Chase Brownstein is an incoming graduate student at Yale University. He is a evolutionary biologist and paleontologist primarily interested on how the biogeography of extinct species can inform ideas about contingency and determinism in evolutionary theory. Here, Chase shares his recent work on the biogeography of extant lungfishes.
Institute. Yale University, USA.
Academic life stage. Incoming graduate student (just graduated college).
Major research themes. Biogeography; ancient vicariance; phylogenetics.
Current study system. I study members of so-called living fossil lineages, which are ancient, species-poor clades of living things! What makes these so interesting? A) They can potentially store information about ancient biogeographic events in their evolutionary history, B) they might show distinctive patterns of speciation and rates of evolution, and C) occasionally so-called living fossil species provide a window into past diversity and disparity. Think the coelacanth for helping us understand early lobe-finned fish evolution.
Recent JBI paper. Brownstein, C.D., Harrington, R.C., and Near, T.J. (2023). The biogeography of extant lungfishes traces the breakup of Gondwana. Journal of Biogeography, 50(7): 1191-1198. https://doi.org/10.1111/jbi.14609
Motivation behind this paper. When my advisor (Thomas Near) was in graduate school, he and his colleagues often discussed the need for a strong test of the biogeography of lungfishes, a clade of sarcopterygians consisting of six living species. Lungfishes are cool for being the closest relatives to Tetrapoda, which consists of the four-limbed vertebrates (like you and me)! The living species are distributed across the southern hemisphere, with the South American and African species more closely related to each other than either is to the Australian form. So, the question was, do these splits among lungfishes track the breakup of the southern continents?
Key methodologies. We used Bayesian methods to make time-calibrated trees (phylogenies) of lungfishes based on data from both living species and fossils, which allowed us to provide what I think is the strongest hypothesis yet of the age of living lungfish clades and the timescale of their origination.
Bayesian tip-dated phylogeny and historical biogeography of extant lungfishes, using mitogenomic and nuclear gene sequence data with 16 fossil taxa. Bars indicate 95% CI intervals for divergence times, boxes at nodes indicate inferred ancestral ranges, and continent silhouettes and shaded regions indicate timing of major Gondwanan fragmentation events (yellow shaded region indicates the isolation of eastern Gondwana, including southeastern Asia, and the orange shaded region indicates the separation of Africa and South America). The position of the pan-lepidosireniform clade †Lavocatodidae is indicated following Longrich (2017), as fossils of this clade are too fragmentary for inclusion in the morphological phylogenetic analyses conducted in this paper. Numbers indicate posterior support values at nodes; note that only nodes subtending extant clades are shown, as posterior values on this tree are affected by the use of tip-dating to constrain monophyly.
Unexpected challenges. This one was pretty straightforward!
Major results. In contrast to other studies that did not use the fossil record in conjunction with mitochondrial and nuclear gene data, we found that lungfish phylogeny shows a clear pattern of vicariance across continents (Australasia-South America + Africa, followed by South America-Africa) that is the age when the corresponding splits happened among the continents of the southern hemisphere! So, lungfish phylogeny apparently tracks Gondwanan breakup.
Next steps for this research. We are continuing to study the biogeography, phylogeny, and ecology of living fossil species! Stay tuned!
If you could study any organism on Earth, what would it be? I started out working on dinosaur fossils from eastern North America, and still have soft spots for squamates (lizards and snakes), early birds, and of course tyrannosaurs. So, it really depends on the questions I am interested in exploring!