There are many more species of insects (>850,000) than of their putative sister taxon (Entognatha, 7500 species) (Mayhew 2002). More than 1600 species of birds have been recorded near the Equator in the New World compared with 300–400 species at latitudes around 40° North or South (Gaston & Blackburn 2000). Mammalian families with average body sizes around 10 g have nearly 10 times as many species as those with average body sizes around 3 kg (Purvis et al. 2003). In a catch of 15,609 moths of 240 species over 4 years of light trapping at Rothamsted, England, the majority of species (180) were represented by 50 individuals or less (Fisher et al. 1943). These observations illustrate the highly uneven distribution of the world's biological diversity. They are examples of four well-known patterns: species richness varies among clades; it varies spatially, with the latitudinal gradient being a classic example; it is higher in small animals than large ones; and rare species are more numerous than common ones. Documenting and explaining such patterns is a major enterprise of ecology (Gaston & Blackburn 2000).

In their introduction to a previous British Ecological Society (BES) Symposium Volume, Blackburn and Gaston (2003) identified three evolutionary processes that underlie large-scale patterns of biodiversity: speciation, extinction and range changes. Anagenetic change might also contribute to some patterns, for example if there is a general tendency for size increase among mammalian lineages (Alroy 1998).