Species (abbreviated sp.; plural, spp.) in the abstract can be thought of as lineages represented at any given time by populations of interbreeding individuals all of whom share a common ancestor (and so are monophyletic). In practice, however, even though species are considered the fundamental units of evolution it is difficult or impossible to come up with a single basis for deciding what is a species. This is because we cannot know with certainty the genealogical relationships between individuals sampled at different times and places from a population of usually unknown size and distribution. Nevertheless, we can extrapolate from what we know about patterns of morphological, anatomical, and chemical variation, about plant genetics and breeding systems, as well as about plant distributions and plant ecology to arrive at what are in most cases useable, working definitions of species. Inevitably, in almost all of the vascular plants in this checklist the species concept employed is basically a morphological one, although in many cases it is bolstered by data from other sources. What this means is that most species will be recognizable from their morphology because they exhibit either several characteristics that are altogether unique, or else unique patterns of covariation between characteristics that individually might show overlapping variation between species.
Subspecies (ssp.) are commonly used to recognize the existence of regional variants of a species, that is some degree of morphological differentiation (not as great as that between species) accompanies geographic (or ecological, or temporal) separation from other subspecies. Subspecies are employed frequently in the checklist of Ontario vascular plants by Morton and Venn (1990).
Varieties (var.) are often used to recognize the existence of local variants, although the choice between subspecies and varietal status in some groups is sometimes more a matter of personal taste than of adherence to a strict definition. Varieties are frequently employed in the account of the orchids of Ontario by Whiting and Catling (1986).
Forma (f.) is used most often to recognize variants that occur sporadically across the range of a species and that are distinguished by only one or a few features, such as an unusual flower color. Forms are used frequently in Gray's Manual (Fernald 1970). as in the rare, white-flowered Aquilegia canadense forma albiflora.
Checklists such as the one you are looking at are built upon the accumulated experience and knowledge of not only its authors, but also of all the workers over the past three or more centuries who have progressively refined the concepts of the taxa presented here. As the checklist is completed you will be able to see, if you care to, how in some places the authors may have agreed with one taxonomic opinion, and disagreed with others, so that the way in which names are used, or the taxonomic level at which they are used, is not necessarily the same as in other treatments of the same plants. Nevertheless, this checklist remains a valid description of the plants of Muskoka District for the following reasons: (1) it is specimen-based, i.e. all of the taxa listed are documented by herbarium specimens, so that identifications can be checked and the use of names verified; (2) the scientific names used are given in full, with authorities, so that a particular taxon concept can be traced back to the original author responsible for it; and (3) the authors provide a complete bibliography of the taxonomic and floristic references consulted in the course of producing their treatment.
Sources, and material for further reading:
Davis, P. H. & V. H. Heywood (1965), Heslop-Harrison (1967), Stebbins (1993)
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