 Palaeontological data suggest that high speciation rates correlate with high extinction rates in almost all major taxa.This correlation has been attributed to a number of ecological factors, but it may result also from a Red Queen situation, in which each speciation event in a clade deteriorates the fitness of coexisting species in the same clade (provided that there is phylogenetic niche conservatism).Discussions of the evolution of sex was not part of Van Valen's Red Queen hypothesis, which addressed evolution at scales above the species level.