 At first, the idea that species are homeostatic systems may have been a way of addressing critics of PE, who wondered what sort of biological mechanisms could maintain stasis over long stretches of geological time. However, it is a short step from thinking of species as homeostatic systems to thinking of them as the sort of thing that could be a unit of selection. Eldredge and Gould thought that variation introduced amongst organisms is typically isolated within the population and constrained by stabilizing selection, unless geographic separation creates the conditions for rapid evolutionary change. During allopatric speciation events, the adaptive forces negate the stabilizing ones effectively enough to disrupt the stasis of the species and cause the formation of a new lineage. Eventually though, the stabilizing forces (re)assert themselves upon the emergent species and (re)establish stasis. Most biologists think of stabilizing selection as an ordinary microevolutionary process (Schmalhausen 1949); however, the suggestion that a species is a homeostatic system makes it look like some form of stabilizing selection might be operating (in some sense) as a mechanism that maintains equilibrium at the level of populations.