Evolutionary grade
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A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly
phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...
unit.


Definition

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 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 ancestral group will not be phylogenetically complete (i.e. will not form a clade), so will represent a paraphyletic taxon. In order to fully understand evolutionary grades, one must first get a better understanding of
Phylogenetics In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups ...
, defined as "''In
biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary i ...
, is the study of the
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
ary history and relationships among individuals or groups of
organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells (cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and ...
s (e.g.
species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...
, or
population Population typically refers to the number of people in a single area, whether it be a city or town, region, country, continent, or the world. Governments typically quantify the size of the resident population within their jurisdiction using a ...
s). These relationships are discovered through
phylogenetic inference Computational phylogenetics is the application of computational algorithms, methods, and programs to phylogenetic
methods that evaluate observed heritable traits, such as DNA sequences or Morphology (biology), morphology under a model of evolution of these traits"'' The most commonly cited example is that of reptiles. In the early 19th century, the French naturalist Latreille was the first to divide
tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids ( pelycosaurs, extinct t ...
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
scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...
or
scutes A scute or scutum (Latin: ''scutum''; plural: ''scuta'' " shield") is a bony external plate or scale overlaid with horn, as on the shell of a turtle, the skin of crocodilians, and the feet of birds. The term is also used to describe the anterio ...
, and having a ' cold-blooded' 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 microbiology, taxa that are thus seen as excluded from their evolutionary grade parent group are called
taxa in disguise In 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 a similar or lower rank, making the parent unit paraphyletic. That happens whe ...
. 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
polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...
.


Grades in systematics

The traditional Linnaean 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 rank In biological classification, taxonomic rank is the relative level of a group of organisms (a taxon) in an ancestral or hereditary hierarchy. A common system consists of species, genus, family, order, class, phylum, kingdom, domain. While ...
s, though generally avoided at
family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...
level and below. In phylogenetic nomenclature evolutionary grades (or any other form of paraphyly) are not accepted. Where information about phylogenetic relationships is available, organisms are preferentially grouped into
clades A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...
. 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
fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
s are often fragmentary and difficult to interpret. Thus, traditional palaeontological works are often using evolutionary grades as formal or informal taxa, including examples such as
labyrinthodonts "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally consi ...
,
anapsid An anapsid is an amniote whose skull lacks one or more skull openings (fenestra, or fossae) near the temples. Traditionally, the Anapsida are the most primitive subclass of amniotes, the ancestral stock from which Synapsida and Diapsida evolve ...
s,
synapsid Synapsids + (, 'arch') > () "having a fused arch"; synonymous with ''theropsids'' (Greek, "beast-face") are one of the two major groups of animals that evolved from basal amniotes, the other being the sauropsids, the group that includes reptil ...
s,
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s,
ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...
,
eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is l ...
s,
lobopodia The lobopodians, members of the informal group Lobopodia (from the Greek, meaning "blunt feet"), or the formally erected phylum Lobopoda Cavalier-Smith (1998), are panarthropods with stubby legs called lobopods, a term which may also be used as ...
ns and many of the more well known taxa of
human evolution Human evolution is the evolutionary process within the history of primates that led to the emergence of '' Homo sapiens'' as a distinct species of the hominid family, which includes the great apes. This process involved the gradual development o ...
. Organizing organisms into grades rather than strict clades can also be very useful to understand the evolutionary sequence behind major diversification of both animals and plants. Evolutionary grades, being united by gross morphological traits, are often eminently recognizable in the field. While
taxonomy Taxonomy is the practice and science of categorization or classification. A taxonomy (or taxonomical classification) is a scheme of classification, especially a hierarchical classification, in which things are organized into groups or types. ...
seeks to eliminate paraphyletic taxa, such grades are sometimes kept as formal or informal groups on the basis of their usefulness for laymen and field researchers. In
bacteriology Bacteriology is the branch and specialty of biology that studies the morphology, ecology, genetics and biochemistry of bacteria as well as many other aspects related to them. This subdivision of microbiology involves the identification, classificat ...
, the renaming of species or groups that turn out to be evolutionary grades is kept to a minimum to avoid misunderstanding, which in the case of
pathogen In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
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 nomenclature, the use of evolutionary grades as formal
taxa In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...
has come under debate. Under a strict phylogenetic approach, only monophyletic taxa are recognized. This differs from the more traditional approach of
evolutionary taxonomy 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 relati ...
. 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 palaeontology Vertebrate paleontology is the subfield of paleontology that seeks to discover, through the study of fossilized remains, the behavior, reproduction and appearance of extinct animals with vertebrae or a notochord. It also tries to connect, by u ...
and
botany Botany, also called , plant biology or phytology, is the science of plant life and a branch of biology. A botanist, plant scientist or phytologist is a scientist who specialises in this field. The term "botany" comes from the Ancient Greek w ...
. The difference between the statement "B is part of A" (phylogenetic approach) and "B has evolved from A" (evolutionary approach) is, however, one of
semantics Semantics (from grc, σημαντικός ''sēmantikós'', "significant") is the study of reference, meaning, or truth. The term can be used to refer to subfields of several distinct disciplines, including philosophy, linguistics and comp ...
rather than of phylogeny. Both express the same phylogeny, but the former emphasizes the phylogenetic continuum while the latter emphasizes a distinct shift in anatomy or ecology in B relative to A.


Examples

*
Fish Fish are aquatic, craniate, gill-bearing 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. Approximately 95% of ...
represent a grade, in as much as they have given rise to the land vertebrates. In turn, the three traditional classes of fish (
Agnatha Agnatha (, Ancient Greek 'without jaws') is an infraphylum of jawless fish in the phylum Chordata, subphylum Vertebrata, consisting of both present (cyclostomes) and extinct (conodonts and ostracoderms) species. Among recent animals, cyclosto ...
,
Chondrichthyes Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue. ...
and Osteichthyes) all represent evolutionary grades. Romer, A.S. & T.S. Parsons. 1977. ''The Vertebrate Body.'' 5th ed. Saunders, Philadelphia. (6th ed. 1985) * Amphibians in the biological sense (including the extinct
Labyrinthodonts "Labyrinthodontia" (Greek, 'maze-toothed') is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras (about 390 to 150 million years ago). Traditionally consi ...
) represent a grade, since they are also the ancestors of the amniotes. * Reptiles represent a grade composed of the cold-blooded amniotes; this excludes birds and
mammals Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
. *
Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
s were proposed to be the ancestors of birds as early as the 1860s. Yet the term sees popular use as an evolutionary grade excluding birds, though most scientists use a monophyletic Dinosauria. * Lizards represent an evolutionary grade, defined by their retention of limbs relative to
snake Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more j ...
s and amphisbaenians. However, defining lizards by the presence of limbs is incorrect, as there are many species of
legless lizards Legless lizard may refer to any of several groups of lizards that have independently lost limbs or reduced them to the point of being of no use in locomotion.Pough ''et al.'' 1992. Herpetology: Third Edition. Pearson Prentice Hall:Pearson Educatio ...
, which are considered true lizards. * Green algae represent a grade, since they are the ancestors of
land plants The Embryophyta (), or land plants, are the most familiar group of green plants that comprise vegetation on Earth. Embryophytes () have a common ancestor with green algae, having emerged within the Phragmoplastophyta clade of green algae as sist ...
. *
Prokaryote A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Conne ...
s, which include cellular organisms lacking a nucleus, represent a grade, since they are the ancestors of eukaryotes, which includes
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s,
plant Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclu ...
s,
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
, and
protist A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the exc ...
s; and the last of these four groups also represents a grade, since it excludes the previous three groups. *
Crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group can ...
s represent a grade, since they are the ancestors of hexapods, which includes
insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...
s and related taxa. * Monkeys represent a grade, since they include the ancestors of
ape Apes (collectively Hominoidea ) are a clade of Old World simians native to sub-Saharan Africa and Southeast Asia (though they were more widespread in Africa, most of Asia, and as well as Europe in prehistory), which together with its sister g ...
s (including
human Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, cultu ...
s). *Likewise, apes represent a grade in common usage, but are a clade if humans are included.


References

{{Phylogenetics Evolutionary biology concepts Evolutionary biology terminology Obsolete taxa Taxonomy (biology)