Dr. Sally Otto at McGill
Dr. Sarah P. (“Sally”) Otto is presently visiting McGill for a couple of days. Yesterday we had two CAMBAM-sponsored events with her; thanks to everybody who turned out for those!
The first was a CAMBAM students’ roundtable lunch with Dr. Otto. A handful of us had pizza and soda with her while talking about our research projects and bouncing around related ideas. This was quite fun for me, since I’m not a CAMBAM member and don’t know much about what you folks do; it was great to hear about heart arrhythmias and neuron chemistry and asthma and actin and myosin and all the rest! I hope it was also fun for the CAMBAM folks to hear about my models of floral morphology and pollen dispersal and reproductive isolation. Mathematical biology contains such a diversity of ideas!
The second was Dr. Otto’s CAMBAM talk, “Inferring the past for traits that alter speciation and extinction rates”. The talk was about a method called BiSSE (“Binary-State Speciation and Extinction”) for estimating evolutionary parameters given information about extant species. For example, suppose you have a set of related species – a “clade”, like primates – and you know the value of some binary trait for all of these species – whether they live in trees or on the ground, for example. Suppose you also have a phylogenetic tree for your clade: based upon genetic sequences, probably, you have a tree that expresses which species are most closely related and long long ago their common ancestor lived, all the way back to the ancestor that the whole clade descends from. BiSSE (and related methods) will allow you to answer questions about the evolution of that clade and the way that it is related to the binary trait of interest. For example, what is the speciation rate and the extinction rate over the clades’ history, and do those rates differ significantly between lineages that live in trees and lineages that live on the ground? What is the rate of evolutionary transition from tree-living to ground-living, and vice-versa? And was the common ancestor of the clade tree-living (and thus ground-living was a later evolutionary innovation), or ground living (making tree-living the more recent development)?
The really neat thing about her talk, to me, was that she didn’t just present the method as “received wisdom”; she showed us exactly how the likelihood equations underlying it were derived, based upon an analysis of the ways that the binary states of lineages could change over infinitesimal time intervals. Once you understand how a model like BiSSE is constructed, you can construct your own models with additional constraints or additional parameters – traits that are not just binary, ancestral morphological states constrained by fossils, all sorts of possibilities. The power of this sort of method is immense – you can use information about extant species to tease apart what happened millions of years ago as those species evolved and diverged!
Today Dr. Otto has been giving a workshop on modeling in ecology and evolution; we’ve got more than 50 participants at that, so that’s a wonderful success. Tomorrow (Thursday December 5th), don’t forget that Dr. Otto will be giving a talk at the Redpath Museum’s auditorium, on “The Evolutionary Enigma of Sex”, at 3:00, with a wine and cheese social afterwards. I hope to see you there!
(the original paper on BiSSE:)
Maddison, W., P. Midford, and S. Otto. 2007. Estimating a binary character’s effect on speciation and extinction. Systematic Biology 56:701–710. doi: 10.1080/10635150701607033
(a paper on a later related method, BISSE-ness, that develops the primate example I used above, with fascinating results:)
Magnuson-Ford, K., and S. Otto. Linking the Investigations of Character Evolution and Species Diversification. 2012. American Naturalist 180:225–245. doi: 10.1086/666649