Absence of population structure across elevational gradients despite large phenotypic variation in mountain chickadees (Poecile gambeli)

Abstract

Montane habitats are characterized by predictably rapid heterogeneity along elevational gradients and are useful for investigating the consequences of environmental heterogeneity for local adaptation and population genetic structure. Food-caching mountain chickadees inhabit a continuous elevation gradient in the Sierra Nevada, and birds living at harsher, high elevations have better spatial memory ability and exhibit differences in male song structure and female mate preference compared to birds inhabiting milder, low elevations. While high elevation birds breed, on average, two weeks later than low elevation birds, the extent of gene flow between elevations is unknown. Despite phenotypic variation and indirect evidence for local adaptation, population genetic analyses based on 18 073 single nucleotide polymorphisms across three transects of high and low elevation populations provided no evidence for genetic differentiation. Analyses based on individual genotypes revealed no patterns of clustering, pairwise estimates of genetic differentiation (FST, Nei's D) were very low, and AMOVA revealed no evidence for genetic variation structured by transect or by low and high elevation sites within transects. In addition, we found no consistent evidence for strong parallel allele frequency divergence between low and high elevation sites within the three transects. Large elevation-related phenotypic variation may be maintained by strong selection despite gene flow and future work should focus on the mechanisms underlying such variation.

Divergent selection along elevational gradients promotes genetic and phenotypic disparities among small mammal populations

“…As altitude increases, steep environmental changes promote spatially dynamic biotic and abiotic interactions. Individuals living in distinct elevation zones are thus expected to show phenotypic and genetic disparities owing to strong divergent ecological selection (Branch, Jahner, Kozlovsky, Parchman, & Pravosudov, 2017;Kawecki & Ebert, 2004;Keller et al, 2013;Ohsawa & Ide, 2008). For example, in southeastern Asian mountains, evergreen broadleaf forests occur at 1,000 m, and alpine landscapes occur at 3,500 m (Ohsawa, 1991(Ohsawa, , 1993.…”

“…Such mixed effects of contrasting climatic variables on a short scale reflect marked heterogeneous vegetation zonation along mountain slopes, in some cases mirroring tropical-temperate forest transitions. In contrast, elevational gradients over a small geographic scale may facilitate migration, especially in species with a high dispersal capacity, thereby constraining morphological and genetic divergence (Branch et al, 2017;Kawecki & Ebert, 2004;Keller et al, 2013;Nosil et al, 2008;Ohsawa & Ide, 2008;Sexton, Hangartner, & Hoffmann, 2014;Waterhouse, Erb, Beever, & Russello, 2018). Individuals living in distinct elevation zones are thus expected to show phenotypic and genetic disparities owing to strong divergent ecological selection (Branch, Jahner, Kozlovsky, Parchman, & Pravosudov, 2017;Kawecki & Ebert, 2004;Keller et al, 2013;Ohsawa & Ide, 2008).…”

“…The presence of animals (particularly those with high vagility) in contrasting habitats on a short geographic scale makes such animals attractive candidates with which to test for ecologically induced genetic alterations. Thus, the existence of limited gene flow between mammalian populations across mountain zones would indicate a strong role of heterogeneous landscapes in shaping the genetic structure of natural populations (Branch et al, 2017;Kierepka & Latch, 2015;Wang & Bradburd, 2014). Thus, the existence of limited gene flow between mammalian populations across mountain zones would indicate a strong role of heterogeneous landscapes in shaping the genetic structure of natural populations (Branch et al, 2017;Kierepka & Latch, 2015;Wang & Bradburd, 2014).…”

Genomic data reject the hypothesis of sympatric ecological speciation in a clade of Desmognathus salamanders

“…); pupfishes (Martin and Feinstein )], but failed to do so in others [e.g., mountain chickadees (Branch et al. ); Mimulus sp. (Twyford and Friedman ); Littorina sp.…”

“…Instead, morphologies associated with ecological differentiation may be tied to genomic islands of divergence (Turner et al 2005;Guerrero and Hahn 2017;Wolf and Ellegren 2017), where genetic divergence is highly localized to key areas of the genome under selection (e.g., Twyford and Friedman 2015;Marques et al 2016;Toews et al 2016). In other cases there may be no consistent pattern of genetic differentiation, suggesting plasticity alone could be responsible for ecological and mor-phological differences, resulting in phenotypic differentiation without lineage divergence (e.g., Dowle et al 2014;Mason and Taylor 2015;Branch et al 2017;Brock et al 2017). Thus, both ecological and morphological differentiation may act as poor proxies for genetic divergence across the speciation continuum.…”