Evolutionary grade

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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 is 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]


An evolutionary grade is a group of species united by morphological or physiological traits, that has given rise to another group that has major differences from the ancestral 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. The ancestral group will not be phylogenetically complete (i.e. will not form a clade), so will represent a paraphyletic taxon.

In order to fully understand evolutionary grades, one must first get a better understanding of Phylogenetics, defined as "In biology, is the study of the evolutionary history and relationships among individuals or groups of organisms (e.g. species, or populations). These relationships are discovered through phylogenetic inference methods that evaluate observed heritable traits, such as DNA sequences or morphology under a model of evolution of these traits"

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 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, traditional 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. Organizing organisms into grades rather than strict clades can also be very useful to understand the evolutionary sequence behind major diversification of both animals[6] and plants.[7]

Evolutionary grades, being united by gross morphological traits, are often eminently recognizable in the field. 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.[5] In bacteriology, 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 of pathogens 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.

Grades and phylogenetic nomenclature

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.

Tools For Finding Evolutionary Grades

  • There are many tools for creating trees and finding different Grades for the evolution,
    • BioNumerics
      • BioNumerics was released in 1996 and is still today a platform for the management, storage and (statistical) analysis of all types of biological data. BioNumerics is used by several networks around the globe to share and identify strain information. Pulsenet, a network run by the Centers for Disease Control and Prevention (CDC), uses BioNumerics to distinguish pulsed field gel electrophoresis (PFGE) patterns from different strains of organisms  Calicinet, an outbreak surveillance network for noroviruses, uses BioNumerics to submit norovirus sequences and basic epidemiologic information to a central database[11]
    • Clustal
      • All variations of the Clustal software align sequences using a heuristic that progressively builds a multiple sequence alignment from a series of pairwise alignments. This method works by analyzing the sequences as a whole, then utilizing the UPGMA/Neighbor-joining method to generate a distance matrix. A guide tree is then calculated from the scores of the sequences in the matrix, then subsequently used to build the multiple sequence alignment by progressively aligning the sequences in order of similarity. Essentially, Clustal creates multiple sequence alignments through three main steps:
        1. Do a pairwise alignment using the progressive alignment method
        2. Create a guide tree (or use a user-defined tree)
        3. Use the guide tree to carry out a multiple alignment These steps are carried out automatically when you select "Do Complete Alignment". Other options are "Do Alignment from guide tree and phylogeny" and "Produce guide tree only".[12]
    • DendroScope
    • HyPhy
      • HYPHY is a free multiplatform (Mac, Windows and UNIX) Computational phylogenetics software package intended to perform maximum likelihood analyses of genetic sequence data and equipped with tools to test various statistical hypotheses. The HYPHY name is an abbreviation for "HYpothesis testing using PHYlogenies". As of March 2018, about 2,000 peer-reviewed scientific journal articles cite HYPHY.is a free multiplatform (Mac, Windows and UNIX) Computational phylogenetics software package intended to perform maximum likelihood analyses of genetic sequence data and equipped with tools to test various statistical hypotheses. The HYPHY name is an abbreviation for "HYpothesis testing using PHYlogenies". As of March 2018, about 2,000 peer-reviewed scientific journal articles cite HYPHY.[14]
    • MEGA
      • Molecular Evolutionary Genetics Analysis (MEGA) is computer software for conducting statistical analysis of molecular evolution and for constructing phylogenetic trees. It includes many sophisticated methods and tools for phylogenomics and phylomedicine. It is licensed as proprietary freeware. The project for developing this software was initiated by the leadership of Masatoshi Nei in his laboratory at the Pennsylvania State University in collaboration with his graduate student Sudhir Kumar and postdoctoral fellow Koichiro Tamura. Nei wrote a monograph (pp. 130) outlining the scope of the software and presenting new statistical methods that were included in MEGA. The entire set of computer programs was written by Kumar and Tamura. The personal computers then lacked the ability to send the monograph and software electronically, so they were delivered by postal mail. From the start, MEGA was intended to be easy-to-use and include solid statistical methods only.[15]
    • PHYLIP
    • SplitsTree







Mosses, though similar in looks and physiology, represent an evolutionary grade united by their lack of vascular tissue.
  • 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.[18] 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.[19]
  • 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.[20]
  • Amphibians in the biological sense (including the extinct Labyrinthodonts) represent a grade, in that they are also the ancestors of the amniotes.[20]
  • 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.[21] Yet the term sees popular use as an evolutionary grade excluding birds, though most scientists use a monophyletic Dinosauria.
  • Lizards as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakes and Amphisbaenans.[22] However, defining lizards by the presence of limbs is incorrect, as there are many species of legless lizards, which are considered true lizards.


  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 Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0198604262. 
  4. ^ Lan, R; Reeves, PR (2002). "Escherichia coli in disguise: molecular origins of Shigella". Microbes and infection / Institut Pasteur. 4 (11): 1125–32. doi:10.1016/S1286-4579(02)01637-4. PMID 12361912. 
  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. doi:10.2307/2446408. JSTOR 2446408. 
  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. Bibcode:2007GSLSP.286..355S. doi:10.1144/SP286.25. Archived from the original (PDF) on 20 December 2009. Retrieved 22 August 2012. 
  7. ^ Donoghue, Michael J. (1 June 2005). "Key innovations, convergence, and success: macroevolutionary lessons from plant phylogeny" (PDF). Paleobiology. 31 (sp5): 77–93. doi:10.1666/0094-8373(2005)031[0077:KICASM]2.0.CO;2. 
  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. doi:10.2307/2992353. JSTOR 2992353. 
  9. ^ Mayr, Ernst & Bock, W.J. (2002). "Classifications and other ordering systems" (PDF). J. Zool. Syst. Evol. Research. 40 (4): 169–94. doi:10.1046/j.1439-0469.2002.00211.x. 
  10. ^ Benton, M. J. (2000). "Stems, nodes, crown clades, and rank-free lists: is Linnaeus dead?". Biological Reviews. 75 (4): 633–648. doi:10.1111/j.1469-185X.2000.tb00055.x. ISSN 0006-3231. PMID 11117201. Archived from the original on 5 June 2011. 
  11. ^ "BioNumerics". Wikipedia. 2018-03-07. 
  12. ^ "Clustal". Wikipedia. 2018-05-01. 
  13. ^ "Dendroscope". Wikipedia. 2017-07-08. 
  14. ^ "HYPHY (software)". Wikipedia. 2018-03-23. 
  15. ^ "MEGA, Molecular Evolutionary Genetics Analysis". Wikipedia. 2018-03-16. 
  16. ^ "PHYLIP". Wikipedia. 2018-03-17. 
  17. ^ "SplitsTree". Wikipedia. 2016-12-14. 
  18. ^ 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. Bibcode:2006PNAS..10315511Q. doi:10.1073/pnas.0603335103. PMC 1622854Freely accessible. PMID 17030812. 
  19. ^ 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. Bibcode:1996Geo....24...55S. doi:10.1130/0091-7613(1996)024<0055:NEFLPF>2.3.CO;2. Retrieved 2008-05-06. 
  20. ^ a b Romer, A.S. & T.S. Parsons. 1977. The Vertebrate Body. 5th ed. Saunders, Philadelphia. (6th ed. 1985)
  21. ^ Huxley, Thomas H. (1870). "Wikisource-logo.svg". Quarterly Journal of the Geological Society of London. 26: 12–31. doi:10.1144/GSL.JGS.1870.026.01-02.08. 
  22. ^ Gibbons, J. Whitfield; Gibbons, Whit (1983). Their Blood Runs Cold: Adventures With Reptiles and Amphibians. Alabama: University of Alabama Press. p. 164. ISBN 978-0-8173-0135-4. 
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