Systematics & Taxonomy
Systematics and taxonomy are scientific disciplines focused on the study of biological diversity and the classification of living organisms. Systematics aims to describe organisms, establish their evolutionary relationships, and create a classification system reflecting these connections. It involves several key phases, starting with identification, where unknown organisms are matched with previously named groups using diagnostic features. Nomenclature, the uniform naming system, typically employs Latin binomials to facilitate clear communication among scientists.
Taxonomy, a subset of systematics, specifically deals with the naming and categorization of organisms. A crucial aspect of systematics is distinguishing between homologous characters—traits with a common evolutionary origin—and analogous characters, which arise from convergent evolution. This distinction is vital for creating accurate classifications that truly represent evolutionary histories. Phylogenetic systematics takes this further by emphasizing speciation events and the genealogical relationships of organisms, often visualized through phylogenetic trees. These trees illustrate the branching patterns of evolution, helping scientists understand the interconnectedness of life forms. Overall, systematics and taxonomy provide foundational frameworks for comprehending the complexity of Earth's biological diversity.
Systematics & Taxonomy
Categories: Classification and systematics; disciplines
In a formal scientific sense, systematics is the study of the diversity exhibited by living organisms and their evolutionary history. It involves the accurate and precise description of organisms and their diagnostic (distinguishing) features, the use of a uniform system for assigning names to organisms, and the development of an appropriate classification scheme to reflect the evolutionary relationships among the organisms being considered.
Systematics itself can be subdivided into several phases. The first phase of systematics, identification, involves the determination of whether an unknown plant belongs to a known, previously named group of plants. This is often achieved by examination of a diagnostic manual for plant identification, consultation with reference collections of plants (termed herbaria), and collaboration with an authority who possesses expertise with a particular group. The uniform system for naming organisms is referred to as nomenclature and typically involves using a Latin binomial (a genus name followed by a species name) to designate a particular organism’s species name. The use of a uniform nomenclature is arrived at through consensus and greatly facilitates communication among scientists when discussing organisms.
The final, and perhaps most elusive goal of systematics, classification, entails assigning an organism or group of organisms to a particular category in a logical hierarchical scheme that accurately reflects underlying patterns of natural (that is, evolutionary) relationships. This hierarchical scheme typically consists of large inclusive groupings (such as classes and orders) containing less inclusive, progressively nested groups (such as families, genera, and species). A group of organisms at any hierarchical level can be abstractly referred to as a taxon.
In a strict sense, taxonomy can be defined as the science of assigning names to groups of organisms. The major difference between systematics and taxonomy is evolutionary, in that systematics encompasses all that taxonomy strives for and also attempts to re-create or elucidate the evolutionary history of the organisms under investigation. In essence, the ultimate goal of systematics is the accurate description of the evolutionary history of organisms.
Homologous versus Analogous Characters
Diagnostic features of an organism that are used in its identification and subsequent classification are termed characters. The different manifestations of the characters are character states. Characters can involve any aspect of morphology, anatomy, biochemistry, and the genetic composition of an organism. The more reliable characters used for systematics must have a genetic basis; that is, they must be inherited in a predictable and reliable fashion and be subject to a minimal amount of variation by nongenetic factors. Superficial characters, which should be excluded from consideration, are subject to environmental modification or lack a predictable genetic basis. For example, the height of a plant or overall size of leaves typically are not good characters, as a number of environmental factors, such as nutrients, water availability, or soil depth and texture, readily influence these traits.
One difficulty faced by systematists is in determining the true nature of character similarities among different groups of organisms. Homologous characters have a direct evolutionary relationship (that is, a common origin). An example of such characters is the placentation of the ovaries in different taxa of the superorder Caryophyllales. Placentation is the arrangement of the placentas (the structures to which the ovules are attached) in the ovary. All Caryophyllales have free central placentation, basal placentation, or some form in between. It is presumed this kind of placentation arose first in the common ancestor to all members of this superorder.
In contrast, analogous characters have different origins but are similar due to convergent evolution. An example is the presence of the succulent habit (fleshy stems and highly reduced, absent, or modified leaves) in members of two families of different evolutionary origins, the Euphorbiaceae that has many species in Asia and the Cactaceae (cactus family) of the Americas. For an evolutionarily sound classification scheme, one needs to emphasize homologous characters and be extremely cautious in using analogous characters.
Evolution and Classification
Some previous classification schemes were highly artificial, in that they did not reflect true evolutionary relationships but rather grouped different organisms together on the basis of superficial similarities. One example of this is the classification of plants based on their growth form, such as grouping all woody plants together, all herbaceous plants in a separate group, and shrubs in yet a third group. The publication of Charles Darwin’s On the Origin of Species by Means of Natural Selection in 1859 prompted systematists to revise their thinking and cast their efforts at classification in an evolutionary context. This has been manifest in the efforts of systematists to elucidate the phylogeny of related groups of organisms.
In essence, phylogeny refers to the evolutionary history of a group of organisms. This evolutionary history entails an understanding of the genealogy of a group or groups of organisms, their patterns of ancestry and descent through time. This conceptual approach is analogous to reconstructing a person’s family tree or genealogy from often fragmentary and indirect evidence.
Phylogenetic Systematics
Phylogenetic systematics focuses on evolutionary processes and speciation events as core components of classification. The objective is to describe the results of speciation events (the species themselves) and to document the events and processes that have led to the present state of biological diversity. Classification is an attempt to reflect the evolutionary history of the living organisms and their lineages. A group of organisms that resemble one another and have a common evolutionary origin is termed a lineage. This concept often includes the ancestral population that first gave rise to this group of organisms and all individuals, both extant and extinct, that are members of that particular group. To achieve this goal, systematists rely on observable features and traits of the organisms and distinguish between the different means by which these characters might arise in different groups of organisms.
Most systematists use character similarities as a basis for grouping organisms together, but this can cause some difficulty in terms of homologous versus analogous characters. However, not all homologous characters are of an identical nature in terms of origin and persistence through time. This problem of distinguishing the true nature of character similarities has been taken to a higher level by phylogenetic systematists through a methodology termed cladistics. In this framework, the nature of homologous characters is further distinguished. Character states that are present in the evolutionary ancestor or ancestral population of a particular organism or set of organisms are referred to as ancestral. Character states that are absent in the ancestor but present in descendants are referred to as derived. Ancestral states that are shared by both ancestral and descendant, or derived, organisms are termed symplesiomorphic. Derived character states not present in the ancestral organisms but shared by two or more lineages are termed synapomorphic. A novel character state that is present in only one lineage, and therefore has little use in classification outside that lineage, is termed autapomorphic.
A key tenet of phylogenetic systematics is that only monophyletic taxa is formally recognized. A monophyletic group consists of an ancestral taxon and all its descendant taxa. A polyphyletic group is composed of two or more ancestral taxa and their descendant taxa and is not an evolutionarily appropriate grouping. Some traditional classifications use polyphyletic groups, and there is much discussion regarding the scientific validity and utility of such schemes.
Phylogenetic Trees
A common way of communicating phylogenetic relationships or patterns is through phylogentic trees, which are diagrammatic representations of the genealogy of taxa or patterns of relationships. Typically, a decision is made as to which taxa or character states are of recent origin and which occurred in the past. The origin of the tree is referred to as the root, and the character changes across the tree (from ancestral to derived taxa) are given a directionality (ancestral versus derived) that is termed polarity. In general, phylogenetic trees are rooted (and therefore the directionality of evolutionary change determined) using a near-relative taxon, termed the outgroup, of the group under consideration.
Ideally, all aspects of the phylogenetic tree should be testable, as with any sound scientific hypothesis. Great progress has been made in evaluating the evolutionary relationships of the flowering plants through phylogenetic systematics. Undoubtedly, with the use of new characters and the development of improved methods of data analysis, further progress in this rapidly changing field is certain.
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