Editing Affinity (taxonomy)

{{Short description|Term in taxonomy}}
'''Affinity (taxonomy)''' – mainly in [[life sciences]] or [[natural history]] – refers to resemblance suggesting a [[common descent]], [[Phylogenetics|phylogenetic relationship]], or type.<ref name="isbn0-19-861271-0">{{cite book |author=Brown, Lesley |title=The New shorter Oxford English dictionary on historical principles |publisher=Clarendon |location=Oxford [Eng.] |year=1993 |isbn=0-19-861271-0 |url=https://archive.org/details/newshorteroxford00lesl }}</ref> The term does, however, have broader application, such as in [[geology]] (for example, in descriptive and theoretical works<ref name="isbn1-86239-220-X">{{cite book |author1=Gibbons, Wes |author2=Teresa Moreno |title=The Geology of Chile |publisher=Geological Society of London |location=London |year=2007 |isbn=978-1-86239-220-5 }}</ref><ref name="isbn1-86239-246-3">{{cite book |author =Mader, Hermann |title=The Geology of Central Europe - Volume 1 Precambrian and Palaeozoic (The Geological Society of London) |publisher=Geological Society of London |location=London |year=2008 |isbn=978-1-86239-246-5 }}</ref>), and similarly in [[astronomy]] (for example, see "[[Centaur object]]" in the context of [[2060 Chiron]]'s close affinity with icy comet nuclei.<ref>"Centaur object." Encyclopædia Britannica. Encyclopædia Britannica 2007 Ultimate Reference Suite . Chicago: Encyclopædia Britannica, 2011.</ref>)

==Basis==
In [[taxonomy]] 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 [[Variable and attribute (research)|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]]s, but irrelevant in comparing a ten-limbed [[Cephalopoda|cephalopod]] with a ten-limbed [[Solifugae|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, although 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."<ref name="BotGloss">Jackson, Benjamin, Daydon; A Glossary of Botanic Terms with their Derivation and Accent; Published by Gerald Duckworth & Co. London, 4th ed 1928</ref> 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}}

[[Category:Taxonomy]]
[[Category:Life sciences]]
[[Category:Natural history]]