• adaptation;
  • biosystematics;
  • ecological race;
  • genecology;
  • reproductive isolation;
  • speciation

Recent interest in the role of ecology in species formation has led to renewed discussion of the stages in the process of speciation. Although attempts to classify the stages in the process of species formation date back at least as far as Alfred Russel Wallace, one of the most intense debates on the subject occurred among botanists during the mid-20th Century. The present review outlines the progression of the historical debate about stages in the evolution of species, which was instigated by the genecological classification scheme of Göte Turesson in the 1920s, championed in the mid-century by Jens Clausen, and then brought under harsh scrutiny by many in the 1960s and 1970s. At the heart of the controversy is the question of whether speciation occurs rapidly on a local scale or gradually through the formation of geographically widespread ecotypes that evolve as precursors to species. A corollary to this debate is the question of whether speciation is reversible and, if so, how does it become irreversible? A third wave of interest in stages in the process of speciation is currently underway, thus making a modern historical narrative of the debate important. Both contemporary and past evolutionary biologists have argued that viewing speciation as being composed of stages can free researchers from concerns over species definitions and focus attention on the mechanisms involved in the process. How speciation becomes irreversible and whether ecogeographically isolated ecotypes are integral to this process remain as important unresolved issues. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 106, 241–257.