Measuring how sexual selection history impacts on population viability under genetic stress

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Sexual selection occurs when individuals of one sex (usually males) compete for reproductive success. Darwin first recognised sexual selection as an force which could cause the evolution of elaborate traits that helped their bearers to gain individual reproductive success, either through male:male competition, female choice, or both. Since these early revelations, we have come to realise that sexual selection probably acts on most of an individual's traits and genes, because achieving reproductive success in the face of environmental stress requires a whole range of functional traits to be as adaptive as possible. Because of this, sexual selection might be relatively effective at filtering out individuals from a population who carry imperfect forms of genes or deleterious mutations.We now know a great deal about how traits evolve through sexual selection, and what factors explain the dynamics of individual reproductive success, but we have placed far less effort into understanding the population consequences of sexual selection. This lack of attention needs to be changed, because it is population viability which underpins biodiversity and ecosystem stability, both of which are now suffering unprecedented and accelerating environmental stress as a result of human activities. This project will therefore measure the importance of sexual selection for creating populations that have superior viability in the face of stress. Our general approach will be to use some unique lines of Tribolium flour beetles which we have carefully maintained under controlled conditions for 5 years at UEA, and where the only factor we have experimentally varied is the intensity and form of sexual selection. These lines will have therefore been subjected to experimental evolution across 65 generations, and they have already revealed some important insights into how individual males and females become adapted to different levels of sexual selection. We now propose to use these lines to measure how histories of high, medium, low and no sexual selection render populations resistant to extinction when they are placed under the genetic stress of inbreeding. Under inbreeding, when genetically related individuals are forced to reproduce, deleterious mutations are more likely to be exposed in offspring. This situation therefore provides a revealing measure of the underlying mutational load that a population is carrying. Inbreeding is now recognised to be a relevant force in the natural environment as populations become stressed and depleted, contributing to an 'extinction vortex' where populations can completely disappear.Our hypothesis is that experimental populations which have been maintained under higher levels of sexual selection, where there are greater opportunities for male:male competition and female choice, should have been more effectively purged of deleterious genetic mutations, and so will maintain higher productivity and greater resistance to extinction under the stress of inbreeding. Our tests of productivity will be to measure the number of offspring generated per male-female pair in standard and poor environments, and our test of viability will be to measure extinction rates. As we increase the inbreeding level through sib-sib pairings down advancing generations, we will see our experimental populations becoming less viable as they reduce their productivity, and eventually go extinct. We predict that populations with a past history of heightened sexual selection should maintain productivity and resist extinction for longer than populations with no or low histories of sexual selection.Our findings will not only provide valuable experimental insight into how sexual selection can filter deleterious mutations from the genome, but will also inform on the importance of sexual selection as a valuable force in the natural environment for maintaining the genetic health of small populations that are subjected to increasing stress.