ORCID Identifier(s)

0000-0002-5197-4791

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Quantitative Biology

Department

Biology

First Advisor

Matthew K Fujita

Abstract

Species are fundamental entities in biology because their existence represents the cohesion that binds populations into a single unit. One facet of evolutionary biology is to identify the processes that cause the origin of species and maintain this cohesion apart from closely-related lineages. Two aims that speciation research attempts to answer are: (1) How do lineages diverge to form new species? and, (2) What prevents the merging of nascent species? One useful approach to address both questions is to study areas of contact where independent populations coexist and exchange genetic material. By investigating the size of contact zones, the extent of genetic exchange between lineages (gene flow), and the identity of introgressed genes; we can understand why and how species remain distinct despite ongoing gene flow between closely-related species. My dissertation focuses on uncovering the dynamics that gene flow has on potentially early and late stage speciation events. To address these goals, I employed next-generation sequencing to quantify species boundaries and the extent of gene flow in two widespread Australian gecko lizard systems, Lialis burtonis and Heteronotia binoei. I found that L. burtonis has four distinct populations across Australia, with limited gene flow between groups, even when in close proximity. Each population appears to associate with an ecoregion in Australia. Currently there are four recognized lineages within the species: a population in the interior of Australia’s arid zone, with a population in the Pilbara region (known for its high rates of endemism in native flora and fauna), the northern monsoonal tropic population, and a population to the occupying the eastern mesic zone bordering the arid zone. Migration is high within populations but not between populations. Findings show that migration between populations only happens where distributions overlap or are adjacent to one another. Migration within populations is likely because L. burtonis is an active and highly mobile predator. Heteronotia binoei is hyperdiverse across its range, with over 60 mitochondrial populations discovered and more found every field season. Heteronotia binoei is constrained to outcrops, does not venture far from the established habitat of its home range. Broadly, this species colonizes and disperses across Australia. However, as lineages expand, isolation begins among distinct populations, allowing for lineages to diverge independently, erecting genetically distinct demes. Comparing 16 lineages across three contact zones at varying degrees of divergence, I found that more closely-related lineages, in overlapping range, exhibit greater gene flow compared to more divergent lineages. Secondary contact between divergent lineages occurs when expanding ranges between varying niche overlap. In this study, these lineages are not sister and have low or no gene flow when there is co-occurrence. These lineages do not encounter any gene flow between them and continue to diversify independently. In this scenario, it is most probable that a speciation event happened, or late-stage speciation is currently occurring. Heteronotia binoei lineages that are sister taxa have continued gene flow or ancestral genotypes still within the population. Isolation by distance through parapatric speciation is the most probable cause for relatively recent lineage splits, where individuals close in geography have higher gene flow than those on the periphery of the range. To test whether there is habitat partitioning driving isolation, niche models were constructed using vegetation, radiation, temperature, precipitation, topography, and elevation variables. All models had no strong support for niche separation, confirming that H. binoei is a generalist gecko occupying most areas of Australia, where temperatures do not go below freezing. Australia’s climatic and geologic history, after splitting from Gondwana, has influenced the evolutionary history of its flora and fauna. Both Lialis and Heteronotia have their origins diversifying in isolation throughout the changing landscape and habitat of Australia. These two genera, however, are quite contrasting from each other. Lialis burtonis only has four distinct lineages, while Heteronotia binoei is hyperdiverse, with new lineages continually being discovered. Very little gene flow and high population structure and fixation indexes in Lialis indicate independent populations. Within Lialis populations fixation indexes are low. By contrast, Heteronotia has many lineages at varying degrees of divergence. Many lineages in contact allow the migration of alleles between lineages, suggesting potential early speciation events. More deeply diverged lineages in secondary contact do not show allele migration or gene flow at overlapping ranges. These lineages experience the same biogeographic conditions but different evolutionary histories. Varying life histories between the species is presumably the root cause for the incredibly different demographies. Lialis is, comparatively, a large gecko, legless, and an active predator of secondary consumers (small squamates). Heteronotia does not cover relatively large distances to forage; it is confined to small home ranges as an insectivorous secondary consumer. The NSF EAPSI Fellowship funded this work to gather sequence capture data and the Lewis and Clark Grants for international fieldwork allowed for the collection of samples, vouchering at James Cook University, and sequencing.

Keywords

Speciation, Heteronotia, Lialis, Genomics, Niche model

Disciplines

Biology | Life Sciences

Comments

Degree granted by The University of Texas at Arlington

Included in

Biology Commons

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