A New Look at the Penguin Evolutionary Tree
This week, a new study of the penguin evolutionary tree officially appeared in Systematic Biology (though the proof has been available online ahead of formal publication). This represents the first paper from an exciting new project aiming to develop Bayesian methods for combining data from fossils and living species. I’ll be teaming with the exceptional duo of Dr. Tracy Heath of Iowa State University and Dr. Rob Meredith of Montclair State University on this project, and penguins will be one of the “test pilot” groups to try out the new methods.
Our first foray into this realm was to re-analyses a dataset that was originally developed for a parsimony analysis of the giant fossil penguin Kairuku in 2012. This dataset includes 245 morphological characters and >6000 molecular characters. Our re-appraisal was led by PhD student Sasha Gavryushkina of Auckland University. We applied the Fossilized Birth-Death Process, a model pioneered by Dr. Heath that explicitly acknowledges that extant species and fossils are representatives of the same macroevolutionary process, in a tip-dating framework which allows fossil ages to be incorporated into the tree directly. Our analyses thus allow the ages of the fossils to directly impact the shape of the tree, and in a new wrinkle also allow for the possibility that some fossils species may be ancestors to one or more other species.
The results are shown below. One pattern that emerges immediately is that penguins are a very old group, extending back past 60 million years, but that crown penguins only started radiating between 13 and 14 million years ago. Thus, we have evidence for a recent wholesale replacement of “primitive” penguins by “modern” forms. Our work also benefits from some recent geological work, which for example has shown that Spheniscus muizoni, a fossil penguin thought to be 11-13 million years old, is actually close to 9 million years in age. In one reversal of previous findings, the fossil penguin Madrynornis mirandus is pulled outside the Eudyptes–Megadyptes clade in our study.
To me, the most intriguing message comes from the dates: modern penguins are young. Our dates are much younger that those recovered by past studies that looked only at DNA from living penguins. Adding the fossil data has a major effect, and it is very important: because about 3/4 of all known penguin species are now extinct, ignoring the fossils is like looking at just one small piece of a large puzzle. Our new dates place the origin of modern penguins at a really interesting time in Earth history: the Middle Miocene Transition. This transition marks the start of a global shift from warmer to cooler climates that ultimately leads to the glacial-interglacial cycles of our modern world. The accompanying expansion of Antarctic ice sheets may have opened up new habitats for penguins. In deed, we find that the peaking of glacial advance-retreat cycles in the Pleistocene may have been a driver of penguin evolution: 12 of the 18 living species likely arose in the last 2 million years according to our results.
Over the next three years, our team will be working on improving these methods and expanding their capacity to incorporate other types of data, such as the geographic locations of fossils and how frequently fossils occur within their overall stratigraphic ranges.
Gavryushkina, A., T.A. Heath, D.T. Ksepka, T. Stadler, D. Welch, and A.J. Drummond. 2017. Bayesian total evidence dating reveals the recent crown radiation of penguins. Systematic Biology 66 (1): 57-73.