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A clade is by definition monophyletic, meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. [note 1] [10] [11] The ancestor can be known or unknown; any and all members of a clade can be extant or extinct.
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.
From the perspective of ancestry, clades are simple to define in purely phylogenetic terms without reference to clades previously introduced: a node-based clade definition, for example, could be "All descendants of the last common ancestor of species X and Y". On the other hand, polyphyletic groups can be delimited as a conjunction of several ...
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria: the grouping contains its own most recent common ancestor (or more precisely an ancestral population), i.e. excludes non-descendants of that common ancestor
Clade T is a total group or pan-group, its extant species organized into crown groups C1 and C2.Within clade T are sister clades T1 and C2. Clade T1 is composed of crown group C1 and the stem group S1 which is the set of extinct species that are closer to C1 than any other crown group of extant species.
The clade shown by the dashed lines in each figure is specified by the ancestor X. Using the hypothesis that the relationships are as in the left tree diagram, the clade includes X, A, B and C. Using the hypothesis that the relationships are as in the right tree diagram, the clade includes X, A and B.
Streptophyta (/ s t r ɛ p ˈ t ɒ f ɪ t ə, ˈ s t r ɛ p t oʊ f aɪ t ə /), informally the streptophytes (/ ˈ s t r ɛ p t ə f aɪ t s /, from the Greek strepto 'twisted', for the morphology of the sperm of some members), is a clade of plants.
The rosids are members of a large clade (monophyletic group) of flowering plants, containing about 70,000 species, [2] more than a quarter of all angiosperms. [3] The clade is divided into 16 to 20 orders, depending upon circumscription and classification. These orders, in turn, together comprise about 140 families. [4]