Animals exhibit extraordinary behavioral diversity. How species variation in complex motor behaviors is encoded by evolutionary changes in genes and neurons is unclear. To fill in this knowledge gap, my colleagues and I take advantage of the diversification of Drosophila courtship songs as a model system to explore the molecular and neural mechanisms underlying species variation in behavior. We investigate the lineage-specific loss of sine song in Drosophila yakuba by comparing the underlying neural circuit across species. I find that the loss of sine song correlates with a reduction of TN1 neuron number, which is a group of male-specific motor patterning neurons. Consistent with this neuroanatomical change, optogenetic activation of TN1 neurons in D. yakuba triggers a typical wing extension related to sine generation but no song. Neuroanatomical and single-cell transcriptomic comparisons of TN1 neurons between D. yakuba and Drosophila melanogaster support a loss of the TN1A neuronal subtype, which is essential for sine generation in D. melanogaster. To explore the molecular mechanism of TN1 neuron divergence, I investigate the role of the sex determination gene doublesex (dsx) in TN1 neuronal development and find that dsx negatively regulates TN1 neuron number in D. yakuba but has no effect in D. melanogaster. Furthermore, blocking apoptosis in dsx neurons also results in a gain of TN1 neurons to the same extent as knocking out dsx, indicating that dsx has evolved a new function to promote apoptosis among TN1 neurons in D. yakuba. Together, our work reveals an important role of cell type changes in motor patterning circuits driving the evolution of complex behaviors and the evolutionary lability of sex-determination genes in reconfiguring the cellular makeup of neural circuits.