Convergent evolution and the problem of homoplasy in morphological and fossil data

It is increasingly easy to generate phylogenetic trees from molecular data, with analyses routinely concatenating sequences from many tens or hundreds of genes. Despite their data-rich underpinnings, new molecular trees often contain surprises, and frequently imply relationships that are at odds with established and traditional hypotheses derived from morphological data. One such example is the phylogeny of mammal orders: the ‘new mammal phylogeny’ overturns many ‘traditional’ groups. Another example is the arthropods. How are we to view these conflicts? Should we necessarily conclude that the new molecular trees are likely to be the correct ones; swayed by the mass of molecular data and greater complexity of analytical models? To do so implies that many decades of comparative anatomy and morphological scholarship have yielded data that are – at best – simply more noisy and error prone than the data from molecules. At worst, it may indicate that convergence within morphological data implies relationships that are positively misleading.

Since phylogeny cannot be known with certainty, there can be no objective test of accuracy. However, it is possible to assess the congruence of competing trees with independent sources of data on evolutionary history. Firstly, where the stratigraphic first occurrence dates of terminals (species, genera and higher taxa) are known with reasonable accuracy, the extent of implied ghosts lineages (relative to theoretical limits) offers an index of the ‘goodness of fit’ to the fossil record. Secondly, the biogeographical or palaeobiogeographical distributions of many groups contain a residual evolutionary signal, and this can also be tested for its fit to competing trees. While neither stratigraphy nor biogeography necessarily offers foolproof discrimination in any particular case, their application to a large statistical sample of cases – different major clades, different taxonomic levels, different ages of radiations – allows for some level of generality. The project will also identify which clades under which circumstances are most prone to morphological convergence, and therefore most suitably analysed with molecular data.

The student will acquire an interdisciplinary and transferrable skill set: bioinformatics, phylogenetics, statistics and palaeontology

Planned start date: Monday 2 October 2017