance for physiological traits.
Methods: Herbarium specimen records were used to detect phenological differences
between sister taxa independent of the effects on flowering time of long‐term
variation in the climate across collection sites. In the field, physiological divergence in
photosynthetic rate, transpiration rate, and instantaneous water‐use efficiency were
recorded during vegetative growth and flowering in 13 field populations of two taxon
pairs of Clarkia, each comprising a self‐pollinating and a outcrossing taxon.
Results: Historically, each selfing taxon flowered earlier than its outcrossing sister
taxon, independent of the effects of local long‐term climatic conditions. Sister taxa
differed in all focal traits, but the degree and (in one case) the direction of divergence
depended on life stage. In general, self‐pollinating taxa had higher gas exchange rates,
consistent with their earlier maturation. In 6 of 18 comparisons, patterns of selection
were concordant with the phenotypic divergence (or lack thereof) between sister taxa.
Conclusions: Patterns of selection on physiological traits measured in heterogeneous
conditions do not reliably reflect divergence between sister taxa, underscoring the
need for replicated studies of the direction of selection within and among taxa.