What is Evolutionary grade?
[[File:Clade-grade II.svg|thumb|334px|Cladogram (family tree) of a biological group. The green box may represent an ''evolutionary grade'', a group united by anatomical and physiological traits rather than phylogeny. The red and blue boxes are ''[[clade]]s'' (i.e., complete [[monophyly|monophyletic]] subtrees). Also, the blue and green boxes together are one clade.]] An '''evolutionary grade''' is a group of species at the same level of organisation.<ref>Futuyma D.J. 1986. ''Evolutionary biology''. 2nd ed, Sinauer. p288</ref><ref>Futuyma D.J. 2005. ''Evolution''. Sinauer. p84 and figure 4.17</ref> It refers to a group of animals or plants which are very similar, but which are not a [[clade]]. They have a similar [[morphology (biology)|morphological]] or [[Physiology|physiological]] complexity, and they are often [[adapted]] to the same way of life. The term was coined by [[Julian Huxley]], to contrast with [[clade]], which is a strictly [[Phylogenetics|phylogenetic]] unit.<ref>Huxley J.S. 1958. Evolutionary processes and taxonomy with special reference to grades. Uppsala Univ. Arsskr. (1958) 21{{ndash}}39</ref><ref>Huxley J. 1959. Clades and grades. In Cain A.J. (ed) ''Function and taxonomic importance''. Systematics Association, London.</ref> The use of 'grade' as a term in biology was strongly supported by [[Ernst Mayr]],<ref name=Ernst>Mayr, Ernst 1982. ''The growth of biological thought''. Harvard. p614</ref> who also pointed out that the term had been used for some time before [[cladism]].<ref>Bather F.A. 1927. Biological classification: past and present. ''Proc. Geol. Soc.'' '''83''', 62{{ndash}}104</ref> == Background == An evolutionary grade is a group of species united by similarities. These similarities led to their being given a name. But now to be a clade, the whole group from the first known member to the present day must be included. The most commonly cited example is that of [[reptiles]]. In the early 19th century, [[tetrapod]]s were put in the four familiar classes of [[amphibians]], reptiles