Sympatric speciation occurs when some members of a population undergo behavioural adaptations (e.g. change in courtship song, or change in circadian rhythm and therefore mating time) or physiological or anatomical adaptations (e.g. change in genitalia) even though they are not geographically isolated from the rest of the population. This can occur via a random mutation or via a mutation/change in behavior in response to a selection pressure in order to give a selective advantage. In any case, those with the mutation will survive and reproduce (they are viable, thus there is good gene flow), increasing the allele frequency of the mutated allele in the population. This results in an increase in genetic diversity, and an increase in the gene pool as the population size of those with the mutated allele increases. If the adaptation is behavioural the same principal applies; those who adapt will survive and reproduce as they are viable and there is good gene flow which increases the population size of those with the adaptation in the population, meaning they have a greater chance of survival and are more likely to reproduce further (viability increases). Either way, the population size of those with the adaptation increases and tltwo distinct groups/gene pools emerge and there is restricted gene flow between the two groups due to a specific reproductive barrier such as incompatible genitalia or different mating times/circadian rhythms. This gives rise to two groups of organisms that are so phenotypically different (have different characteristics/outward appearance due to the interaction between genes and their environment) that they do not reproduce to produce fertile offspring. This means that two different species have emerged as sympatric speciation has occurred via the mechanism of natural selection resulting in the evolution of a species via physiological/behaviuoral/anatomical adaptations.