20th century neo-Darwinian evolutionary theory, i.e. the ‘Modern Synthesis’ between Darwinian selectionism and Mendelian genetics, studied the evolution of organismal phenotypes, genetically represented as genotypes. It black-boxed the complex developmental processes leading from genotype to phenotype, as such offering a coarse-grained abstraction of evolution. Emerging in the 1980s, evolutionary developmental biology (abbreviated ‘evo-devo’), as the evolutionary study of the developmental process itself, opened up the black box between genotype and phenotype. The major goal of 21st century evolutionary biology and bio-philosophy is the construction of an ‘Extended Evolutionary Synthesis’ (EES) incorporating evo-devo’s research findings. In the present research project we will investigate the explanatory potential of the gene-selectionist or selfish gene version of neo-Darwinism to contribute to such a synthesis. Contrary to classical neo-Darwinism (which works with genotypically represented fitness differences between whole organisms), gene-selectionism has the capacity to work with fitness differences between genes within the same organism or genome and hence should, in principle at least, be able to offer a much more detailed and fine-grained description of the evolution of developmental processes. In a first research part, we will work towards a conceptual integration among the different gene concepts involved in gene-selectionism and evo-devo. In the following research parts then, we will elaborate a gene-selectionist conceptualization of by evo-devo revealed processes and entities mediating between the genetic and the phenotypic level, and explore the explanatory and predictive power of the gene-selectionist models by confronting them with the empirical evo-devo literature.