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Affinity (taxonomy) – mainly in
life sciences This list of life sciences comprises the branches of science that involve the scientific study of life – such as microorganisms, plants, and animals including human beings. This science is one of the two major branches of natural science, ...
or
natural history Natural history is a domain of inquiry involving organisms, including animals, fungi, and plants, in their natural environment, leaning more towards observational than experimental methods of study. A person who studies natural history is cal ...
– refers to resemblance suggesting a
common descent Common descent is a concept in evolutionary biology applicable when one species is the ancestor of two or more species later in time. According to modern evolutionary biology, all living beings could be descendants of a unique ancestor commonl ...
, phylogenetic relationship, or type. The term does, however, have broader application, such as in
geology Geology (). is a branch of natural science concerned with the Earth and other astronomical objects, the rocks of which they are composed, and the processes by which they change over time. Modern geology significantly overlaps all other Earth ...
(for example, in descriptive and theoretical works), and similarly in
astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...
(for example, see " Centaur object" in the context of
2060 Chiron 2060 Chiron is a ringed small Solar System body in the outer Solar System, orbiting the Sun between Saturn and Uranus. Discovered in 1977 by Charles Kowal, it was the first-identified member of a new class of objects now known as centaurs— ...
's close affinity with icy comet nuclei."Centaur object." Encyclopædia Britannica. Encyclopædia Britannica 2007 Ultimate Reference Suite . Chicago: Encyclopædia Britannica, 2011.)


Basis

In
taxonomy image:Hierarchical clustering diagram.png, 280px, Generalized scheme of taxonomy Taxonomy is a practice and science concerned with classification or categorization. Typically, there are two parts to it: the development of an underlying scheme o ...
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 attributes 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
crustacean Crustaceans (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...
s, but irrelevant in comparing a ten-limbed
cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...
with a ten-limbed solifugid (including its
pedipalp Pedipalps (commonly shortened to palps or palpi) are the secondary pair of forward appendages among Chelicerata, chelicerates – a group of arthropods including spiders, scorpions, horseshoe crabs, and sea spiders. The pedipalps are lateral to ...
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 are freshwater vertebrates belonging to the class Dipnoi. Lungfish are best known for retaining ancestral characteristics within the Osteichthyes, including the ability to breathe air, and ancestral structures within Sarcopterygii, inc ...
and a
porpoise Porpoises () are small Oceanic dolphin, dolphin-like cetaceans classified under the family Phocoenidae. Although similar in appearance to dolphins, they are more closely related to narwhals and Beluga whale, belugas than to the Oceanic dolphi ...
as having closer (but largely different) affinities to a cow than to a
tuna A tuna (: tunas or tuna) is a saltwater fish that belongs to the tribe Thunnini, a subgrouping of the Scombridae ( mackerel) family. The Thunnini comprise 15 species across five genera, the sizes of which vary greatly, ranging from the bul ...
, and a
bat Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...
as having closer affinities to a
banteng The banteng (''Bos javanicus''; ), also known as tembadau, is a species of wild Bovinae, bovine found in Southeast Asia. The head-and-body length is between . Wild banteng are typically larger and heavier than their Bali cattle, domesticated ...
than to a bird or a butterfly, although a banteng has no "wings". These are considerations arising from the principles discussed in articles on
Homology (biology) In biology, homology is similarity in anatomical structures or genes between organisms of different taxa due to shared ancestry, ''regardless'' of current functional differences. Evolutionary biology explains homologous structures as retained her ...
and
Analogy (biology) Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
. 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."Jackson, Benjamin, Daydon; A Glossary of Botanic Terms with their Derivation and Accent; Published by Gerald Duckworth & Co. London, 4th ed 1928 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.


References

{{Reflist Taxonomy Life sciences Natural history