Evolutionary grade

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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.[1] However, they have 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 cannot classify into extant groups.[2] When refering to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.

[edit] References

  1. ^ Qiu, Y.L.; Li, L.; Wang, B.; Chen, Z.; Knoop, V.; Groth-malonek, M.; Dombrovska, O.; Lee, J.; Kent, L.; Rest, J.; Others, (2006). "The deepest divergences in land plants inferred from phylogenomic evidence". Proceedings of the National Academy of Sciences 103 (42): 15511. 
  2. ^ e.g. Strother, P.K.; Al-hajri, S.; Traverse, A. (1996). "New evidence for land plants from the lower Middle Ordovician of Saudi Arabia". Geology 24 (1): 55-58.