Despite the triumphs of modern sexual selection research, there are still many topics that need to be addressed. For example, some models of the evolution of mate choice enjoy only limited empirical support, and for the most part we are not sure which model explains the majority of choice evolution within or between systems. With respect to factors determining the intensity of sexual selection, there may be even more confusion. We are still in the process of building connections between reproductive ecology and selection differentials. Finally, there seems to be a lack of connections between theory related to mate-choice evolution and theory related to sexual selection intensity. There are many other unanswered questions, many of which will require new theory and empirical work.
Overall, the study of sexual selection has been a rich and exciting endeavor, especially in the last several decades. We owe a lot to Darwin for establishing a framework for all modern work in this area. However, we are far from complete resolution on many topics, so the next several decades should be at least as exciting as the recent past.
We thank John Avise and Francisco Ayala for inviting us to participate in the Arthur M. Sackler Colloquium of the National Academy of Sciences, In the Light of Evolution III: Two Centuries of Darwin. We also are grateful to John Avise, Steve Shuster, and an anonymous referee for helpful comments on this manuscript.