We can recognize and identify many species by their appearance. Linnaeus described hundreds of species on the basis of their appearance morphological species concept. Species form over time: each species starts at a speciation event and ends at either extinction or another speciation event, at which it produces two daughter species. If individuals of a population mate with one another, but not with individuals of other populations, they constitute a distinct group within which genes recombine. Allopatric speciation requires almost complete genetic isolation: speciation that results when a populations is divided by a physical barrier. Prezygotic barriers operate before fertilization: mechanisms that operate before fertilization prezygotic reproductive barriers may prevent individuals of different species or populations from interbreeding, habitat isolation, temporal isolation, mechanical isolation, gametic isolation, behavioural isolation. If individuals of two different populations lack complete prezygotic reproductive barriers, postzygotic reproductive barriers may still prevent gene exchange: low hybrid zygote viability, low hybrid adult viability, hybrid infertility.