Affinity (taxonomy)
Encyclopedia
Affinity — mainly in life sciences
or natural history
- resemblance suggesting a common descent
, phylogenetic relationship
, or type. The term does however have broader application, such as in geology
(for example, in descriptive and theoretical works), and similarly in astronomy
(for example, see "Centaur object" in the context of 2060 Chiron
's close affinity with icy comet nuclei.)
the basis of any particular type of classification is the way in which objects in the domain resemble each other. A resemblance of a type that seems appropriate to the classification that we propose, we may call an affinity, and when we decide how to classify say, a specimen of rock or butterfly, we justify our decision according to the affinities that we observe.
Other resemblances we dismiss as being out of context or at least non-cogent; for example, in deciding whether to classify a lizard as having closer affinities to a snake than to a table, biologists rely on affinities such as the scales, blood, physiology, vertebral anatomy, and reproductive system as being more relevant than the possession of four "feet".
of the object, is to determine the affinities and evaluate their significance.
The number of legs might well be a significant affinity in comparing different types of related organisms such as crustaceans, but irrelevant in comparing a ten-limbed cephalopod with a ten-limbed solifugid
(including its pedipalp
s as limbs). Such a comparison would be no more cogent than the foregoing example of the lizard and the table.
There are many such examples in nature; we see both a lungfish
and a porpoise
as having closer (but largely different) affinities to a cow than to a tuna
, and a bat
as having closer affinities to a banteng
than to a bird or a butterfly, in spite of the fact that a banteng has no "wings". These are considerations arising from the principles discussed in articles on Homology (biology)
and Analogy (biology)
.
It is clear that there is an element of subjectivity to the recognition of affinities; that is implicit in such dictionary definitions as: ""Affinity: the closeness of relation between plants as shown by similarity of important organs." That definition is over a century old, but it is typical of the basis on which taxonomists had to work till recently, and in practice still must use; it is not practical to sequence the genome of every specimen. Nucleic acid analyses are eroding many difficulties, but there is a long way to go.
Life sciences
The life sciences comprise the fields of science that involve the scientific study of living organisms, like plants, animals, and human beings. While biology remains the centerpiece of the life sciences, technological advances in molecular biology and biotechnology have led to a burgeoning of...
or natural history
Natural history
Natural history is the scientific research of plants or animals, leaning more towards observational rather than experimental methods of study, and encompasses more research published in magazines than in academic journals. Grouped among the natural sciences, natural history is the systematic study...
- resemblance suggesting a common descent
Common descent
In evolutionary biology, a group of organisms share common descent if they have a common ancestor. There is strong quantitative support for the theory that all living organisms on Earth are descended from a common ancestor....
, phylogenetic relationship
Phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...
, or type. The term does however have broader application, such as in geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
(for example, in descriptive and theoretical works), and similarly in astronomy
Astronomy
Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...
(for example, see "Centaur object" in the context of 2060 Chiron
2060 Chiron
2060 Chiron is a minor planet in the outer Solar System. Discovered in 1977 by Charles T. Kowal , it was the first-known member of a new class of objects now known as centaurs, with an orbit between Saturn and Uranus.Although it was initially classified as an asteroid, it was later found to...
's close affinity with icy comet nuclei.)
Basis
In taxonomyTaxonomy
Taxonomy 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...
the basis of any particular type of classification is the way in which objects in the domain resemble each other. A resemblance of a type that seems appropriate to the classification that we propose, we may call an affinity, and when we decide how to classify say, a specimen of rock or butterfly, we justify our decision according to the affinities that we observe.
Other resemblances we dismiss as being out of context or at least non-cogent; for example, in deciding whether to classify a lizard as having closer affinities to a snake than to a table, biologists rely on affinities such as the scales, blood, physiology, vertebral anatomy, and reproductive system as being more relevant than the possession of four "feet".
Application and obstacles
Analysing and determining the proper classification of an organism, a rock, or an astronomic object according to a particular system is often a difficult and treacherous procedure. Problems in such fields of study have tripped up whole generations of workers in recent centuries. When the position is not clear from an early stage, the first step after beginning to determine, evaluate, and describe the attributesVariable and attribute (research)
In science and research, attribute is a characteristic of an object . Attributes are closely related to variables. Variable is a logical set of attributes. Variables can "vary" - for example, be high or low...
of the object, is to determine the affinities and evaluate their significance.
The number of legs might well be a significant affinity in comparing different types of related organisms such as crustaceans, but irrelevant in comparing a ten-limbed cephalopod with a ten-limbed solifugid
Solifugae
Solifugae are an order of Arachnida, known as camel spiders, wind scorpions and sun spiders or solifuges, comprising more than 1,000 described species in about 153 genera...
(including its pedipalp
Pedipalp
Pedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s as limbs). Such a comparison would be no more cogent than the foregoing example of the lizard and the table.
There are many such examples in nature; we see both a lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
and a porpoise
Porpoise
Porpoises are small cetaceans of the family Phocoenidae; they are related to whales and dolphins. They are distinct from dolphins, although the word "porpoise" has been used to refer to any small dolphin, especially by sailors and fishermen...
as having closer (but largely different) affinities to a cow than to a tuna
Tuna
Tuna is a salt water fish from the family Scombridae, mostly in the genus Thunnus. Tuna are fast swimmers, and some species are capable of speeds of . Unlike most fish, which have white flesh, the muscle tissue of tuna ranges from pink to dark red. The red coloration derives from myoglobin, an...
, and a bat
Bat
Bats are mammals of the order Chiroptera "hand" and pteron "wing") whose forelimbs form webbed wings, making them the only mammals naturally capable of true and sustained flight. By contrast, other mammals said to fly, such as flying squirrels, gliding possums, and colugos, glide rather than fly,...
as having closer affinities to a banteng
Banteng
The banteng , also known as tembadau, is a species of wild cattle found in Southeast Asia.Banteng have been domesticated in several places in Southeast Asia, and there are around 1.5 million domestic banteng, which are called Bali cattle. These animals are used as working animals and for their meat...
than to a bird or a butterfly, in spite of the fact that a banteng has no "wings". These are considerations arising from the principles discussed in articles on Homology (biology)
Homology (biology)
Homology forms the basis of organization for comparative biology. In 1843, Richard Owen defined homology as "the same organ in different animals under every variety of form and function". Organs as different as a bat's wing, a seal's flipper, a cat's paw and a human hand have a common underlying...
and Analogy (biology)
Analogy (biology)
An analogy is a trait or an organ that appears similar in two unrelated organisms. The cladistic term for the same phenomenon is homoplasy, from Greek for same form. Biological anologies are often the result of convergent evolution....
.
It is clear that there is an element of subjectivity to the recognition of affinities; that is implicit in such dictionary definitions as: ""Affinity: the closeness of relation between plants as shown by similarity of important organs." That definition is over a century old, but it is typical of the basis on which taxonomists had to work till recently, and in practice still must use; it is not practical to sequence the genome of every specimen. Nucleic acid analyses are eroding many difficulties, but there is a long way to go.