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Evolutionary grade

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Title: Evolutionary grade  
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Subject: Phylogenetics, Taxon in disguise, Wastebasket taxon, Clade, Prosimian
Collection: Evolutionary Biology Terminology, Obsolete Taxa, Taxonomy, Taxonomy (Biology)
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Evolutionary grade

Cladogram (family tree) of a biological group. The green box (central) may represent an evolutionary grade (paraphyletic), a group united by conservative anatomical and physiological traits rather than phylogeny. The flanking red and blue boxes are clades (i.e., complete monophyletic subtrees).

In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.[1]

Contents

  • Definition 1
  • Grades in systematics 2
  • Grades and phylogenetic nomenclature 3
  • Examples 4
  • References 5

Definition

An evolutionary grade is a group of species united by morphological 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 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 or scutes, 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.[3] In microbiology, taxa that are thus seen as excluded from their evolutionary grade parent group are called taxa in disguise.[4]

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 genus Australopithecus is ancestral to Homo, yet actively in use in palaeoanthropology.

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.[5]

Where information about phylogenetic relationships is available, organisms are preferentially grouped into

  1. ^ Huxley J. 1959. Clades and grades. In Cain A.J. (ed) Function and taxonomic importance. Systematics Association, London.
  2. ^ 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.
  3. ^ a b  
  4. ^ Lan, R; Reeves, PR (2002). "Escherichia coli in disguise: molecular origins of Shigella". Microbes and infection / Institut Pasteur 4 (11): 1125–32.  
  5. ^ 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): 741–752.  
  6. ^ Sperling, E. A.; Pisani, D.; Peterson, K. J. (1 January 2007). "Poriferan paraphyly and its implications for Precambrian palaeobiology" (PDF). Geological Society, London, Special Publications 286 (1): 355–368.  
  7. ^ Donoghue, Michael J. (1 June 2005). "Key innovations, convergence, and success: macroevolutionary lessons from plant phylogeny" (PDF). Paleobiology 31 (sp5): 77–93.  
  8. ^ Kevin de Queiroz & Jacques Gauthier (1990). "Phylogeny as a central principle in taxonomy: phylogenetic definitions of taxon names". Syst. Zool. 39 (4): 307–322.  
  9. ^  
  10. ^ Benton, M. J. (2000). "Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead?". Biological Reviews 75 (4): 633–648.  
  11. ^ Qiu, Y.L.; Li, L.; Wang, B.; Chen, Z.; Knoop, V.; Groth-malonek, M.; Dombrovska, O.; Lee, J.; Kent, L.; Rest, J.; et al. (2006). "The deepest divergences in land plants inferred from phylogenomic evidence". Proceedings of the National Academy of Sciences 103 (42): 15511–6.  
  12. ^ 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.  
  13. ^ a b Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  14. ^  
  15. ^ Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians.  

References

  • Bryophytes were long considered a natural group, defined as those land plants which lacked vascular systems. Molecular evidence shows that the bryophytes are not monophyletic since mosses, liverworts and hornworts are in fact separate lineages, with hornworts closest to vascular plants.[11] 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.[12]
  • 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.[13]
  • Amphibians in the biological sense (including the extinct Labyrinthodonts) represent a grade, in that they are also the ancestors of the amniotes.[13]
  • Reptiles are composed of the cold-blooded amniotes, this excludes birds and mammals.[3]
  • Dinosaurs were proposed to be the ancestors of birds as early as the 1860s.[14] Yet the term sees both popular and scientific use as an evolutionary grade excluding birds, though some scientists insist on using "dinosaur" to cover birds too.
  • Lizards as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakes and Amphisbaenans.[15]
Mosses, though similar in looks and physiology, represent an evolutionary grade united by their lack of vascular tissue.
embryophytes



 tracheophytes


 hornworts 



 mosses 



 liverworts 


bryophytes

Examples

With the rise of phylogenetic nomenclature, the use of evolutionary grades as formal taxa has come under debate. Under a strict phylogenetic approach, only monophyletic taxa are recognized.[8] This differs from the more traditional approach of evolutionary taxonomy.[9] The difference in approach has led to a vigorous debate between proponents of the two approaches to taxonomy, particularly in well established fields like vertebrate palaeontology and botany.[10] The difference between the statement "B is part of A" (phylogenetic approach) and "B has evolved from A" (evolutionary approach) is, however, one of semantics 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.

Grades and phylogenetic nomenclature

Evolutionary grades, being united by gross morphological traits, are often eminently recognizable in the field. While

[7] and plants.[6]

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