"Living organisms exhibit great variation in size and shape, and display many morphological innovations. Recent progress has been made in identifying the genetic basis for the evolution of some traits but there remains a paucity of knowledge concerning the evolution of complex morphological traits, and how the underlying changes arose and spread in populations. This proposal will address this by establishing a research program to investigate the genetic basis for the evolution of intra- and inter- specific differences in a complex sensory organ, the insect compound eye. Drosophila mauritiana has significantly larger eyes than its sibling species D. simulans. However, there is also considerable eye size variation between D. simulans populations. Taking advantage of our knowledge of eye development in flies, this proposal will describe the developmental basis for eye size differences and investigate the number and distribution of photoreceptor subtypes (which are sensitive to different light wavelengths) between species and populations with eye size variation. In addition, the proposal will take advantage of cutting edge methods to map the molecular basis for eye size evolution between D. mauritiana and D. simulans, and compare this to the genetic architecture of eye size differences within D. simulans. Population genetic approaches will then be applied to test evolved sequences for directional selection. Thus, this project will not only reveal the genetic changes in development underlying the evolution of eye size and resolve the contribution of standing genetic variation for eye size differences within species to differences between species, but also establish this trait as a model for future studies of morphological evolution."