Title: The role of balancing and linked selection in determining levels of genetic variation in humans

Abstract
One of the central aims of population genetics is to understand the maintenance of genetic variation. Genetic variation varies both between species and along the genome of a single species. I will talk about two projects which impinge upon both of these questions. In the first half of my talk I will introduce a new test of balancing selection, that is simple to apply, and which, under simplifying assumptions gives an estimate of the number of polymorphisms that are directly subject to balancing selection. Application of this test to human data suggests that several hundred polymorphisms are maintained by balancing selection between human populations. In the second half of my talk I will discuss a project in which we have attempted to understand what causes variation in diversity across the human genome. The level of neutral genetic diversity should be a function of the mutation rate, the genealogy length and the effective population size. Surprisingly we find there is less variation in SNP density than there is in the mutation rate. We suggest that this is because linked selection depends on the mutation rate. Modelling this suggests that linked selection is extremely common in the human genome, such that diversity is depressed by an average of more than 40%.