Coevolution can occur at different levels: ecologically linked species have linked evolutionary trajectories, but species lineages can also have linked histories. Tangled Trees is a collection of a dozen papers on this latter case, sometimes referred to as cospeciation and best known from host-parasite associations.

For a non-specialist it has the common flaws of a collection of papers: there's a fair bit of repetition, only few of the contributions attempt broad syntheses, and the style is rarely inspiring. But the subject material is fascinating and the topic is one of broad importance: Tangled Trees is not just for phylogeneticists.

The first four papers focus on algorithms, methodologies, and models. They deploy largely similar terminology and notation, but take different approaches to relating host and parasite trees: using parsimony, using "jungle" representations of all mappings between the trees, building statistical measures of tree similarity and stochastically modelling host switching, and using a population genetics approach with gene trees.

The remaining papers include studies of rodents and their pinworms and of retroviruses and their hosts, but most consider the coevolution of lice with avian and mammalian hosts. Answering questions about specific taxa is not, however, the primary focus. The broader topics tackled include the factors behind incongruence (dispersal and establishment, extinction), the ecological factors affecting coevolutionary histories, the likelihood of sorting events (when parasite species die out on a host species), and the use of information from multiple parasite lineages.

September 2005

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