EXPERIMENTAL STUDIES OF ADAPTATION IN clarkia xantiana:II. FITNESS VARIATION ACROSS A SUBSPECIES BORDER

Abstract

Abstract. Because the range boundary is the locale beyond which a taxon fails to persist, it provides a unique opportunity for studying the limits on adaptive evolution. Adaptive constraints on range expansion are perplexing in view of widespread ecotypic differentiation by habitat and region within a species' range (regional adaptation) and rapid evolutionary response to novel environments. In this study of two parapatric subspecies, Clarkia xantiana ssp. xantiana and C. x. ssp. parviflora, we compared the fitness of population transplants within their native region, in a non-native region within the native range, and in the non-native range to assess whether range expansion might be limited by a greater intensity of selection on colonists of a new range versus a new region within the range. The combined range of the two subspecies spans a west-to-east gradient of declining precipitation in the Sierra Nevada of California, with ssp. xantiana in the west being replaced by ssp. parviflora in the east. Both subspecies had significantly higher fitness in the native range (range adaptation), whereas regional adaptation was weak and was found only in the predominantly outcrossing ssp. xantiana but was absent in the inbreeding ssp. parviflora. Because selection intensity on transplants was much stronger in the non-native range relative to non-native regions, there is a larger adaptive barrier to range versus regional expansion. Three of five sequential fitness components accounted for regional and range adaptation, but only one of them, survivorship from germination to flowering, contributed to both. Flower number contributed to regional adaptation in ssp. xantiana and fruit set (number of fruits per flower) to range adaptation. Differential survivorship of the two taxa or regional populations of ssp. xantiana in non-native environments was attributable, in part, to biotic interactions, including competition, herbivory, and pollination. For example, low fruit set in ssp. xantiana in the east was likely due to the absence of its principal specialist bee pollinators in ssp. parviflora's range. Thus, convergence on self-fertilization may be necessary for ssp. xantiana to invade ssp. parviflora's range, but the evolution of outcrossing would not be required for ssp. parviflora to invade ssp. xantiana's range.

Interactions between Soil Habitat and Geographic Range Location Affect Plant Fitness

“…replacement) at both sites in the beyond edge region, the 95% confidence intervals of seed production included values below one (not shown), indicating that even these populations, though best-adapted to this region, may not maintain themselves here based on data from this year of study. This is consistent with other studies that find adaptation of populations to conditions at the range edge, though absolute fitness below that necessary to sustain the populations when transplanted beyond the range edge [38], [39]. By contrast, other studies have not found a reduction in fitness beyond the range edge [40]–[42].…”

Parallel evolution of phenological isolation across the speciation continuum in serpentine-adapted annual wildflowers

“…This proposed route to FT divergence is an example of countergradient selection leading to genetic compensation, wherein selection in the opposite direction of a plastic response leads to genetic change decreasing the amount of plasticity [21]. Countergradient selection in FT has been documented in other annual plants [59,60], including a study of ecotypes on and off of edaphically harsh mine tailings in Thalspi caerulescens [61].…”

Microgeographic Patterns of Genetic Divergence and Adaptation across Environmental Gradients in Boechera stricta (Brassicaceae)

“…Yet, the studies available for that meta-analysis likely did not assess variation in fitness at very early life history stages. Indeed, few reciprocal transplant studies have investigated the seed to seedling transition; therefore, we lack a comprehensive understanding of local adaptation in germination success (Donohue et al 2010; Geber and Eckhart 2005; Kim and Donohue 2013; Wilczek et al 2014). Germination is a critical life history transition, as seeds that fail to germinate have no future fecundity.…”