Evolutionary History of the Patagonian Liolaemus fitzingerii Species Group of Lizards
Grummer, Jared Anthony
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The majority of the world’s land mass and biota reside in the Northern Hemisphere. However, even when land area is accounted for, we know disproportionately less about Southern Hemisphere flora and fauna than their Northern Hemisphere counterparts. The South American biota is extremely unique with high levels of endemism due to a long history of geologic and evolutionary isolation. A prime example of South American endemism is the Squamate family Liolaemidae. In this family, the sole genus Liolaemus has one of the widest elevational, latitudinal, and climatic distributions of any lizard genus anywhere. The 258 described species (at the time of this dissertation) in this genus are distributed across 40 of latitude, from southern Peru to Tierra del Fuego, and from sea level to more than 16,000’ in elevation. The genus Liolaemus is composed of two subclades, Liolaemus (sensu stricto) and Eulaemus, and it is in the second clade that we find the Liolaemus fitzingerii species group. The L. fitzingerii group is ⇠5 million years old and is distributed in the Patagonian shrub-steppe of central Argentina from approximately 37-50 S latitude. Due to its abundance in the field, high morphological diversity, and broad distribution, this species group has been the subject of many taxonomic, ecological and evolutionary studies. Taxonomic studies of the group began in the mid- 19th century when Charles Darwin collected the L. fitzingerii holotype; nine species are currently recognized in the group. Approximately a decade ago in 2006, Avila and colleagues performed an in-depth phylogeographic analysis of this species group where they inferred hybridization and post-Pleistocene glacial range expansion in some of the species in this group. In light of previous studies, I addressed three specific goals that I partitioned into the three chapters of my dissertation: 1) infer evolutionary relationships between described and candidate species in the Liolaemus fitzingerii group, 2) determine the number and geographic extent of genetically distinct populations in the group as a function of geologic features and historic climatic events, and 3) compare evolutionary patterns and processes across independently formed hybrid zones in this group. Each chapter had a distinct molecular dataset. For the first chapter, I collected DNA sequence data for 580 nuclear loci and full mitochondrial genomes of 27 individuals. The dataset for chapter 2 was 178 individuals that were sequenced for ⇠1,500 genome-wide SNPs (single nucleotide polymorphisms). And for chapter 3, I sampled 267 individuals that were sequenced for 2,000 SNPs and the mitochondrial cytochrome B gene. I performed a variety of phylogenetic reconstruction techniques in chapter 1, including multispecies coalescent and concatenation approaches. Because hybridization was inferred from previous research on this species group, I also conducted network analyses that consider reticulate evolutionary relationships. Although these methodologies are quite distinct, they all revealed low support for relationships between species. Furthermore, the network analyses supported at least two instances of interspecific hybridization. My conclusion is that the poor phylogenetic support reported across analyses indicates a rapid radiation from a common ancestor, but this signal may also be exacerbated by poor taxonomy and an over-description of species. In chapter 2, I sought to determine the effects of landscape features and Pleistocene glacial cycling (e.g., over the last ⇠2.6 million years) on the distribution of populations in the Liolaemus fitzingerii group. With 178 individuals covering the known distribution of this group, analyses revealed six distinct populations that are arranged predominantly in east-west bands. In the north, the Somuncura Plateau marks the interface between two populations, as does the Canquel Plateau in the south. Similarly, the Chubut River forms a nearly complete barrier between two populations in the center of the group’s distribution. Migration analyses bolstered these results, with low levels of migration inferred around these landscape features. An expected effect of late-Pleistocene glaciations is that genetic diversity should be highest in the east and north where refugial populations were predicted to inhabit. The estimates of genetic diversity support this, with higher genetic diversity in the east and north, and conversely lower genetic diversity in the west and south. My analyses of demographic models also support glacial refugia, in that all populations went through a population bottleneck and only very recently have population sizes begun to recover. These results show the importance of geographic features and climatic events in shaping the evolutionary history of the L. fitzingerii species group, and add much needed data to our relatively poor understanding of taxa in this region of the world. My aims for chapter 3 were to characterize suspected hybrid zones in the Liolaemus fitzingerii species group and assess selection on both nuclear and mitochondrial genomes in a comparative manner. Initial analyses revealed a completely unexpected result where four species are connected through three hybrid zones, and two species, L. melanops and L. xanthoviridis each hybridize with two other species. I calculated linkage disequilibrium coefficients for the SNP data and estimated clines for both nuclear and mitochondrial DNA, which allowed me to calculate selection and compare the strength of selection acting on the same species in the different hybrid zones. In all three hybrid zones, the mitochondrial cline was to the south of the nuclear cline, indicating that either the hybrid zones are moving to the north, or that a northward male-biased dispersal occurs in each of these hybrid zones. When comparing levels of selection acting on the same species in each of the three hybrid zones, L. melanops was under stronger selection when hybridizing with L. xanthoviridis as compared to L. shehuen. This is potentially due to limited dispersal abilities of northern L. melanops individuals as compared to individuals in the southern part of the range, or stronger exogenous selection in the south due to differing ecologies of L. xanthoviridis and L. melanops. In the second comparison, the strength of selection against L. xanthoviridis is higher when hybridizing to the south with L. fitzingerii than L. melanops to its north. The higher selection in the south could be due to differing habitats that these two species occupy, or that the low genetic diversity of L. fitzingerii mathematically inflates the selection estimate. In summary, my research supports the notion that species in the Liolaemus fitzingerii group are the result of a rapid evolutionary radiation, and that this signal is likely strengthened by taxonomic inflation. Prominent geologic features such as plateaus and rivers seem to have strongly influenced the spatial distribution of populations in this group, in tandem with glacial cycling over the past 2.5 million years. Hybridization is commonplace where distinct populations meet, which has provided a unique opportunity to study independent replicates of the evolutionary process. My research on the Patagonian Liolaemus fitzingerii species group has helped reduce the knowledge gap of phylogeographic and evolutionary studies between Northern and Southern Hemisphere taxa.
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