Phylogenies reveal new interpretation of speciation and the Red Queen

Unnatural selection? Leigh Van Valen's famous Red Queen hypothesis is firmly established in evolutionary biology textbooks. It states that species accumulate small changes to keep up with a continually changing environment: as Lewis Carroll's monarch has it, “it takes all the running you can do, to keep in the same place”. Speciation, on this model, occurs at a constant rate when enough changes accumulate. Remarkably, this claim has never been tested against competing models that make virtually identical predictions, but imply different underlying processes or varying rates of speciation. Now a test based on the lengths of branches from the phylogenetic trees of more than 100 groups of species has been used to compare the traditional Red Queen with four alternative models. Surprisingly, the simplest model of constant speciation provides not only the best description of the branch lengths in 80% of the phylogenies, but also a startlingly good one. This new model offers an alternative to the metaphor of species losing a race against a deteriorating environment — the pressure of natural selection — with a view linking speciation to rare random events that cause reproductive isolation.