Tangled Trees: Phylogeny, cospeciation and coevolution edited by Roderick Page, University of Chicago Press, $75, ISBN 0226644669 Reviewed by Adrian Barnett
“Two bird species can nest next to each other and exchange parasites, but not their heads.” With this statement, veteran biologist Ernst Meyr undermined the cosy assumption that hosts and their parasites automatically co-evolve along parallel evolutionary tracks.
Blame the confusion on North America’s pocket gophers and their lice. The gophers live in ultra-isolated populations where transmission of lice to fellow colonists is almost guaranteed, making them the perfect model for host-parasite co-evolution. The evolutionary trees of louse and gopher match almost exactly, whether derived from morphology or from mitochondrial DNA. Whenever a gopher population speciated, the lice followed suit. As one of the first systems studied in detail, it became the textbook classic. Unknowing biologists thought this represented all host-parasite systems. It didn’t. They can be teasingly different.
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Tangled Trees covers a wide range of relationships, from that of the malarial parasites of lizards to mammalian viruses and the feather lice of pigeons. It explores the interplay between population biology, ecology and pure accident during evolutionary history. Parasites may cross species, fail to speciate when the host does, die out or simply not colonise a potential host. And all this may happen several times over.
Finding ways to sort out the histories of different host-parasite associations is a challenging task. In meeting it, these 24 scientists display considerable ingenuity and technical skill. It is fine to propose comparing the evolutionary trees of feather lice living on a pigeon’s body with those restricted to the wings. But imagine actually doing the work. If you met any of these scientists at a party, what would be your reaction to their research topic? More likely spit out your drink in disbelief than choke in admiration.
As the doubtless cocktail-flecked Page points out, host-parasite systems provide models with applications in both medicine – the diseases the parasites carry – and conservation – host-parasites as models of larger communities.
This is a resolutely scientific tome. The general reader, happy perhaps with the biology, is liable to be a bit put off. This is not a fault, since Tangled Trees is not trying to be anything other than a technical treatise. As such it succeeds brilliantly and will doubtless be highly influential in shaping research in the field. But I can only hope it will also inspire a publisher to commission a popular version. This extraordinary and fascinating subject deserves it.