Research

Speciation

One of the fundamental mysteries in biology from the time of Darwin to the present day is how to explain the vast scope of biodiversity, estimated today at around 10 million species. Species must often develop and maintain their identity in the face of interbreeding and genetic exchange, known as gene flow. Gene flow poses a fascinating challenge for evolutionary theorists because it counters the forces driving differentiation between species. 

 A primary research focus in the lab centers on determining the evolutionary mechanisms that produce and maintain biodiversity when there is gene flow. One of the most powerful mechanisms responsible for generating and maintaining species integrity for co-occurring animal species is mate choice.  Mate choice generally (but not always) generates sexual selection, which has dual roles in speciation.  A recent research focus has been exploration of a generalization that is often made about the relationship between sexual selection and speciation: that sexual selection “drives” the speciation process. This inference is tempting because closely related species often differ in sexually selected characters, but that is not a sufficient reason to jump to a conclusion of causality, and research in the lab shows that often sexual selection can instead inhibit species divergence.  It can also contribute to an underappreciated aspect of speciation with gene flow -- that it may stop at a stable intermediate level of reproductive isolation. 

Sexual selection

Research in the lab has also focused on several different aspects of sexual selection, in addition to its role in speciation.  A major area of work (among many others, see “Publications”) has been on male mate choice, an intriguing yet understudied topic. It has long been recognized that males generally have more mating opportunities than females, because females invest greater amounts of energy into each reproductive attempt. This leads to the general outcome of females being the choosy sex, while males compete over females. Indeed the vast majority of models of sexual selection make the simplifying assumption that males court indiscriminately. There has been increasing realization, however, highlighted by reviews in recent years, that although “sperm is cheap”, males do in fact seem to choose from among available females in polygynous mating systems more often than was previously appreciated. 

In the Servedio lab, we been asking the question of what would happen evolutionarily to an allele for male courtship choice in a polygynous system. Interestingly, females often chose males on the basis of traits such as plumage characteristics or songs that have no obvious bearing on the “quality” of the potential mate (so called “arbitrary” traits). Males seldom, however, seem to choose on the basis of similar traits in females. Early models in the lab provided unexpected, yet logical and very interesting explanations for why male mate choice for “arbitrary” traits should not be able to evolve in polygynous systems, namely that the existence of male preferences will inherently cause direct selection against male preference alleles (see Servedio and Lande 2003).  Additional studies in the lab have concentrated on determining evolutionary circumstances under which this direct selection against male preferences can be overcome, leading to the evolution of male choice. 

Learning

Although it has long been recognized that both mating signals and mating preferences can be affected by learning, the vast majority of theoretical models of mate choice and its role in speciation have assumed that these traits are genetically controlled. Learning, however, can have a profound effect on the evolutionary outcome of these processes. This fascinating topic is gaining attention as more and more cases of learning affecting mate choice decisions are accumulating in a wide variety of systems, including models systems for speciation research such as Darwin’s finches and Drosophila. Several studies in the lab have concentrated on the evolution and evolutionary effects of both learned mating preferences and learned mate choice cues.