They are reproductively active, but less fecund than large queens. Without genetic exchange, geographically and reproductively isolated populations may undergo genetic drift. Males emerge as reproductively capable adults. Common mating or reproductively motivated systems include monogamy, polygyny, polyandry, polygamy and promiscuity. Sexual creatures will then inevitably tend to group themselves into reproductively isolated species. It is not known whether the two populations are reproductively isolated. In North America, European earwigs comprise two sibling species, which are reproductively isolated. Males become reproductively active during their second year. This female choice has caused the two species to diverge and become reproductively isolated. Higher-ranking females are more reproductively successful than lower-ranking ones. Minnows are more reproductively resilient than anticipated. Bobcats remain reproductively active throughout their lives. Such crosses are almost always reproductively sterile. There are multiple factors that determine whether a worker bee will become reproductively active. The average age of reproductively active individuals is 21 years.