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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 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.
The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley, an early advocate of evolutionary theory, proposed a revised taxonomy based on a concept strongly resembling clades, [7] although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley.
Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship (shared descent), progenitor-descendant relationship (serial descent), and degree of evolutionary change.
Willi Hennig 1972 Peter Chalmers Mitchell in 1920 Robert John Tillyard. The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); but the terms "cladistics" and "clade" were popularized by other researchers.
Evolution is the change in the heritable characteristics of biological populations over successive generations. [1] [2] It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. [3]
One study has suggested that Euparkeriidae may not represent a true evolutionary grouping or clade. Instead, the family may represent an evolutionary grade of small archosauriforms (making it paraphyletic ) or a group of species that each evolved small body sizes through evolutionary convergence (making it polyphyletic ). [ 1 ]
Being an evolutionary grade rather than a clade, the prosimians are united by being primates with traits otherwise found in non-primate mammals. Their diets typically are less dominated by fruit than those of the simians, and many are active arboreal predators, hunting for insects and other small animals in the trees. [ 5 ]
[1] [2] That happens when rapid evolution makes a new species appear so radically different from the ancestral group that it is not (initially) recognised as belonging to the parent phylogenetic group, which is left as an evolutionary grade. While the term is from bacteriology, parallel examples are found throughout the tree of life.