genetic data indicate the "molestus" form in the london underground appear to have a common ancestry, rather than the population at each station being related to the nearest above-ground population. byrne and nichols' working hypothesis was that adaptation to the underground environment had occurred locally in london once only – many hurdles must be overcome to become adapted to the subterranean environment, and understandably it would occur rarely. this hypothesis implies that local adaptation would be expected in different locations around europe and beyond, as each local population evolved an offshoot that overcame the problems of living underground.