20th century neo-Darwinian evolutionary theory black-boxed the complex developmental processes leading from genotype to phenotype. Emerging in the 1980s, evo-devo, i.e. 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. We investigate the explanatory potential of gene-selectionist or selfish gene evolutionary theory to contribute to this synthesis, and elaborate a gene-selectionist conceptualization of evo-devo.