In
alpha taxonomyAlpha taxonomy is the discipline concerned with finding, describing and naming species of living or fossil organisms. This field is supported by institutions holding collections of these organisms, with relevant data, carefully curated: such institutes include natural history museums, herbaria and...
, a
grade refers to a taxon united by a level of
morphologicalIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
or physiological complexity. The term was coined by British biologist
Julian HuxleySir Julian Sorell Huxley FRS was an English evolutionary biologist, humanist and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century evolutionary synthesis...
, to contrast with
cladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
, a strictly
phylogeneticIn biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
unit.
Definition
An evolutionary grade is a group of species united by
morphologicalIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
or
physiologicalPhysiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
traits, that has given rise to another group that differs markedly from the ancestral condition, and is thus not considered part of the ancestral group. The ancestral group will not be phylogenetically complete (i.e. will not form a
cladeA clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
), so will represent a
paraphyleticA group of taxa is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants...
taxon.
The most commonly cited example is that of reptiles. In the early 19th century, the French naturalist
LatreillePierre André Latreille was a French zoologist, specialising in arthropods. Having trained as a Roman Catholic priest before the French Revolution, Latreille was imprisoned, and only regained his freedom after recognising a rare species he found in the prison, Necrobia ruficollis...
was the first to divide
tetrapodTetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s into the four familiar classes of amphibians, reptiles, birds and mammals. In this system, reptiles are characterized by traits such as laying membranous or shelled eggs, having skin covered in
scalesIn most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
or scutes, and having a '
cold-bloodedA poikilotherm is an organism whose internal temperature varies considerably. It is the opposite of a homeotherm, an organism which maintains thermal homeostasis. Usually the variation is a consequence of variation in the ambient environmental temperature...
' 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. In
microbiologyMicrobiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell , cell clusters or no cell at all . This includes eukaryotes, such as fungi and protists, and prokaryotes...
, taxa that are thus seen as excluded from their evolutionary grade parent group are called
taxa in disguiseIn bacteriology a taxon in disguise is a species, genus or higher unit of biological classification whose evolutionary history reveals has evolved from another unit of similar or lower rank, making the parent unit paraphyletic...
.
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
polyphyleticA polyphyletic group is one whose members' last common ancestor is not a member of the group.For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded...
.
Grades in systematics
The traditional
LinnaeanLinnaean taxonomy can mean either of two related concepts:# the particular form of biological classification set up by Carl Linnaeus, as set forth in his Systema Naturæ and subsequent works...
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 rankIn biological classification, rank is the level in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories...
s, though generally avoided at
familyIn biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
level and below. In
phylogenetic nomenclaturePhylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
evolutionary grades (or any other form of paraphyly) are not accepted.
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
fossilFossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s are often fragmentary and difficult to interpret. Thus, palaeontological works are often using evolutionary grades as formal or informal taxa, including examples such as
LabyrinthodontsLabyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...
,
AnapsidAn anapsid is an amniote whose skull does not have openings near the temples.While "anapsid reptiles" or "anapsida" are traditionally spoken of as if they were a monophyletic group, it has been suggested that several groups of reptiles that had anapsid skulls may be only distantly related...
s,
SynapsidSynapsids are a group of animals that includes mammals and everything more closely related to mammals than to other living amniotes. They are easily separated from other amniotes by having an opening low in the skull roof behind each eye, leaving a bony arch beneath each, accounting for their name...
s, Dinosaurs, Ammonites,
EurypteridEurypterids are an extinct group of arthropods related to arachnids which include the largest known arthropods that ever lived. They are members of the extinct order Eurypterida ; which is the most diverse Paleozoic chelicerate order in terms of species...
, Lobopodes and many of the more well known taxa of
human evolutionHuman evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...
.
Evolutionary grades, being united by gross morphological traits are often eminently recognizable in the field. While paraphyletic taxa are sought eliminated in
taxonomyTaxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
, such grades are sometimes kept as formal or informal groups on basis of their usefulness for laymen and field researchers. In
bacteriologyBacteriology is the study of bacteria. This subdivision of microbiology involves the identification, classification, and characterization of bacterial species...
, renaming of species or groups that turn out to be evolutionary grades are kept to a minimum to avoid misunderstanding, which for
pathogenA pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...
s 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 nomenclaturePhylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...
, the use of evolutionary grades as formal
taxa|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
has come under debate. Under a strict phylogenetic approach, only monophyletic taxa are recognized. This differs from the more traditional approach of
evolutionary taxonomyEvolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship and overall similarity. This type of taxonomy considers taxa rather than single species, so that...
. 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 palaeontologyVertebrate paleontology is a large subfield to paleontology seeking to discover the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord, through the study of their fossilized remains...
and
botanyBotany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...
. The difference between the statement "B is part of A" (phylogenetic approach) and "B has evolved from A" (evolutionary approach) is however one of
semanticsSemantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....
rather than of phylogeny. The two both express the same phylogeny, though the former emphasize the phylogenetic continuum while the latter emphasize a distinct shift in anatomy or ecology in B relative to A.
Examples
- Bryophyte
Bryophyte is a traditional name used to refer to all embryophytes that do not have true vascular tissue and are therefore called 'non-vascular plants'. Some bryophytes do have specialized tissues for the transport of water; however since these do not contain lignin, they are not considered to be...
s were long considered a natural group, defined as those land plants which lacked vascular systemsVascular tissue is a complex conducting tissue, formed of more than one cell type, found in vascular plants. The primary components of vascular tissue are the xylem and phloem. These two tissues transport fluid and nutrients internally. There are also two meristems associated with vascular tissue:...
. Molecular evidence shows that the bryophytes are not monophyleticIn common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...
since mossMosses 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. They do not have flowers or seeds, and their simple leaves cover the thin wiry stems...
es, liverwortsThe Marchantiophyta are a division of bryophyte plants commonly referred to as hepatics or liverworts. Like other bryophytes, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of genetic information....
and hornwortHornworts are a group of bryophytes, or non-vascular plants, comprising the division Anthocerotophyta. The common name refers to the elongated horn-like structure, which is the sporophyte. The flattened, green plant body of a hornwort is the gametophyte plant.Hornworts may be found worldwide,...
s are in fact separate lineages, with mosses closest to vascular plants. 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.
- Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
represent a grade, inasmuch as they have given rise to the land vertebratesTetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
. In fact, the three traditional classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
of fish (AgnathaAgnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....
, ChondrichthyesChondrichthyes or cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone...
and OsteichthyesOsteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...
) all represent evolutionary grades.
- Amphibians in the biological sense (including the extinct Labyrinthodonts
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...
) represent a grade, in that they are also the ancestors of the amnioteThe amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
s.
- Reptiles are composed of the cold-blooded amniotes, this excludes birds and mammals
- Dinosaur
Dinosaurs are a diverse group of animals of the clade and superorder Dinosauria. They were the dominant terrestrial vertebrates for over 160 million years, from the late Triassic period until the end of the Cretaceous , when the Cretaceous–Paleogene extinction event led to the extinction of...
s were proposed to be the ancestors of birds as early as the 1860s. Yet the term sees both popular and scientific use as an evolutionary grade excluding birds, though some scientists insist on using "dinosaur" to cover birds too.
- Lizard
Lizards are a widespread group of squamate reptiles, with nearly 3800 species, ranging across all continents except Antarctica as well as most oceanic island chains...
s as a unit represent an evolutionary grade, defined by their retention of limbs relative to snakeSnakes are elongate, legless, carnivorous reptiles of the suborder Serpentes that can be distinguished from legless lizards by their lack of eyelids and external ears. Like all squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales...
s and AmphisbaenansThe Amphisbaenia are a usually legless suborder of squamates closely related to lizards and snakes. As many species possess a pink body coloration and scales arranged in rings, they have a superficial resemblance to earthworms. They are very poorly understood, due to their burrowing lifestyle...
.