Except that evolution is not cyclical. Mechanism of evolution (i.e, geographic isolation, selective pressures, etc.) will almost never spontaneously disappear, as the pesticides did. If the pesticides had been continually used, the mosquito population would not have reverted to a state of vulnerability. Many of the mosquitoes would still have the vulnerability to pesticides buried within their genes, but any born with this trait would die. Over time, the portion of the population carrying the recessive vulnerability genes would become smaller and smaller. Eventually, they would reach a point where these recessive genes would become so incredibly rare that the population could not ever revert to its previous state of being vulnerable to the pesticide, even if the pesticide were spontaneously removed.