Scott J. Steppan’s research while affiliated with Florida State University and other places

Studies of radiations after invasion often overlook the potential role of climatic, biotic, and geologic triggers, instead focusing largely on the conduit for invasion. For example, studies of the rodent subfamily Sigmodontinae, a clade of over 500 species that radiated throughout South America during the Great American Biotic Interchange, have historically focused more on invasion than potential triggers or subsequent environmental change. Here, we put the timing and transitions of this radiation in context of changing climatic, biotic, and geologic factors by reconstructing the biogeography of the radiation. To accomplish this, we generated the largest genomic phylogeny of Sigmodontinae that include over 80% of the genera and 40% of the known species, including all incertae sedis taxa and produced a fossil-calibrated chronogram. Results indicate a single invasion of South America at the base of Sigmodontinae (~ 10.46 million years ago [mya]) with two waves of increased lineage generation and biogeographic transition rates, the first of which occurred after a four-million-year lag following invasion. The timing and location of this initial radiation (6.61 - 5.78 mya) coincided with the spread of montane cloud forest during the Late Miocene Cooling and sigmodontines did not spread throughout the continent until the Mid-Pliocene Faunal Turnover (4.5 – 3.0 mya), a period of high extinction of South American mammals. A comprehensive classification for the subfamily is provided that accounts for the new results.

Marmots (Marmota spp.) comprise a lineage of large-bodied ground squirrels that diversified rapidly in the Pleistocene, when the planet quickly transitioned to a drier, colder, and highly seasonal climate-particularly at high latitudes. Fossil evidence indicates the genus spread from North America, across Beringia, and into the European Alps over the course of only a few million years, beginning in the late Pliocene. Marmots are highly adapted to survive long and severely cold winters, and this likely favored their expansion and diversification over this time period. Previous phylogenetic studies have identified two major subgenera of marmots, but the timing of important speciation events and some species relationships have been difficult to resolve. Here we use ultraconserved elements and mitogenomes, with samples from all 15 extant species, to more precisely retrace how and when marmots came to inhabit a vast Holarctic range. Our results indicate marmots arose in North America in the mid Miocene (∼16.3 Mya) and dispersed across the Bering Land Bridge in the late Pliocene (∼3-4 Mya); in addition, our fossil-calibrated timeline is suggestive of the rise and spread of open grasslands as being particularly important to marmot diversification. The woodchuck (M. monax) and the Alaska marmot (M. broweri) are found to be more closely related to the Eurasian species than to the other North American species. Paraphyly is evident in the bobak marmot (M. bobak) and the hoary marmot (M. caligata), and in the case of the latter the data are highly suggestive of a second, cryptic species in the Cascade Mountains of Washington.

Biologists have long pondered the extreme limits of life on Earth, including the maximum elevation at which species can live and reproduce. Here we review evidence of a self-sustaining population of mice at an elevation that exceeds that of all previously reported for mammals. Five expeditions over 10 years to Volcán Llullaillaco on the Argentina/Chile border observed and collected mice at elevations ranging from 5,070 m at the mountain’s base to the summit at 6,739 m (22,110 feet). Previously unreported evidence includes observations and photographs of live animals and mummified remains, environmental DNA, and a soil microbial community reflecting animal activity that are evaluated in combination with previously reported video recordings and capture of live mice. All of the evidence identifies the mouse as the leaf-eared mouse Phyllotis vaccarum, and it robustly places the population within a haplotype group containing individuals from the Chilean Atacama Desert and nearby regions of Argentina. A critical review of the literature affirms that this population is not only an elevational record for mammals but for all terrestrial vertebrates to date, and we further find that many extreme elevations previously reported for mammals are based on scant or dubious evidence.

Quantitative genetics is a powerful tool for predicting phenotypic evolution on a microevolutionary scale. This predictive power primarily comes from the Lande equation (Δz̅ = Gβ), a multivariate expansion of the breeder’s equation, where phenotypic change (Δz̅) is predicted from the genetic covariances (G) and selection (β). Typically restricted to generational change, evolutionary biologists have proposed that quantitative genetics could bridge micro- and macroevolutionary patterns if predictions were expanded to longer timescales. While mathematically possible, making quantitative genetic predictions across generations or species is contentiously debated, principally in assuming long-term stability of the G-matrix. Here we tested stability at a macroevolutionary timescale by conducting full- and half-sib breeding programs in two species of sigmodontine rodents from South America, the leaf-eared mice Phyllotis vaccarum and P. darwini and estimated the G-matrices for eight pelvic traits. To expand our phylogenetic breadth, we incorporated two additional G-matrices measured for the same traits from Kohn & Atchley’s 1988 study of the murine rodents Mus musculus and Rattus norvegicus. Using a phylogenetic comparative framework and four separate metrics of matrix divergence or similarity, we found no significant association between evolutionary divergence among species G-matrices and time, supporting the assumption of stability for at least some structures. However, the phylogenetic sample size is necessarily small. We suggest that small fluctuations in covariance structure can occur rapidly, but underlying developmental regulation prevents significant divergence at macroevolutionary scales, analogous to an Ornstein–Uhlenbeck pattern. Expanded taxonomic sampling will be needed to test this suggestion.

Based on previously published molecular (mitochondrial) and herein provided morphological (qualitative and quantitative data) evidence, we describe a new species of leaf-eared mouse of the genus Phyllotis. The new species is morphometrically distinct when compared with other phylogenetically or geographically close species of Phyllotis, showing several quantitative differences in their external and craniodental characters (e.g., proportionally broader nasals and interorbital region, and proportionally smaller tympanic bullae). The new species is endemic to central Argentina, occurring on rocky grasslands at elevations of 650–2,800 m a.s.l. This is the only species of Phyllotis inhabiting the Central Sierras, a mountain system of medium elevation, isolated from the Andes by low elevation arid and semiarid environments.

The initial vertebrate conquest of land by stegocephalians (Sarcopterygia) allowed access to new resources and exploitation of untapped niches precipitating a major phylogenetic diversification. However, a paucity of fossils has left considerable uncertainties about phylogenetic relationships and the eco-morphological stages in this key transition in Earth history. Among extant actinopterygians, three genera of mudskippers (Gobiidae: Oxudercinae), Boleophthalmus, Periophthalmus and Periophthalmodon are the most terrestrialized, with vertebral, appendicular, locomotory, respiratory, and epithelial specializations enabling overland excursions up to 14 hours. Unlike early stegocephalians, the ecologies and morphologies of the 45 species of oxudercines are well known, making them viable analogs for the initial vertebrate conquest of land. Nevertheless, they have received little phylogenetic attention. We compiled the largest molecular dataset to date, with 29 oxudercine species, and 5 nuclear and mitochondrial loci. Phylogenetic and comparative analyses revealed strong support for two independent terrestrial transitions, and a complex suit of ecomorphological forms in estuarine environments. Furthermore, neither Oxudercinae nor their presumed sister-group the eel gobies (Amblyopinae, a group of elongated gobies) were monophyletic with respect to each other, requiring a merging of these two subfamilies and revealing an expansion of phenotypic variation within the “mudskipper” clade. We did not find support for the expected linear model of ecomorphological and locomotory transition from fully aquatic, to mudswimming, to pectoral-aided mudswimming, to lobe-finned terrestrial locomotion proposed by earlier morphological studies. This high degree of convergent or parallel transitions to terrestriality, and apparent divergent directions of estuarine adaptation, promises even greater potential for this clade to illuminate the conquest of land. Future work should focus on these less-studied species with “transitional” and other mud-habitat specializations to fully resolve the dynamics of this diversification.

Rodents are the largest order of mammals and contain several model organisms important to scientific research in a variety of fields, yet no large set of genomic markers have been designed for this group to date, hindering evolutionary studies into relationships of the group as a whole. Here we present a genomic probe set designed and optimized for rodents with a protocol easy to replicate with little laboratory investment. This design utilizes an anchored hybrid enrichment approach specifically targeting rodents to generate longer loci with a higher substitution rate than existing vertebrate probes to provide utility at various taxonomic levels. Using a test set of rodents from all five suborders we successfully obtained alignments for 416 of the 418 target loci with an average of 1,379 base pairs per locus and a total alignment of more than half a million base pairs. This genomic dataset performed well in all phylogenetic analyses, especially in recent phylogenetic splits, with ample parsimoniously‐informative sites within genera and even within species, showing more than four times as many single nucleotide polymorphisms per locus than a recent vertebrate ultra‐conserved elements study. Additional support is provided in resolving deeper clades in Rodentia. By providing this probe design, we hope that more labs can easily generate data for answering questions in rodents from species delimitation to understanding relationships among families in rapid radiations.