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I am a PhD candidate in Chris Austin’s Lab at the Museum of Natural Science at Louisiana State
University. Feel free to follow the links for more information.
Nathan Jackson
Museum of Natural Science
119 Foster Hall
Louisiana State University
Baton Rouge, LA 70803
Email: njacks4@lsu.edu
Phone: 225-578-5393
RESEARCH
I am an evolutionary biologist primarily interested in understanding the mechanisms that govern
population divergence and speciation. My interests encompass population genetics, systematics,
phylogeography, speciation, landscape genetics, and conservation biology.
The focus of my dissertation is to understand how organisms and their genes interact with the
environment (e.g. geographical features, climate, etc.) to generate and maintain diversity. A
major goal is also to utilize emerging species delimitation approaches to define hierarchical
population structure in a recent radiation. Specific projects I am currently working on are listed
below:
Phylogeographic History of a Common North American lizard
The ground skink, Scincella lateralis, is one of the most abundant reptiles in the southeastern
United States. Occupying a continuous distribution across most of its range and exhibiting no
apparent morphological differences among its populations, this lizard is not the first organism
from which you would expect to observe complex genetic fragmentation. Yet, this species
exhibits pronounced genetic structure at both mitochondrial and autosomal loci. The geographic
distribution of the observed genetic diversity and structure relative to both latitude and major
rivers suggests a river-refugial basis for diversification of the group, where riverine barriers,
strengthened when populations were trapped within southern refugia in the recent past, are
responsible for the isolation and divergence of populations.
To further explore this hypothesis, I plan to collect north-south sample transects along both sides
of several Gulf Coast rivers. If the river-refugial hypothesis is correct, I would expect that 1) the
delineation of distinct lineages by rivers should be most precise for samples near the coast and
blur as you sample northward and 2) the latitude at which rivers cease to delineate lineage
boundaries should lie north of the species’ southern-most paleo-climate-based niche distribution.
Delimiting Lineage Structure and Boundaries in a Recent Radiation
For populations or lineages in the early stages of speciation, the history of evolutionary
divergence is infamously difficult to reconstruct, in part due to the presence of shared alleles
among populations that can result from ongoing gene flow or the retention of ancestral
polymorphisms in one or both groups. Preliminary analysis of multilocus genetic data for S.
lateralis indicates that both of these processes (along with intra-locus recombination, high
heterozygosity and discordance between mitochondrial and nuclear histories) are confounding
estimation of the “true” lineage tree.
I am currently taking advantage of recent advances in population genetic and phylogenetic
methodology that allow for the explicit modeling of gene flow and lineage sorting stochasticity
in a coalescent framework to better track and define the hierarchical structure of populations, as
well as disentangle the underlying evolutionary processes that gave rise to it. In doing so, I also
aim to compare inferences using several “species tree” and migration-divergence methods as
well as explore the sensitivity of these methods to sampling strategy and recombination.
Testing the Role of River Channel Migration in Population Genetics
The major tenet of the riverine barrier hypothesis is that rivers are sufficiently long-lived and
impenetrable to would-be migrants as to play a significant role in species diversification
wherever they occur. While major rivers are likely an impediment to migration for many
organisms, evidence of their wide-spread effect on speciation is mixed. One characteristic of
large rivers often cited as important for reducing their isolating effect is the recurrence of
channel migration due to meander cutoff or “oxbowing.” Most large meandering rivers undergo
a dynamic process of erosion and deposition that leads to lateral shifting of channels and the
formation of meander loops. These loops eventually expand to the point of being cut off at the
neck by the formation of a more direct channel, leaving behind an ever-receding oxbow lake.
This process may facilitate the passive transport of organisms inhabiting the meander loop from
one side of the river (before cutoff) to the other (after cutoff), thereby breaking down an
otherwise significant dispersal barrier.
Scincella lateralis is a great species in which to empirically test the predictions of this migrationby-oxbow-transfer model given that major rivers have been shown to delineate lineages in this
group, albeit with some mixing near their boundaries (which suggests that rivers are an
important—but leaky—long -term dispersal barrier for ground skinks). Using data from hypervariable microsatellites analyzed using a variety of coalescent and cluster-based approaches, I
am currently comparing past and current migration rates and direction between sampled sites
along the Mississippi River to determine whether gene flow across the river is heightened at sites
near oxbow lakes relative to control sites. Alternative explanations for the observed low-level
gene flow across the Mississippi River (such as wholesale channel-switching of the Mississippi
River Delta) are also being investigated.
Understanding the Effects of Landscape Heterogeneity and Recent History on Dispersal
The introduction of highly-variable genetic data and improved statistical tools to problems in
ecology and evolution has recently stimulated an interest in ever-more fine-scaled investigations
of the historical and current processes that govern the dispersal of individuals, the genetic
structuring of populations, and the genesis of species. These investigations are particularly
welcome when applied to very recent history given that the heterogeneous and ephemeral nature
of many features of the modern landscape (largely due to anthropogenic activity) may shape
dispersal patterns and genetic structure differently than did the (relatively) less fragmented
landscapes of the pre-historical past.
Scincella lateralis is ideal for a fine-scale landscape genetic study due to its high abundance and
availability, short generation time, and apparent philopatry. I have collected tail-tips from over
600 lizards in a ~650 km2 area west of Baton Rouge, Louisiana (an area that has undergone
documented and extensive fragmentation of forest habitat over the past two centuries) in order to
investigate the effects of landscape heterogeneity on ground skink dispersal using microsatellite
data. Very little is known about the ability of these lizards, or small ground dwelling, dispersallimited organisms generally, to traverse the variety of matrix habitats that characterizes the
typical landscape.
I will compare the fit of a variety of alternative landscape dispersal models (constructed from
Euclidean, least-cost, and resistance distances) to genetic data in order to test the permeability of
various landscape features (e.g. roads, waterways, agricultural fields, geographical distance) to
the dispersal of S. lateralis. I also plan to investigate the time-scale of the genetic response to
landscape change by comparing the fit of genetic data to landscape models based on the current
landscape versus models based on landscapes that existed in the past as inferred from historical
aerial photographs and old maps.
Investigating Species Boundaries in a Complex of Southern Appalachian Salamanders
As a master’s student, I investigated phylogeographic history in a very interesting species
complex of Appalachian salamanders which includes Desmognathus quadramaculatus, D.
marmoratus, and D. folkertsi. Results from this study have yet to be published (for some
complex reasons), but should finally be submitted in the very near future. Look for it!