Cladogenesis and morphological diversification in passerine birds

Morphological diversity tends to increase within evolving lineages over time1,2, but the relative roles of gradual evolutionary change (anagenesis)3 and abrupt shifts associated with speciation events (cladogenesis, or ‘punctuated equilibrium’)4 have not been resolved for most groups of organisms5. However, these two modes of evolution can be distinguished by the fact that morphological variance increases in proportion to time under anagenesis6, and in proportion to the logarithm of the number of species under cladogenesis7. Although species and time are themselves correlated, multiple regression analysis provides a statistical framework for partitioning their relative contributions. In this study, I use multiple regressions to evaluate the effects of time and species number on morphological diversity within clades of passerine birds. The results show clearly that number of species exerts a strong influence on morphological variance independent of time, but that time has no unique effect. Thus, morphological evolution in birds seems to be associated with cladogenesis. How lineage splitting promotes morphological diversification poses an important challenge to ecologists and evolutionary biologists.