Ecology is in principle tied to evolution, since communities and ecosystems result from evolution, while ecological conditions in turn determine fitness values, hence evolution. Yet, as disciplines, evolution and ecology were not unified along the 20th century. The Modern Synthesis intended to invest ecology, but its major ideas, namely the primacy of selection and the key role of gene frequencies, did not directly translate into ecology. However, the architects of the Modern Synthesis, starting with Huxley who mentored Elton, constantly pushed for such integration, like Fisher who supported Ford's 'ecological genetics', or Mayr's supporting Lack's views on clutch-size during debates on density-dependent regulation of populations. I’ll consider four stages through which the MS got integrated into ecology, and distinguish between various ways in which a possible integration was gained, focusing on the way the questions of population regulation and of species coexistence (or diversity) were two successive crucial issues through which the Synthesis' key ideas were brought in contact with diverse families of ecologists. Starting with Elton’s animal ecology (1927), I consider successively Ford’s ecological genetics in the 1940s, the textbook Principles of animal ecology edited by Allee and colleagues (1949) as the expression of a sort of Clements-Wright synthesis, and then the debates over the role of competition in population regulation in the 1950s, ending with Hutchinson's formulating the niche concept as both a solution to the density-dependence debate, and an overture towards group-selection-free approach to the coexistence question. I'll emphasise throughout this story the involvement of Synthesis architects
