Speciation and biogeography of subterranean organisms

Divergence with gene flow

Speciation in subterranean organisms is generally thought to follow one of two main models: the climate-relict model which invokes allopatric speciation and the adaptive-shift model where parapatric speciation occurs in the face of gene flow. However, speciation in the face of gene flow is generally thought to be difficult because gene flow constrains differentiation, thereby preventing the evolution of reproductive isolation. In collaboration with Ben Fitzpatrick (University of Tennessee), we are investigating divergence with gene flow between the cave-dwelling Gyrinophilus salamanders and their close relative, the Spring Salamander (G. porphyriticus). We have shown previously using coalescent-based analyses under an "isolation with migration" model that divergence of cave species from their surface relative has occurred with gene flow (Niemiller et al. 2008; Niemiller et al. 2009). We are now applying next-generation sequencing (NGS) to this system to examine the dynamics of gene flow and divergence in these salamanders in greater detail across multiple cave populations where both surface and cave species co-occur and morphologically-intermediate individuals exist.  We are also investigating what factors promote differentiation despite gene flow and rapid adaptation to subterranean habitats. 

Primary Collaborators: Benjamin M. Fitzpatrick (University of Tennessee), R. Graham Reynolds (University of Massachusetts-Boston)

Dispersal and vicariance

Aspelaea Fig3

There has been a long and ongoing debate on the roles of vicariance and dispersal in the biogeography of subterranean organisms. Most subterranean species have highly restricted distributions, which has traditionally been explained by reduced abilities or opportunities for dispersal. However, some species have broader distributions, particularly aquatic taxa, which might reflect greater connectivity of subterranean habitats in some regions. Both dispersal and vicariance events are thought to have influenced the biogeography of subterranean taxa but biogeographic hypotheses have only recently begun to be explored using molecular approaches from many taxonomic groups. 

Determining the factors promoting speciation and divergence in subterranean organisms is of great interest to my research program. In a recent paper, my colleagues and I tested several hypothesis-driven approaches to assess the influence of Pleistocene climatic and geological changes on the biogeography of the Northern Cavefish, Amblyopsis spelaea (Niemiller et al. 2013). This species is distributed just to the south of the southernmost extent of glacial advances in Indiana and Kentucky, with populations occurring both north and south of the Ohio River. Our results showed that the modern Ohio River has been a significant barrier to dispersal in this species and suggest that populations were isolated in two refugia located north and south of the river during the most two recent glacial advances potentially leading to cryptic sister lineages. Climatic changes have promoted colonization of subterranean habitats and speciation from surface-dwelling ancestors, whereas geological processes have facilitated subterranean isolation and divergence.

I am examining speciation and the biogeography of several other subterranean groups that inhabit caves in eastern North America, including cave beetles, crangonyctid amphipods and obligate cave-dwelling salamanders.

Primary Collaborators: Benjamin M. Fitzpatrick (University of Tennessee), Thomas J. Near (Yale University), William Pearson (University of Louisville)

Speciation and possible adaptive radiation in cave beetles

Pseudanophthalmus sp, Pompie Cave, Maury Co, TN, November 2012, No2

Cave-dwelling carabid beetles in the subfamily Trechinae represent a extremely large radiation of subterranean species in the Interior Low Plateau and Appalachian Valley and Ridge karst regions of the eastern United States. Over 150 species are currently recognized, the vast majority of which are restricted to a single or just a few cave systems. In some cases, up to six different species may inhabit the same cave. Consequently, this beetle group is ideal to examine rates and modes of speciation in subterranean organisms, the importance of dispersal and isolation and the factors that facilitate syntopy and community structure in close-related species in subterranean habitats. To date, the systematics of this group has largely been based on subtle morphological variation, primarily in male genitalia. As a first step in understanding speciation and biodiversity in cave carabids, my colleagues and I aim to produce the first molecular phylogeny of cave carabids in the eastern United States to understand the demo and mode of speciation and determine the factors that promote or constrain diversification. In addition, we hope to also elucide the factors driving multi-species beetle assemblages in many cave systems. 

Primary Collaborators: Karen Ober (College of the Holy Cross), Keith Phillips (Western Kentucky University), Kirk S. Zigler (University of the South)

Matthew L. Niemiller, Ph.D. (e-mail:  cavemander17@gmail.com) ©2014-2015