Clades versus grades in primate phylogeny . The uses and limitations of embryological data in assessing the phylogenetic relationships of. GRADES AND CLADES AMONG RODENTS. Albert E. Wood. Biology Department, Amherst College,, Amherst, Massachusetts. Search for more papers by this. The phylogenetic relationships of hornless ruminants are reassessed in light of consideration of the systematic position of the genera Lophiomeryx (previously.
Organizing organisms into grades rather than strict clades can also be very useful to understand the evolutionary sequence behind major diversification of both animals  and plants. While taxonomy 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.
When referring to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.
Grades and phylogenetic nomenclature[ edit ] With the rise of phylogenetic nomenclaturethe use of evolutionary grades as formal taxa has come under debate. Under a strict phylogenetic approach, only monophyletic taxa are recognized.
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.
Examples[ edit ] Fish represent a grade, inasmuch as they have given rise to the land vertebrates. In fact, the three traditional classes of fish AgnathaChondrichthyes and Osteichthyes all represent evolutionary grades. Lizards as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakes and Amphisbaenans.
Green algaerepresent a grade, in that they are the ancestors of land plants. Prokaryoteswhich include cellular organisms lacking a nucleus, represent a grade, in that they are the ancestors of eukaryoteswhich includes animalsplantsfungiand protistswhich itself also represents a grade, especially since it excludes the first three aforementioned groups.
Crustaceansrepresent a grade, in that they are the ancestors of hexapodswhich includes insects and related taxa. The Variety of Life. American Journal of Botany. Botanical Society of America.
Geological Society, London, Special Publications.
Grades and Clades
Archived from the original PDF on 20 December Retrieved 22 August Archived from the original on 5 June Now consider a genealogy of the Versions of theNew Testament. Please note that this is much simplified and very theoretical;not everyone agrees with this.
Indeed, I don't agree with it myself. Butwe have to do something for demonstration purposes. A primaryversion is taken directly from the Greek, a secondary is taken from aprimary version rather than the Greek, and a tertiary is taken from asecondary version rather than from the Greek or a primary version.
There can be, in some cases, some real use to this grade classification --for instance, if you are dealing with grammatical questions about the originaltext, a primary version is much more likely to carry useful information thana secondary or tertiary, simply because such distinctions will become progressivelymore blurred with each language the text is translated through.
So thedistinction between primary and secondary versions is quite important for, say,the Hebrew Bible, where the versions are almost the only sources independentof the MT. But we don't need such help very much in connection with the GreekNew Testament, where we have so many independent witnesses. Which means that grades of versions aren't much use in NT textualcriticism. The Old Church Slavonic may be a primary version in terms ofits origin -- but it is translated from the Byzantine text.
Even if youare a believer in Byzantine priority, it's pretty useless, because thereare older and better Byzantine sources in Greek. On the other hand, theArmenian may be a secondary version this is of course much disputed ,but it is a secondary version from an interesting and early source.
Grades and Clades
Thusthe Armenian version, though of an inferior grade, is of a superior clade. If that is all that knowledge of grades and clades told us, it would be justa bit of minor terminology that we could all ignore. The true importance, however,lies in what we can learn from the biologists. The essential point is thatclades are much more useful than grades. Especially if you want to knowsomething about earlier ancestors.Taxonomy: Life's Filing System - Crash Course Biology #19
In an evolutionary context which theprogressive change of manuscripts clearly isgrades are meaningful only ifall change is in the same direction. The chart above, for reptiles, showsthis.
The grade "reptile" is meaningful if you think evolutionhad a goal, to get from amphibians to reptiles to mammals. But it is demonstrablethat this was not the goal; this is demonstrated by the fact that manythousands of unevolved reptile species still exist, and the reptiles alsoevolved into birds.
Reptiles were not some sort of way station from amphibians tosomething "higher"; they aresimply a stage.
To treat them as if they are something to progress pastis equivalent to saying that, because you drivethrough Baltimore on your way from Washington, D.