 A different criticism of the population-genetic approach to evolution is that it ignores embryological development; this criticism really applies to the evolutionary theory of the ‘modern synthesis’ era more generally, which had population genetics at its core. As we have seen, population-genetic reasoning assumes that an organism’s genes somehow affect its phenotype, and thus its fitness, but it is silent about the details of how genes actually build organisms, i.e., about embryology. The founders of the modern synthesis treated embryology as a ‘black box’, the details of which could be ignored for the purposes of evolutionary theory; their focus was on the transmission of genes across generations, not the process by which genes make organisms (see the entry on  evolution and development).  This strategy was perfectly reasonable, given how little was understood about development at the time. But since the 1990s, great strides have been made in molecular developmental genetics, which has renewed hopes of integrating the study of embryological development with evolutionary theory and has led to the emergence of the discipline of ‘evolutionary developmental biology’, or evo-devo (see Arthur 2021 for a recent overview or the entry on  evolution and development).  It is sometimes argued that evo-devo is in tension with traditional neo-Darwinism (e.g., Amundson 2007), but it is more plausible to view them as complementary ways of studying evolution that have different emphases.