misuse of mixed collections in bryophyte taxonomy

The use of mixed collections has become widespread in moss taxonomy as a short-cut for more rigorous examination of the genetic basis of morphological characters. The reasoning behind this approach is that closely related but morphologically different plants growing inter-mixed must be experiencing the same environment. Therefore, the morphs must be genetically distinct and can be treated as separate taxa. The method is simple to apply and has been extended from field observations of mixed populations to mixed specimens in herbarium packets. There are serious problems with the logic and methodology of this approach. It overlooks the possibility of microenvironmental variation and requires unreasonably restrictive assumptions about genetic variability in bryophyte populations. The mixed collection method should be abandoned entirely, and more rigorous experimental tests, such as uniform garden cultivation and reciprocal field transplants, employed.