Genetic linkage and molecular evolution

The different models make similar predictions which makes it difficult to ascribe an observation to just one process. Are there any observations that allow one to identify which process is operating? It turns out that the distribution of frequencies of neu-tral variants in a sample of sequences (the frequency spectrum) is model-dependent. A large distortion of the frequency spectrum towards rare variants is more likely with selective sweeps (Fig. 1Fig. 1). And when there is some recombination, so that initially rare variants are not necessarily swept to complete fixation, a transient sig-nature of a sweep is provided by the occurrence of an excess of ‘derived’ variants at a high frequency within a sample. (One can infer if a variant is derived or ancestral from the sequence of a closely related species.) Other features of genetic variation, such as patterns of linkage disequilibrium, may also be useful in testing alternative models. Perhaps the best way to quantify the relative importance of these processes is to get solid estimates of the rate at which deleterious mutations occur, and of the distribution of their effects on fitness. Although this is a simple question to ask, it is hard to answer. But given such information, and using the increasing amount of information on DNA sequence variation and evolution, one can perhaps try to answer an even harder question: what is the rate at which advantageous mutations occur, and what are their effects on fitness?Key references