Although we know generally a lot about natural selection, very recent investigations have shown that reliable inference and detailed analysis of selective events depend critically on our knowledge of the population or species in which selection occurred. For example, the effects of natural selection and of population subdivision and demography on the genome are difficult to distinguish if only single (or a small number of) genes are analyzed. This difficulty arises because the effects of positive directional selection and population size expansion on the patterns of polymorphism are very similar, as are those of balancing selection and population subdivision with limited gene flow. However, the situation has drastically improved with the advent of population genomics.
The notion is that natural selection usually occurs at individual genes, while the structure of a population (subdivision or demography) affects the entire genome. Thus, our ability to survey a large number of loci and even re-sequence entire genomes from the same species greatly facilitates the study of selection in natural populations. These technical developments will impact greatly on the research that is planned.