Morphotaxonomy
Take a look at the three species of Amanita pictured here. There can be little doubt that the first, with the greenish cap (A. phalloides), differs in color and stature from the second (A. rubescens). Besides, if one were to eat the first, one would be dead in a week, while the second is edible and enjoyed by Europeans and Americans alike. The third, with a rich bronze cap and white patches (A. regalis) is a variety of the fly agaric (A. muscaria), causes hallucinations and moderate poisoning. All three photos are of European specimens.
 Amanita phalloides |
 Amanita rubescens |
 Amanita regalis |
Remember the motto, "if they look different, they ARE different." No further investigation is required, although other more subtle observations may be necessary in some cases (i.e. spore measurements, cystidial shapes, etc.). But if this motto is correct, then some factor must operate to KEEP these species from looking like each other. That factor, of course, is called genetics, or the genes of each species dictating the essential traits (taxonomists call them "characters") of each species. So although the idea is seldom pronounced out loud, we assume that because these species don't look alike, they are therefore separated by some GENETIC hiatus or gulf. The implication is that A. phalloides remains A. phalloides because it cannot interbreed with any other species of Amanita, a truism assumed for each species of the genus. The reason that "horse" remains "horse" is that it cannot interbreed with "duck." If it could, we would have horses with feathers, and ducks with hoofs.
On the other hand, we all know that mushrooms WITHIN one species can vary, for example, in size (tall, short, etc.), stature (stout, slender), color (blood red, patriotic red). We assume that such differences can be caused, or at least influenced, by ground moisture, temperature, richness of humus or other factors. These traits (size, shape, color, etc.) are all grouped under the term "phenetics."
Thus there is a tension between two causes of variability: "genetic variation" is caused by the GENES of the organism, while "phenetic plasticity" is the variation which reflects environmental pressures.
So... if one mushroom is purple and another puce, we ask whether this difference is genetic variability, and thus dictates two "species," or whether it is merely variation WITHIN a single species.