Evolutionary Grade Information
In alpha taxonomy, a grade refers to a level of morphological and/or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.[1]
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Definition
An evolutionary grade is a group of species united by morphological and/or physiological traits, that has given rise to another group that differs markedly from the ancestral condition, and is thus not considered part of the ancestral group. The ancestral group will not be phylogenetically complete (i.e. will not form a clade), so will usually represent a paraphyletic taxon.
The most commonly cited example is that of reptiles. In the early 19th century, the French naturalist Latreille was the first to divide tetrapods into the four familiar classes of amphibians, reptiles, birds and mammals.[2] In this system, reptiles are characterized by traits such as laying membranous or shelled eggs, having skin covered in scales and/or scutes, and having a 'cold-blooded' metabolism. However, the ancestors of mammals and birds also had these traits and so were reptiles in this sense, making reptiles a grade rather than a clade.[3]
Paraphyletic taxa will often, but not always, represent evolutionary grades. In some cases paraphyletic taxa are united simply by not being part of any other groups, and give rise to so called wastebasket taxa which may even be polyphyletic.
Grades in systematics
The traditional Linnaean way of defining taxa is through the use of anatomical traits. When the actual phylogenetic relationship is unknown, well defined groups sometimes turn out to be defined by traits that are primitive rather than derived. In Linnaean systematics, evolutionary grades are accepted in higher taxonomic ranks, though generally avoided at family level and below. In phylogenetic nomenclature evolutionary grades (or any other form of paraphyly) are not accepted.[4]
Where information about phylogenetic relationships is available, organisms are preferentially grouped into clades. Where data is lacking, or groups of uncertain relationship are to be compared, the cladistic method is limited and grade provides a useful tool for comparing organisms. This is particularly common in palaeontology, where fossils are often fragmentary and difficult to interpret. Thus, palaeontological works are often using evolutionary grades as formal or informal taxa, including examples such as Labyrinthodonts, Anapsids, Synapsids, Dinosaurs, Ammonites, Eurypterid, Lobopodes and many of the more well known taxa of human evolution.
Evolutionary grades, being united by gross morphological traits are often eminently recognizable in the field. While paraphyletic taxa are sought eliminated in taxonomy, such grades are sometimes kept as formal or informal groups on basis of their usefulness for laymen and field researchers.[4] When referring to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.
Examples
- Bryophytes were long considered a natural group, defined as those land plants which lacked vascular systems. However, molecular evidence shows that the bryophytes are not monophyletic since mosses, liverworts and hornworts are in fact separate lineages.[5] However, the three clades 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 cannot always be classified into extant groups.[6]
- Fish represent a grade, inasmuch as they have given rise to the land vertebrates. In fact, the three traditional classes of fish (Agnatha, Chondrichthyes and Osteichthyes) all represent evolutionary grades.[7]
- Amphibians in the biological sense (including the extinct Labyrinthodonts) represent a grade, in that they are also the ancestors of the amniotes.[7]
- Dinosaurs were proposed to be the ancestors of birds as early as the 1860s.[8] Yet the term sees both popular and scientific use, the dinosaurs representing an easily recognizable group.
- Lizards as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakes and Amphisbaenans.[9]
References
- ^ Huxley J. 1959. Clades and grades. In Cain A.J. (ed) Function and taxonomic importance. Systematics Association, London.
- ^ Latreille, P.A. (1804). Nouveau Dictionnaire à Histoire Naturelle, xxiv; cited in Latreille, P.A. (1825).Familles naturelles du règne animal, exposés succinctement et dans un ordre analytique.
- ^ Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.
- ^ a b Grant, Verne (1998), "Primary Classification and Phylogeny of the Polemoniaceae, with Comments on Molecular Cladistics", American Journal of Botany (Botanical Society of America) 85 (6): 741, doi:10.2307/2446408, http://www.amjbot.org/cgi/content/abstract/85/6/741
- ^ 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. doi:10.1073/pnas.0603335103. PMID 17030812. PMC 1622854. http://www.pnas.org/cgi/content/abstract/103/42/15511. Retrieved 2008-05-06.
- ^ 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. doi:10.1130/0091-7613(1996)024<0055:NEFLPF>2.3.CO;2. http://geology.geoscienceworld.org/cgi/content/abstract/24/1/55. Retrieved 2008-05-06.
- ^ a b Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
- ^ Huxley, Thomas H. (1870). "Further evidence of the affinity between the dinosaurian reptiles and birds". Quarterly Journal of the Geological Society of London 26: 12–31. doi:10.1144/GSL.JGS.1870.026.01-02.08.
- ^ Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians. Alabama: University of Alabama Press. pp. 164. ISBN 978-0817301354.
Categories: Taxonomy | Evolutionary biology terminology
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