Diversification and Local Adaptation in Western Fence Lizards, Sceloporus occidentalis
Bouzid, Nassima Mahdjouba
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I am fascinated by intraspecific variation–by both its sources and its potential implications for how organisms interact with their environments. The importance of intraspecific variation for predicting species responses to climate change has recently become a research priority. Differences in the sources of intraspecific variation–genetic divergence, phenotypic plasticity, and drift–can have profoundly different outcomes for species responses. Variation in traits produced by heritable differences in genes will be sensitive to future selection, while variation produced by phenotypic plasticity may be buffered. Time and again, mechanistic studies of species responses have highlighted the importance of considering trait variation to predict idiosyncratic responses, and the sources of trait variation must also be considered. I studied intraspecific variation in Western Fence Lizards (Sceloporus occidentalis) at three spatial scales and three levels of organization. In Chapter 1 I investigated species-wide phylogeographic patterns and demographic scenarios throughout western North America. In Chapter 2 I characterized clinal variation in genotypes and phenotypes and gene flow along an elevation gradient in Yosemite National Park. In Chapter 3 I disentangled the genetic and plastic constituents of divergent phenotypes in a lab rearing experiment. My dissertation research provides an integrative framework for studying local adaptation in a polymorphic and well-established vertebrate system. Chapter 1 is the culmination of over two decades of research on phylogeographic structure within S. occidentalis. We sampled 108 individuals from 83 localities throughout the range in western North America. We used 4,555 SNPs from ddRADseq to characterize population structure and estimate demographic history. We found five genetically distinct populations including: one in the southwest, south of the Transverse Ranges; two west of the Sierra Nevada-Cascades cordillera, separated from north-to-south just north of San Francisco Bay; and two east of the Sierra Nevada-Cascades cordillera, separated from east-to-west in the Great Basin desert. The branching pattern of populations suggests that populations south of the Transverse Ranges and west of the Sierra Nevada-Cascades cordillera are divergent from populations east of the Sierra Nevada-Cascades cordillera. The predominant mechanism of population divergence is allopatric divergence and contemporary secondary contact, which supports Quaternary glacial cycles as drivers of intraspecific genetic divergence. Chapter 2 builds on foundational work by LeachÃ© et al. (2010) to characterize genetic and phenotypic clines along an elevation gradient in Yosemite National Park. At high elevations lizards are larger and more melanistic, while at low elevations lizards are smaller and lighter-colored. We sampled 78 individuals from a 21 km stretch of the Grand Canyon of the Tuolumne River in northern Yosemite. The elevation gradient spanned 1321 m from N Hetch Hetchy Reservoir (37.9168 N, 119.6595 W, 1167 m) in the west to E Glen Aulin (37.9076 N, 119.4196 W, 2488 m) in the east. We used 721 SNPs from ddRADseq to characterize genetic clines and estimate demographic history of populations along the elevation gradient. We found evidence for additional population structure and genetic divergence between phenotypically divergent individuals; one genetically distinct population corresponds to low elevation individuals and another corresponds to high elevation individuals. Analyses of SNPs, maximum size (snout-vent length, SVL), and coloration (ventral patch area) confirm that genes and phenotypes vary clinally, and not discretely, along the elevation gradient. Genetically distinct populations diverged in allopatry, but contemporary gene flow between populations is asymmetric. Genes flow uphill, with five times as many migrants entering the high elevation population from low elevation than the converse. Chapter 3 delves into the underlying sources of trait divergence between low and high elevation individuals from the Grand Canyon of the Tuolumne River elevation gradient. While low and high elevation lizards mature at the same age, high elevation lizards are larger and more melanistic than low elevation lizards. We disentangled the genetic and environmental constituents of phenotypic variation by rearing hatchling lizards under controlled lab conditions. We collected five gravid females from low elevation (N Hetch Hetchy Reservoir [37.96 N, -119.78 W, ca. 1200 m]) and eight gravid females from high elevation (Glen Aulin [37.91 N, -119.42 W, ca. 2400 m]), who produced 36 and 51 hatchling lizards, respectively. We evenly distributed hatchlings from both populations among two treatments that varied in potential activity time: short activity period (6 hrs) and long activity period (12 hrs). We varied activity time by limiting access to heat-lamp-produced thermal gradients, which are necessary for thermoregulation. We found evidence that differences in size are genetically-based; high elevation hatchlings were larger than low elevation hatchlings, regardless of treatment. We found evidence that differences in color are at least partially produced by phenotypic plasticity; high elevation hatchlings were capable of plastically lightening to a color that was lighter than low elevation hatchlings. We found evidence that differences in behavior are genetically-based; high elevation hatchlings spent more time engaged in active behaviors. Overall, our findings are suggestive of local adaptation of high elevation hatchlings to restricted activity periods at high elevation.
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