 To see why it makes sense to think of multiple modes of evolution, consider again one of the definitions of evolution presented above, where evolution is understood as “any change in the frequency of alleles within a population from one generation to the next”. With natural selection, the frequency of alleles that confer greater fitness would tend to increase over those which confer lesser fitness. Sexual selection would be the same, but with fitness understood strictly in terms of mating ability. With genetic drift, a form of evolution that involves chance (see the entry on  genetic drift   for explanation), there could be an increase in the frequency of alleles that confer greater fitness, an increase in the frequency of alleles that confer lesser fitness, or an increase in the frequency of alleles whose manifestation (if any) was neutral. If organisms migrate from one population to another, it is likely that there will be a change in the frequency of alleles in both populations. And if there is a mutation from one allele to another, then the frequency of alleles in the population will likewise change, albeit by a small amount. Distinguishing these different modes of evolution allows biologists to track the various factors that are relevant to evolutionary changes in a population.