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Evolutionary grade

 

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Evolutionary grade



 
 
In alpha taxonomy
Alpha taxonomy

Alpha taxonomy is the science of finding, describing and categorising organisms, thus leading to the recognition of proposed taxonomic groups, or taxon , which may then be naming conventions....
, a grade refers to a level of morphological
Morphology (biology)

The term morphology in biology refers to form, structure and configuration of an organism. This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs....
 and/or physiological complexity. Organisms may be grouped by the grade of organisation they display without making any implications about their phylogenetic relationship. Due to convergent evolution
Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action....
, most grades contain organisms of differing descent, and are thus paraphyletic.

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






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In alpha taxonomy
Alpha taxonomy

Alpha taxonomy is the science of finding, describing and categorising organisms, thus leading to the recognition of proposed taxonomic groups, or taxon , which may then be naming conventions....
, a grade refers to a level of morphological
Morphology (biology)

The term morphology in biology refers to form, structure and configuration of an organism. This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs....
 and/or physiological complexity. Organisms may be grouped by the grade of organisation they display without making any implications about their phylogenetic relationship. Due to convergent evolution
Convergent evolution

Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action....
, most grades contain organisms of differing descent, and are thus paraphyletic.

Where information about phylogenetic relationships is available, organisms are preferentially grouped into clades. Where data is lacking, or two clades of uncertain relationship are to be compared, the cladistic method is limited; then, the grade provides a useful tool for comparing organisms.

As an example, moss
Moss

Mosses are small, soft plants that are typically 1?10 cm tall, though some species are much larger. They commonly grow close together in clumps or mats in damp or shady locations....
es and liverwort
Liverwort

Liverwort may refer to either* Marchantiophyta, a division of non-vascular plants.* Hepatica, a genus of spring flowers....
s were long thought to represent a clade, but molecular evidence shows the two are in fact separate lineages. However, they 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 we cannot classify into extant groups. When referring to a group of organisms, the term "grade" is usually enclosed in quotation marks to denote its status as a paraphyletic term.