Everything about Evolutionary Grade totally explained
In
taxonomy, a
grade refers to a level of
morphological and/or
physiological complexity. Organisms may be grouped by the grade of organisation they display without making any implications about their phylogenetic relationship. Due to
convergent evolution, most grades contain organisms of differing descent, and are thus
paraphyletic.
Where information about phylogenetic relationships is available, organisms are preferentially grouped into
clades. Where data is lacking, or two clades of uncertain relationship are to be compared, the cladistic method is limited; then, the grade provides a useful tool for comparing organisms.
As an example,
mosses and
liverworts were long thought to represent a clade, but molecular evidence shows the two are in fact separate lineages. However, they've a similar degree of complexity, and the "bryophyte grade" is a useful benchmark when analysing early plants - it contains information about the status of fossils which we can't classify into extant groups. When refering to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.
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