Biosciences Research Seminar - Using simulation to understand (and perhaps resolve) difficult problems in animal phylogeny

Some parts of the evolutionary tree of animals remain difficult to resolve. The relationships of two phyla in particular, the Ctenophora (sea gooseberries) and Xenacoelomorpha (including Xenoturbella) have proved contentious. Ctenophora have been proposed as the most distant relatives of all other animals (Ctenophora-first rather than the traditional Porifera (sponges)-first), implying independent origins of the nerves and muscles they (but not Porifera) share with most animals. Xenacoelomorpha may be simple relatives of other, bilaterally symmetrical animals (Nephrozoa tree) or simplified relatives of complex echinoderms and hemichordates (Xenambulacraria tree).
 
One of the alternative topologies must be a result of systematic errors in tree reconstruction. These can result when aspects of the process of molecular evolution are heterogenous - across species, across alignment sites or across time - yet are not accommodated by the homogeneous models used. This results in underestimating the likelihood of character convergence leading to errors in the tree.
 
I will describe experiments using empirical data and simulations which show that the Ctenophores-first and Nephrozoa topologies (but not Porifera-first and Ambulacraria topologies) are strongly supported by analyses affected by systematic errors. Accommodating this finding suggests that Porifera and not ctenophores are the first branch of the animals and that Xenacoelomorpha are simplified relatives of Ambulacraria.