Agrade is a taxon united by a level ofmorphological orphysiological complexity. The term was coined by British biologistJulian Huxley, to contrast withclade, a strictlyphylogenetic unit.[1]
The concept of evolutionary grades arises in the context ofphylogenetics: the study of theevolutionary history and relationships among or within groups oforganisms. These relationships are determined byphylogenetic inference methods that focus on observedheritable traits, such asDNA sequences,proteinamino acid sequences, ormorphology. The result of such an analysis is aphylogenetic tree—a diagram containing a hypothesis of relationships that reflects the evolutionary history of a group of organisms.[2]
Anevolutionary grade is a group of species united bymorphological orphysiological traits, that has given rise to another group that has major differences from the ancestral group's condition, and is thus not considered part of the ancestral group, while still having enough similarities that we can group them under the same clade.[clarification needed] The ancestral group will not be phylogenetically complete (i.e. is not aclade), and so will represent aparaphyletic taxon.[citation needed]
The most commonly cited example is that ofreptiles. In the early 19th century, the French naturalistLatreille was the first to dividetetrapods into the four familiar classes of amphibians, reptiles, birds, and mammals.[3] In this system, reptiles are characterized by traits such as laying membranous or shelled eggs, having skin covered inscales orscutes, and having a 'cold-blooded' metabolism. However, the ancestors of mammals and birds also had these traits and so birds and mammals can be said to "have evolved from reptiles", making the reptiles, when defined by these traits, a grade rather than a clade.[4] Inmicrobiology, taxa that are thus seen as excluded from their evolutionary grade parent group are calledtaxa in disguise.[5]
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-calledwastebasket taxa which may even bepolyphyletic.
The traditionalLinnaean 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 Linnaeansystematics, evolutionary grades are accepted in highertaxonomic ranks, though generally avoided atfamily level and below. Inphylogenetic nomenclature evolutionary grades (or any other form of paraphyly) are not accepted.[6]
Where information about phylogenetic relationships is available, organisms are preferentially grouped intoclades. 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 inpalaeontology, wherefossils are often fragmentary and difficult to interpret. Thus, traditional palaeontological works are often using evolutionary grades as formal or informal taxa, including examples such aslabyrinthodonts,anapsids,synapsids,dinosaurs,ammonites,eurypterids,lobopodians and many of the more well known taxa ofhuman evolution. Organizing organisms into grades rather than strict clades can also be very useful to understand the evolutionary sequence behind major diversification of both animals[7] and plants.[8]
Evolutionary grades, being united by gross morphological traits, are often eminently recognizable in the field. Whiletaxonomy seeks to eliminate paraphyletic taxa, such grades are sometimes kept as formal or informal groups on the basis of their usefulness for laymen and field researchers.[6] Inbacteriology, the renaming of species or groups that turn out to be evolutionary grades is kept to a minimum to avoid misunderstanding, which in the case ofpathogens could have fatal consequences. When referring to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.
With the rise ofphylogenetic nomenclature, the use of evolutionary grades as formaltaxa has come under debate. Under a strict phylogenetic approach, onlymonophyletic taxa are recognized.[9] This differs from the more traditional approach ofevolutionary taxonomy.[10] The difference in approach has led to a vigorous debate between proponents of the two approaches to taxonomy, particularly in well established fields likevertebrate palaeontology andbotany.[11] The difference between the statement "B is part of A" (phylogenetic approach) and "B has evolved from A" (evolutionary approach) is, however, one ofsemantics rather than of phylogeny. Both express the same phylogeny, but the former emphasizes the phylogenetic continuum while the latter emphasizes a distinct shift in anatomy or ecology in B relative to A.
Huxley, Thomas H. (1870). "Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds".Quarterly Journal of the Geological Society of London. Vol. 26. pp. 12–31.doi:10.1144/GSL.JGS.1870.026.01-02.08 – viaWikisource.| Relevant fields | ||
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