Sensu is a Latin word meaning "in the sense of". It is used in a number of fields including biology, geology, linguistics, and law. Commonly it refers to how strictly or loosely an expression is used in describing any particular concept, but it also appears in expressions that indicate the convention or context of the usage.

Sensu and its common qualifiers [link]

Sensu is a Latin word meaning "in the sense of". It is the ablative case of the noun sensus, here meaning "sense". It is used in a number of fields including biology, geology, linguistics, semiotics, and law, often accompanied by an adjective (in the same case). Three such phrases are sensu stricto – "in the strict sense", abbreviation s.s.;^[1] sensu lato – "in the wide or broad sense", abbreviation s.l.;^[2] and sensu amplo – "in a relaxed, generous (or 'ample') sense", a similar meaning to sensu lato.

When appropriate, comparative and superlative adjectives may also be used to convey the meaning of "more" or "most". Thus sensu stricto becomes sensu strictiore ("in the stricter sense" or "more strictly speaking") and sensu strictissimo ("in the strictest sense" or "most strictly speaking").

Current definitions of the plant kingdom (Plantae) offer a biological example of when such phrases might be used. One definition of Plantae is that it consists of all green plants, all red algae and all glaucophyte algae. A stricter definition excludes the red and glaucophyte algae; the group defined in this way could be called Plantae sensu stricto. An even stricter definition excludes green algae, leaving only land plants; the group defined in this way could be called Plantae sensu strictissimo.^[3] Conversely, where convenient, some authors derive expressions such as "sensu non strictissimo", meaning "not in the narrowest possible sense".^[4]

Another common usage is to follow sensu by a person's name. Thus "sensu Smith" means "in the sense intended by or used by Smith".^[5][6]

A similar form is in use to indicate the sense of a particular context, such as "Nonmonophyletic groups are ... nonnatural (sensu cladistics) in that..."^[7] or "...computation of a cladogram (sensu phenetics)..."^[6]

[edit] Sensu with non-Latin qualifiers; circumscription in biological taxonomy

A related convenient usage is in conjunction with a concept author citation ("sec. Smith", or "sensu Smith"), indicating that the intended meaning is the one defined by that author. (Here the meaning of "sec." is "secundum": "in accordance with".) Such an author citation is different from the (more common) citation of the nomenclatural author citation. The author citation refers only to the type of the name, the specimen or specimens that one refers to in deciding whether other specimens are members of that species or not. Given that an author (such as Linnaeus, for example) was the first to supply a definite type specimen and to describe it, it is to be hoped that his description would stand the tests of time and criticism, but even if it does not, then as far as practical the name that he had assigned will apply. It still will apply in preference to any subsequent names or descriptions that anyone proposes, whether his description was correct or not, and whether he had correctly identified its biological affinities or not. This does not always happen of course; all sorts of errors occur in practice. For example, a collector might scoop a netful of small fish and describe them as a new species; it then might turn out that he had failed to notice that there were several (possibly unrelated) species in the net. It then is not clear what he had named, so his name can hardly be taken seriously, either s.s. or s.l.

After a species has been established in this manner, specialist taxonomists may work on the subject and make certain types of changes in the light of new information. In modern practice it is greatly preferred that the collector of the specimens immediately passes them to specialists for naming; it is rarely possible for non-specialists to tell whether their specimens are of new species or not, and in modern times not many publications or their referees would accept an amateur description.

In any event, the person who finally classifies and describes a species has the task of taxonomic circumscription. Circumscription means in essence that anyone competent in the matter can tell which creatures are included in the species described, and which are excluded. It is in this process of species description that the question of the sense arises, because that is where the worker produces and argues his view of the proper circumscription. Equally, or perhaps even more strongly, the arguments for deciding questions concerning higher taxa such as families or orders, require very difficult circumscription, where changing the sense applied could totally upset an entire scheme of classification, either constructively or disastrously.

Note that the principles of circumscription apply in various ways in non-biological senses. In biological taxonomy the usual assumption is that circumscription reflects the shared ancestry perceived as most likely in the light of the currently available information; in geology or legal contexts far wider and more arbitrary ranges of logical circumscription may apply, not necessarily formally uniform. However, the usage of expressions incorporating sensu remains largely interchangeable among those fields.

Sensu in circumscription [link]

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Readers unfamiliar with technical aspects of taxonomy might find it helpful first to think of everyday examples of the principles. When dealing with groups and parts of groups (subgroups) of different types of things, taxonomists sometimes wish to speak of the full set under consideration, and sometimes just a subset, but almost always want to refer to some particular part, to the exclusion of other elements; in issuing an instruction to poll the opinions of twenty-one members of a village community, a competent pollster would not accept the reactions of two heads of households, three infants, four dogs, five cats, six rats, and a tramcar. That would be taking sensu lato beyond good sense.

Instead the instruction should specify which sense should apply, such as sensu stricto (or strictiore):

* "...all the heads of households on the north side of the stream," or "...all the children in hospital with mumps", or "...the men the district attorney questioned this morning," or "Zachiariah Quenton Henton of 22b Baker Street".

or sensu lato (or latiore):

* "... five of the school football team", or "the first few friendly-looking people you find in the street," or "...some of the people in the district."

The important thing is that in each example the instruction circumscribed the appropriate subjects; that means that the interviewer could tell which people were wanted and correspondingly, which were to be left out.

The circumscription could be in terms of very specific criteria:

(...of all the possible people, only those the DA questioned, and of those, only the adult males, or one specific person only)

or the criterion could be very casual, even vague:

(...as many as you like of the people that looked friendly to you in the street, even if it turns out that the appearance was misleading.)

However simple that may sound, it is fundamental both in formal science and in everyday affairs. Circumscription amounts to the basis for telling things apart, which in turn is the rational basis for all diagnoses, formal or informal.

In biological taxonomy, as the next section describes, the same principles apply, but they deal in various ways with circumscribing living things according to any relevant criterion. In modern biology the criterion usually has something to do with which creature descended from which kind of ancestor, in which ways it changed in the process, and by how much. However, in more general taxonomies, although the principles of circumscription are fundamentally similar, the criteria could be largely different in type as well as in detail.

In short, in every discipline the sense of circumscription in taxonomy must reflect the nature of the subject matter.

Examples in practical taxonomy [link]

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Sensu is used in the taxonomy of living creatures to specify which circumscription of a given taxon is meant, where more than one circumscription can be defined.

Examples:

• "The family Malvaceae s.s. is cladistically monophyletic."

This means more or less that the members of the entire family of plants going under the name Malvaceae, (strictly speaking), which comprises over 1000 species, including the closest relatives of cotton and hibiscus, all descend from a shared ancestor, specifically, that they shared a most recent common ancestor (MRCA). That ancestor might have been a single species of plant, or even possibly a single individual plant. Conversely it also means that this does not necessarily apply to any other species of plants that some people might (broadly speaking or s.l.} have included in the family. In short, the circumscription s.s. includes all and only plants that have descended from that particular ancestral stock.

• "In the broader APG circumscription the family Malvaceae s.l. includes Malvaceae s.s. and also the families Bombacaceae, Sterculiaceae and Tiliaceae."

Here the circumscription is broader, stripped of some of its constraints by saying sensu lato; that is what speaking more broadly amounts to. In this example, by adding other groups of plants to the family Malvaceae s.l., including those related to cacao, cola, durian, and jute, the circumscription omits some of the criteria by which the new members previously had been excluded. Now it is no longer clear that all members of the circumscription descended from that one ancestor. Consequently, we say that Malvaceae s.l. form a polyphyletic group, one that does not share any single ancestor that had no other descendants. Then their most recent common ancestor could have lived perhaps tens of millions of years earlier than the most recent common ancestor of the Malvaceae s.s. alone; also there may be other extant species that are not included in the modern Malvaceae s.l..

• "The 'clearly non-monophyletic' series Cyrtostylis sensu A.S. George has been virtually dismantled..."

This specifies Alex George's particular description of that series. This is a different kind of circumscription; it does not refer directly to the biological nature of the common features of the plants, but to the fact that A.S. George referred to them as a series. "Sensu A.S. George" means that A.S. George discussed the Cyrtostylis in that series, and that members of that series are the ones under discussion in the same sense, — how A. S. George saw them. The current author might or might not approve George's circumscription, but George's is the circumscription currently under consideration.

References [link]

1. ^ "Sensu stricto". FishBase. https://www.fishbase.org/Glossary/Glossary.php?q=sensu+stricto&language=english&sc=is. 
2. ^ W. Greuter, J. McNeill, F. R. Barrie et al.. Regnum Vegetabile Volume 138. International Code of Botanical Nomenclature (St Louis Code). Koeltz Scientific Books, Königstein. ISBN 3-904144-22-7. https://www.bgbm.fu-berlin.de/iapt/nomenclature/code/SaintLouis/0051Ch4Sec3a047.htm. 
3. ^ Spichiger, R-E; Savolainen, Vincent V.; Figeat, Murielle: Systematic Botany of Flowering Plants. Publisher: Science Publishers 2004 ISBN 978-1-57808-373-2
4. ^ Villiger, Mark E. "Customary International Law and Treaties (Developments in International Law, 7)" Publisher: Springer 1985 ISBN 978-90-247-2980-7
5. ^ Sinclair, Bradley J. The Systematics of New World Clinocera. Publisher: National Research Council (Canada) Research Press 2008. ISBN 978-0-660-19800-2
6. ^ ^{a b} Panchen, Alec L. "Classification, Evolution, and the Nature of Biology" Publisher: Cambridge University Press 1992 ISBN 978-0-521-31578-4
7. ^ Wheeler, Quentin & Blackwell, Meredith. Fungus-Insect Relationships: Perspectives in Ecology and Evolution. Publisher: Columbia Univ 1984 ISBN 978-0-231-05695-3

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