The Strange Story of Mosaic Evolution

Mosaic evolution, whereby transitional forms display a mixture of primitive and derived features, is nowadays recognized as a ubiquitous pattern of macroevolutionary change. Less known is that this expression was coined by Gavin de Beer in 1954. I move from the philological symptom of the frequent oblivion of de Beer’s authorship of this notion to investigate the history of its reception by G.G. Simpson and E. Mayr in the context of the “hardening” Modern Synthesis. I argue that the incorporation of mosaic evolution by the Modern Synthesis was the result of a process of selective appropriation, which was accomplished with the overshadowing of de Beer’s original formulation by Mayr in 1963. In fact, mosaic evolution touched several sensitive issues at the junction between embryology and paleontology: the role of natural selection in the origin of complex adaptations, the origin of higher taxa, and the explanation of macroevolutionary trends. As a litmus test about the hardening of Modern Synthesis, mosaic evolution reveals several cracks and fissures in the consolidation of the synthetic framework and sheds a realistic light on the defensive function of the hardening itself, as well as on its uneven success. In fact, the dismissal of developmental explanations for mosaic evolution pushed both Simpson and Mayr to adopt Huxley’s progressivist notion of grade to account for macroevolutionary trends, loosening the strict requirement that adaptation be driven by natural selection.