Quantitative genetic variance and multivariate clines in the Ivyleaf morning glory, Ipomoea hederacea

Abstract

Clinal variation is commonly interpreted as evidence of adaptive differentiation, although clines can also be produced by stochastic forces. Understanding whether clines are adaptive therefore requires comparing clinal variation to background patterns of genetic differentiation at presumably neutral markers. Although this approach has frequently been applied to single traits at a time, we have comparatively fewer examples of how multiple correlated traits vary clinally. Here, we characterize multivariate clines in the Ivyleaf morning glory, examining how suites of traits vary with latitude, with the goal of testing for divergence in trait means that would indicate past evolutionary responses. We couple this with analysis of genetic variance in clinally varying traits in 20 populations to test whether past evolutionary responses have depleted genetic variance, or whether genetic variance declines approaching the range margin. We find evidence of clinal differentiation in five quantitative traits, with little evidence of isolation by distance at neutral loci that would suggest non-adaptive or stochastic mechanisms. Within and across populations, the traits that contribute most to population differentiation and clinal trends in the multivariate phenotype are genetically variable as well, suggesting that a lack of genetic variance will not cause absolute evolutionary constraints. Our data are broadly consistent theoretical predictions of polygenic clines in response to shallow environmental gradients. Ecologically, our results are consistent with past findings of natural selection on flowering phenology, presumably due to season-length variation across the range.

The evolvability of herkogamy: Quantifying the evolutionary potential of a composite trait

“…The link between micro-and macroevolution is subject to a long-standing debate in evolutionary biology (Gould 2002;Hansen 2012;Bolstad et al 2014), but if trait evolvability is important in constraining or facilitating trait divergence, we expect divergence of a given trait, or the direction of divergence in morphospace, to be positively associated with evolvability (Schluter 1996;Hansen and Voje 2011;Bolstad et al 2014). This leads to the prediction that herkogamy should have diverged proportionally more than other floral traits in many systems, which appears to be the case (e.g., Herlihy and Eckert 2007;Stock et al 2014;Opedal et al 2016). Further studies combining accurate estimates of quantitative-genetic parameters with data on trait divergence among populations and species seems a promising avenue toward understanding the links between microand macroevolution (Bolstad et al 2014).…”

Multivariate phenotypic divergence along an urbanization gradient

“…Despite accumulating evidence of evolution in response to urban-driven environmental changes, most studies focusing on phenotypic divergence associated with urbanization have examined just one or a few traits (Cheptou et al 2008; Gorton et al 2018;Winchell et al 2016; but see Yakub and Tiffin 2016). However, theoretical and empirical work in other systems suggests that selection can drive the evolution of multivariate phenotypic clines along environmental gradients (Lohman, Berner, & Bolnick, 2017;Stock, Campitelli, & Stinchcombe, 2014). Multivariate phenotypic analyses incorporate multiple, often correlated, traits to explain overarching shifts in phenotypes across environments.…”

Mating system shifts a species’ range

“…Positive or negative b value means that the optimum value increases or decreases against the geographic distance. For instance, trait means in anther-stigma distance of flower and the days to flowering show negative gradients with latitude in Ipomoea hederacea (Stock et al 2014). The abundant-center model is considered to describe a species' density distribution where population is the most abundant in the central location but gradually declines toward two edges in one-dimensional space.…”

“…In each demographic model, we first examine the case of fixed genetic variance for polygenic quantitative trait, such as the relatively constant genetic variances observed in five trait means exhibiting clines along latitude in Ipomoea hederacea (Stock et al. ). This is a plausible approximation if the genetic basis of the trait consists of infinite number of loci each with infinitesimal effects and selection is weak (Kirkpatrick and Barton ; Hill et al.…”

“…The abundant-center model is considered where population density is high in the central regions but declines in both directions toward species' edges (Gaston 2003). In each demographic model, we first examine the case of fixed genetic variance for polygenic quantitative trait, such as the relatively constant genetic variances observed in five trait means exhibiting clines along latitude in Ipomoea hederacea (Stock et al 2014). This is a plausible approximation if the genetic basis of the trait consists of infinite number of loci each with infinitesimal effects and selection is weak (Kirkpatrick and Barton 1997;Hill et al 2008), or if the causes of genetic variance are not well understood (Barton 2001).…”