Systematics and Taxonomy

Systematics deals with evolutionary, or phylogenetic, relationships among organisms, whereas taxonomy is more involved with the classification, naming, and description of organisms.

In practice, the two terms are often used interchangeably to refer to the study of relationships among organisms,which in turn often derives from their description and drives their naming.

The history of the disciplines of systematics and taxonomy has shifted with the evolution over the years of the state of knowledge about living organisms, their origins, and their relationships. There has been a historical shift from an emphasis on classification (simply naming and identifying organisms) to the study of phylogenetic (evolutionary) relationships.

Classification traditionally focused on defining the relationships among organisms based primarily on their overall similarity in morphology and appearance. Phylogenetics is now the more common approach in studying the relationships among organisms and involves constructing phylogenies, or evolutionary trees, using evidence from evolutionary relationships.

In addition, the advent of genetics and DNA research has significantly changed the way many biologists approach classification, leading in some cases to reconsideration of former taxonomic relationships.

Ancient World and Middle Ages

The roots of taxonomy go back to Greeks, most notably the philosopher Theophrastus in the third century b.c.e., who wrote two treatises on plants, Peri phyton historias (also known as Historia plantarum; “Enquiry into Plants,” 1916) and Peri phyton aition (also known as De causis plantarum; English translation, 1976-1990). Theophrastus’s system and many other early classification systems grouped plants into herbs, undershrubs, shrubs, and trees.

Classification of plants, beyond this more or less simplistic approach, was not attempted until the latter part of the sixteenth century, when Andrea Cesalpino published De plantis libri (1583). Between the time of the Greeks and Cesalpino, most botanical work was done in the name of medicine, and numerous plants were described because of their usefulness as herbs.

Naming of plants was haphazard, at best. Colloquial names were used by some, and Latin phrases not only were used to describe a plant but also served as official names. There was no accepted length for Latin phrase names, and the names carried little information about how a particular plant might be related to others.

Linnaeus and the Birth of Modern Taxonomy

In 1753 Carolus Linnaeus published his Species plantarum, which quickly brought simplicity and order to the naming of organisms, including plants. Linnaeus introduced binomial nomenclature, which standardized the naming of all organisms by using two Latin words, which together were referred to as the species name. The first word in the species name was the genus, which immediately identified how an organism fit into the classification system.

In addition to improving the system of naming, Linnaeus revolutionized the classification system by introducing a hierarchical approach. Similar species were grouped together into genera. Similar genera were grouped into families. In turn, families were grouped into orders, orders into classes, classes into phyla, and phyla into kingdoms, the most inclusive of the categories.

Although his classification of organisms implied no evolutionary relationships, it was useful for bringing some order to taxonomy. All of these hierarchical categories are used for all types of organisms, including plants, although in plants the name division is sometimes used instead of the phylum.

According to Linnaeus, the turnip, Brassica rapa, which is the name Linnaeus gave to this species, is in the same genus as black mustard, Brassica nigra. The genus Brassica is in the mustard family, Brassicaceae, along with related genera such as Raphanus and Arabis. The family Brassicaceae is in the order Capparales, along with related families like Capparaceae and Resedaceae.

Capparales is a member of the class Eudicotyledones, which includes all the other orders commonly referred to as dicots or dicotyledons. Class Eudicotyledones belongs to the division Anthophyta, along with class Monocotyledones.

Anthophyta, along with all other green plants in divisions like Coniferophyta (the gymnopserms) and Pteridophyta (the ferns), belongs in kingdom Plantae. Each of these categories has a standard suffix, such as -phyta for divisions, -opsida for classes, -ales for orders, and -aceae for families, so that the rank of a name is immediately apparent. Rare exceptions to these rules exist.

In addition to the main categories in the hierarchy, many subdivisions are used. For example, between the levels of kingdom and division, there is subkingdom,which would contain within it one or more divisions.

The sub- prefix can be used before any of the categories, so that there are subclasses, subfamilies, and even subspecies. The prefix super can also be used to define additional ranks. For example, a super family contains one or more related families, and a super order contains one or more related orders.

Classification Since Linnaeus

Linneaus’s binomial nomenclature and hierarchical classification system have been used ever since, but when particular taxa have been added, the classification system has undergone great change. The placement of taxa by Linnaeus was done in what is called an artificial manner.

He grouped taxa into categories based on the organisms’ overall similarities and the possession of particular physical characteristics. Linnaeus’s system is called an artificial classification system because he made no attempt to group taxa based on evolutionary relationships.

Although other plant taxonomists since Linnaeus have also produced artificial classifications, after evolution became more generally accepted in science, many attempts were made to produce a “natural,” or phylogenetically based, classification that would reflect, as much as possible, the evolutionary relationships of the taxa.

One of the first, and still highly respected, phylogenetic classifications of plants was published in 1892 by Adolf Engler. It was actually a revision of an earlier classification by AugustWilhelmEichler. With the help of Karl Prantl and others, the system continued to be elaborated until 1911 and became a twenty-volume work called Die natürlichen Pflanzenfamilien (1887-1911; the natural families of plants).

The families and genera, instead of being ordered alphabetically, were ordered within their taxonomic ranks, from most evolutionarily primitive to most advanced. It was so influential that plant specimens storedinmany herbaria are still organized by what is now referred to as the Engler and Prantl system.

As more and more sophisticated phylogenetic studies have been done, many other plant taxonomists have attempted to improve on Engler and Prantl’s system. Some of themore notable plant taxonomists of the twentieth century have included John Hutchinson, Armen Takhtajan, Arthur Cronquist, Robert F. Thorne, and Rolf M. T. Dahlgren.

The differences among the systems proposed by these various taxonomists aremainly due to different opinions about which plant taxa should be considered most primitive and which most advanced. The identification of what the first land plants, first seed plants, and first flowering plants were like is still uncertain, leaving ample roomfor speculation.

Consequently, a number of competing classification systems exist today. Modern information from DNA analysis and cladistics continues to sharpen taxonomists’ understanding of how plants should be classified, but more work remains to be done.

Naming Rules: The Genus and Below

The rules for naming plants are very specific. The International Code of Botanical Nomenclature (ICBN) contains authoritative rules on the correct way to name plants, as well as groups such as algae and fungi, which have traditionally been considered plants in a broad sense. Rules for naming fossil plants are also covered. Revisions to the code take place on a regular basis.

For a plant name to be accepted, it must be validly published. For any new species (or genus) described before 1953, “validly published” could mean anything from publication in a newspaper or catalog to publication in a respected scientific journal or other professional work.

Since 1953, all new names must be published in accepted scientific publications. In addition, all newspecies (or genus) descriptions must include a complete description in Latin, often called the Latin diagnosis.

Sometimes two or more plant taxonomists inadvertently describe the same species, giving it different names. When this happens, the earliest validly published name is given priority and is considered the correct name; any other names are called synonyms.

May 1, 1753, the date Linnaeus published Species plantarum, is considered the starting date for determining priority, and any names published before this date are not considered.

In addition to being validly published, a type specimen must be identified. A type specimen is a preserved plant specimen that is designated by the author as the best representative of the new species.

An author can define more then one type, in which case the first designated specimen is the holotype and duplicates are called isotypes. Each of these is placed in an established herbarium so other plant taxonomists can examine it.

All names of taxonomic groups are treated as Latin, regardless of their source. Proper names and non-Latinwordsmust be Latinized, following specific rules in the ICBN.

Species names always comprise the genus name, with the first letter capitalized, followed by the species epithet, which is not capitalized. Both names must be either italicized or underlined to denote the name as a species name. A complete species name is also followed by the name of the author who named it.

Author names are often abbreviated, and many author names have official abbreviated forms. An example of a species named by Linnaeus is Brassica rapa L. (the L. stands for Linnaeus). The author’s name should not be italicized or underlined.

Once a genus has been referred to in a scientific paper, later references to species within the genus can then be written with the genus abbreviated to just the first letter and the author’s name is left off: for example, Brassica rapa L. becomes B. rapa.

In a species with a lot of variability, subspecies and varieties can also be described. Some plant taxonomists consider subspecies to be of higher taxonomic rank than varieties, whereas others treat them as equivalent.

Often particular taxonomists will use only one of these ranks to describe taxa below the species rank. Any species can be split into two or more varieties or subspecies.

The variety or subspecies that contains the type specimen is always considered the typical variety or subspecies. For example, the species Abies magnifica Andr. Murray (California redfir) has been divided into two varieties. The typical variety is A.magnifica var.magnifica, and the other variety is A. magnifica var. shastensis Lemmon.

Notice that the word “variety” is abbreviated as “var.” and is not italicized or underlined and that the name of the author of the variety follows the variety name (except for the typical variety,where the author is assumed to be the author of the species).

The word “subspecies” is abbreviated as “ssp.” and is also not italicized or underlined. For the sake of simplicity, italics are now often used for taxonomic groups higher than the genus, all the way up to the phylum. However, strictly speaking, only the genus and species names are italicized.

How Names Are Chosen

Names can be chosen for a variety of reasons and can be derived fromanything, as long as the source word is Latinized, if it is not already in Latin. One of themost common name choices is one that describers some obvious characteristic of the plant.

For example, the genus name Trilliumnicely describes the fact that essentially all the plant parts are in three’s (tri-meaning “three”), and the species epithet for T. albidumnicely describes the strikingwhite petals of this species.

Names can also be derived from the geographic location where the plant is found. These kinds of names are most commonly found in species epithets, such as Juniperus californica (California juniper) or Carex norvegica (Scandinavian sedge). In rare cases, a genus will be named after a place, as in Idahoa, a mustard genus found in Idaho and elsewhere in the western United States.

Another popular approach is to name a plant after someone famous, as in the genera Darwinia (after Charles Darwin) and Linnaea (after Carolus Linnaeus). Species epithets are often given the name of the person who collected the plant. Examples of this type include Pseudotsuga menziesii and Iris douglasii.

Some species are named with less originality, using very common Latin epithets. For example, Juncus ambiguus, meaning ambiguous, not only is nondescriptive but also leaves some doubt about what the author intended.

Then there is Fritillaria affinis, where the epithet affinis simply means “like.” Like what? In cases like these, itmay be necessary to refer to the original publication where the species is described to understand why the name was given.

Naming Rules: Above the Genus

Above the genus the type concept is used to determine correct names. All family names must be derived from a genus name within the family. For example, the rose family is called Rosaceae, which is derived from the genus Rosa, and the lily family is called Liliaceae, which is derived from the genus Lilium. Exceptions to this rule are only allowed when acted upon by the International Botanical Congress.

In 2001, therewere only eight exceptions to the family naming rules. These are referred to as conserved family names and are of long-standing usage. These conserved names can be used, but each also has a name derived according to the rules, and the names can be used interchangeably.

The eight conserved names, and their alternatives (in parentheses) are Palmae (Arecaceae); Gramineae (Poaceae); Cruciferae (Brassicaceae); Leguminosae (Fabaceae); Guttiferae (Clusiaceae); Umbelliferae (Apiaceae); Labiatae (Lamiaceae); Compositae (Asteraceae).

Two common ranks between the family and genus are subfamily and tribe. Names for these should also follow the type concept, with their name being derived froma genuswithin them. The proper suffixes for subfamilies and tribes are -oideae and -inae, respectively.

Ranks above the family level can be chosen either by the type concept or by using a common characteristic of members of the taxon. Standard suffixes for these higher ranks are mentioned above.

Using the type concept, the flowering plants, or angiosperms, are phylum Magnoliophyta (based on the genus Magnolia), but a common alternative name is Anthophyta. Likewise, the gymnosperms are phylum Pinophyta (after the genus Pinus), but are also commonly called Coniferophyta. In each of these cases, both names are valid and are used preferentially by different plant taxonomists.

Sometimes, not only the names will differ, but even the suffixesmay not followthe standards. For example, using the type concept, the class names for the monocots and dicots (the two major groups of flowering plants) are Liliopsida and Magnoliopsida, respectively. Alternative names, in common use, are Monocotyledones and Eudicotyledones, respectively.

Why Names Change

Some common reasons that names change are the result of changes in taxonomic opinion, the discovery that the current name is not the oldest published name, or the discovery that it has some other technical problem.

Although such name changes can be annoying and unpopular to some people, they are essential if the ICBN is to be followed. If plant taxonomists and others were to be free to ignore the rules, then confusion would result.

Plant taxonomists are continually studying relationships among plants, and as new discoveries are made, they are incorporated into the classification system. Sometimes it is discovered that a species needs to be split into two species, in which case the plants that include the holo type retain the original name, and the remaining plants are given a new name.

On the other hand, separate species are sometimes found to be so similar that they are reclassified as belonging to the same species, in which case all the plants from both original species are given the name that was published first. These same rules must be applied to all taxonomic levels whenever taxonomic conclusions warrant splitting or joining of taxa.

Changes in classification at the genus level can also affect species names. For example, if two genera are found to be so similar that they end up being combined into one genus, or some of the species from one genus are found to be more related to members of another genus and are therefor removed into it, species names will be affected.

When this happens, the new species name will carry two authors’ names after it (the original author of the old species name and the author of the new species name), and it is considered a newcombination. The species does not have to be redescribed, but the change must be validly published.

Thus, the species Castilleja exserta (A. A. Heller) Chuang & Heckard used to be in the genus Orthocarpus and was called Orthocarpus exsertus A. A. Heller. Note that the author of the original species name appears in parentheses. Also note, in this case, that the ending of the species epithet had to be changed slightly to follow proper rules of Latin grammar.

Similar rules are followed when a taxon changes from a species to a variety (or some other lower rank) or vice versa. For example, Potentilla breweri S. Watson was later determined to be so closely related to the P. drummondii Lehm. that it was changed to a variety of this species, P. drummondii var. breweri (S. Watson) B. Ertter.

Sometimes a simple study of the published names of taxa in a particular plant group reveals that a currently used name is invalid according to ICBN rules.

For example, it may be discovered that the same species name has been published twice, by different authors who have also identified different holo types. In this case the current name is considered illegitimate and cannot be used, and the name must be changed to the next oldest validly published name.

Alternatively, it may be discovered that a currently used species name is not actually the oldest validly published name, in which case the name must be changed to the older name. Such changes can be controversial, especially when the species is very common and is used by many people who are not plant taxonomists themselves.

Nontaxonomists do not often understand the reasons for such changes. A notable example of this problem is for the species Pseudotsuga menziesii (Mirb.) Franco. The name P. douglasii Carr. was used for many years and led to the use of the common name Douglas fir.

This species is extremely important to foresters, and when the name had to be changed, many resisted the name P. menziesii. With the change in scientific name, the common name should probably be Menzies fir, but it remains Douglas fir.

Future of Plant Taxonomy

Plant taxonomy is a field that has completely embraced modern methods and uses data from molecular genetics, biochemistry, and electron microscopy to gain greater insights into plant evolutionary relationships. The use of computers to perform detailed phylogenetic and cladistic analyses has also revolutionized the field.

A greater emphasis on evolutionary relationships and processes has led to a much better understanding of species concepts and relationships but has led others to consider doing away with the species concept as currently used. Continuing studies using modern approaches should lead to ever better classification systems that better reflect the evolutionary history of plants.

1 comment:

mridu saikia said...

sir can u plz tell me relation between taxonomy an cytotaxonomy........

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