In systematics, some generalist, world wide distributed species are in fact constituted of an assemblage of sub-species or forms. This is particularely true for phytophagous species exhibiting a broad range of host plants. These species may be considered as a source of new specialized strain or species on one host plant or one geographical condition just at the outer circle of the species niche. Our aim is to analysed that point by using two models: the aphid Aphis fabae and the mite Tetranychus urticae.