Atlas Journal of Biology 2016, pp. 295–307 doi: 10.5147/ajb.2016.0143 A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) Evolutionarily Degenerate Biological Structures: Terminology Through Time, and the Question of Terminological Consensus Philip J. Senter Department of Biological Sciences, Fayetteville State University, 1200 Murchison Road, Fayetteville, NC 28301, USA. Received: September 17, 2016 / Accepted: November 20, 2016 __________________________________________________ * Corresponding author: psenter@uncfsu.edu 295 Abstract The existence of evolutionarily degenerate biological struc- tures (EDBS) is a major concept in biology. Biologists have often used the terms “vestige,” “rudiment,” and their adjec- tive forms “vestigial” and “rudimentary” for EDBS since the nineteenth century. Some authors have advocated stricter us- age of the terms than others have. For example, some have advocated restriction of the term “rudimentary” to embryonic structures, whereas others have also applied it to postem- bryonic EDBS. Likewise, some have restricted the term “ves- tigial” to putatively functionless structures, whereas others have applied it to structures that retain some function. Here, I sought to determine whether a consensus has been reached in the usage of such terms for EDBS. A sample of 200 articles in primary scientific literature from the twentieth and twenty- first centuries shows that through both centuries it has been more common to call EDBS “rudimentary” than to restrict the term “rudimentary” to embryonic structures, and it has been more common to attribute function or possible function to structures called “vestigial” than to restrict the term “vesti- gial” to putatively functionless structures. The consensus in both centuries has been less-strict usage of such terms; such usage is, in several ways, more logical than strict usage. Keywords: Vestigial structures, Rudimentary structures, Evolu- tion, Scientific terminology. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecom- mons.org/licenses/by/3.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the origi- nal work is properly cited. Introduction Biologists have long recognized the existence of biological structures that have become drastically reduced and/or have lost salient functions during the course of evolution (hereafter called evolutionarily degenerate biological structures or EDBS). Through the last three centuries, different authors have preferred different terms for such structures, but various forms of the terms “vestige” and “rudiment” have been the most popular. Some authors have suggested strict definitions for such terms (Brues, 1903; Lull, 1920; Hall, 2003), while others ignore strict defini- tions and use the terms more loosely (e.g. Miralles et al., 2012; Nweeia et al., 2012; Woon and Stringer, 2012). This study was undertaken to determine whether a consensus has developed in primary scientific literature of the twentieth and twenty-first centuries as to the proper level of strictness for terms for EDBS. Such terms are often spelled the same in English as they are in other languages. Below, therefore, for the sake of clarity, words in languages other than English are italicized. Biologists used the noun “vestige,” its adjective form “vesti- gial,” and their cognates (hereafter collectively called V-terms), as well as the noun “rudiment,” its adjective form “rudimentary,” and their cognates (hereafter collectively called R-terms) for di- minutive biological structures, even before such structures were widely recognized as evolutionary degenerate. In a French- language article, Saint-Hilaire (1798) called the tiny clavicles of the ostrich rudiments and called the flightless wings of the cas- sowary vestiges. Cuvier (1799) used the French terms rudiment and vestige for numerous diminutive animal organs in a trea- tise on animal anatomy. Both authors used the terms rudiments A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) and vestiges synonymously and without implying evolutionary change. Several nineteenth-century biologists also used similar terms without evolutionary implications, implying only that the structures were relatively tiny in comparison to their homologs in related taxa. Examples include the use of the German words Spur (equivalent to “vestige”) and Rudiment by Mayer (1825) for parts of the diminutive hindlimb skeletons of snakes and re- duced-limbed lizards; the use of the Danish word Rudiment by Eschricht and Reinhardt (1861) for the pelves and hindlimbs of whales; and the use of R-terms by Strothers (1881) for whale hindlimbs. The earliest published reference to apparently-degenerate structures as evidence of biological evolution is in Erasmus Dar- win’s 1791 book Botanic Garden. Darwin noted “apparently useless or incomplete appendages to plants and animals, which seem to shew they have gradually undergone changes from their original state” (p. 8). As examples he listed stamens with- out anthers, styles without stigmas, the halteres of flies, the side toes of pigs, and nipples on male mammals. He used no V-term or R-term for such structures. In his French-language book Philosophie Zoologique, Lamarck (1809) was the earliest author to use such a term to express the opinion that such structures were the evolutionary remnants of more fully-expressed structures in the organisms’ ancestors. La- marck called the blind eyes of mole rats and olms vestiges. He used these and other examples to argue that biological evolu- tion occurs and that an organ that is useless in an animal’s envi- ronment degenerates through the generations. Robert Chambers’ (1844) Vestiges of the Natural History of Creation also cited such structures as evidence for biological evo- lution. Despite the book’s title, Chambers used R-terms for such structures. Examples that he cited include ostrich wings, snake hindlimbs, and the human coccyx. In the category “rudimentary organs” he also included structures that appear and then vanish in the embryo (e.g. teeth in baleen whales), and—echoing Eras- mus Darwin—structures that are present in a sex for which they are useless (e.g. male nipples). He considered rudimentary or- gans to be degenerate forms of more fully-developed ancestral organs in some cases and precursors to more fully-developed descendant organs in other cases. Lamarck and Chambers explicitly cited fossil succession as evidence of biological evolution. Richard Owen subsequently did likewise, and to emphasize the point he cited specific ex- amples of fossil species with morphology intermediate between older fossil species and modern species (Owen, 1846, 1849). He used the term “rudiments” for diminutive biological structures with larger homologs in related species (Owen, 1849, 1866), and implied that such diminutive structures are evolutionarily de- generate (Owen, 1849). Charles Darwin (1860, 1871) largely followed Chambers’ terminology. He used R-terms for putatively degenerate struc- tures, for structures that are present in a sex for which they are useless, and for structures that appear and then vanish in the embryo. However, he coined the term “nascent organs” for pre- cursors to more fully-developed descendant organs, rather than calling them “rudiments.” In the second half of the nineteenth century it became com- monplace for biologists to explicitly opine that apparently- degenerate structures were indeed derived from ancestrally more fully-expressed structures. For EDBS Führbinger (1870) used the German terms Rudiment and Spur, Marsh (1879) used “rudiments” and “remnants,” Cope (1864, 1892, 1894) and Rothschild (1900) used R-terms, and some authors interchange- ably used R-terms and V-terms (Morgan, 1891; Bernard, 1893; Wiedersheim, 1895; Wortmann, 1898). By the twentieth century the theory of biological evolution was accepted by most biologists, and the use of R-terms and V-terms for post-embryonic structures was understood to imply evolutionary degeneration. However, some authors insisted that R-terms be restricted to embryonic structures and not applied to EDBS, to which only V-terms should be applied (Brues, 1903; Lull, 1920; Hall, 2003). Another call for terminological strictness came from biologists who included functionlessness in the definition of “vestigial,” im- plying or stating outright that V-terms ought not be used for a structure with a known function (Bellairs, 1950; Kinsky, 1971; Scadding, 1981, 1982). Such terminological strictness began in the twentieth century. In contrast, some eighteenth-century and early twentieth-century authors explicitly opined that an EDBS could maintain a minor function even after having lost a major one (Darwin, 1860, 1871; Brues, 1903; Waddington, 1937; Stickel and Stickel, 1946). In the primary scientific literature of the twenty-first century some authors advocate or employ terminological strictness of the two kinds mentioned above (Hall, 2003; Buckland-Nicks et al., 2011), whereas others do not. Because terminological consensus is important for communication, it would be useful to determine whether a consensus has developed regarding the degree of strictness in the use of terms for EDBS. If a consensus has developed, then it would be advisable for future authors to use terminology as per the consensus so as to communicate with maximum effectiveness. Methods I compiled a sample of 200 publications that mention EDBS, 100 apiece from the twentieth and twenty-first centuries. The twenty-first century was therefore sampled more densely, which is appropriate because of its higher publication rate. For the search I used online search engines such as JSTOR (www.jstor. org), Science Direct (www.sciencedirect.com), and Journal Finder (library.uncfsu.edu/journal-finder), as well as the help of nine graduate students who were instructed in the use of those three search engines. Students using Journal Finder were instructed to search within eight journals with frequent reference to EDBS (Am J Bot, Ann Bot, Evolution, Evolution and Development, Journal of Experimental Biology. Journal of Morphology, J Zool, and Proc Natl Acad Sci USA), which together provided 83 of the 200 articles. A publication was included in the sample only if it satis- fied the following criteria: (1) it is an example of primary scien- tific literature (an article in a peer-reviewed journal), (2) it uses V-terms and/or R-terms in reference to EDBS, (3) it is not written from an explicitly anti-evolution perspective. The third criterion was used because of potential conflict between the second crite- 296 A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) rion and publications written from an anti-evolution perspective. Once the sample of 200 articles was compiled, I examined the use of R-terms and V-terms to determine which articles were rel- evant to the question of prevailing level of strictness in the use of such terms for EDBS. An article was deemed relevant to R-term strictness level if it included explicit or implicit advocacy of re- striction of R-terms to embryonic structures (by outright insistence on such restriction or by unambiguously employing such strict us- age), or if it took the opposite stance by applying R-terms to EDBS. An article was deemed relevant to V-term strictness level if it included explicit or implied advocacy of restriction of V- terms to putatively functionless structures (by outright insistence on such restriction or by characterizing specific EDBS as function- less), or if it took the opposite stance by attributing a known or possible function to EDBS to which V-terms were applied. Results The results are detailed in Table 1 and summarized in Table 2. Of the 200 articles, 64 were deemed relevant to strictness level for R-terms: 39 from the twentieth century and 25 from the twenty-first century. Of these 64 articles, 59 (92%) applied R-terms to EDBS, while five (8%) advocated or applied restric- tion of R-terms to embryonic structures. Among the 39 relevant 297 Table 1. Usage of forms of the terms “vestige” and “rudiment” in reference to evolutionarily degenerate bio- logical structures (EDBS) in 200 twentieth- and twenty-first century publications in primary scientific literature. EA = explicit advocacy of strict usage. F = function (known or possible) admitted for the EDBS in question, or for EDBS in general. L = EDBS in general, or the EDBS in question, characterized as functionless. R = form(s) of the term “rudiment” used for EDBS. SU = strict usage without explicit advocacy. V = form(s) of the term “vestige” used for EDBS. Publication Term used for EDBS Level of strictness for R-terms Level of strictness for V-terms Brues, 1903 V EA F Matthew, 1908 R, V Bechtel, 1921 V Osborn, 1921 R, V Camp, 1923 R Essex, 1927 V Sewertzoff, 1931 R Chubb, 1932 V Dawson, 1936 V Munro, 1937 V Waddigton, 1937 R, V F Fisher, 1940 V Colbert, 1941 V Pavan, 1945 R Stickel and Stickel, 1946 V F Stokely, 1947a V Stokely, 1947b R, V Bellair,s 1950 R L Bellairs and Underwood, 1951 R, V F Colbert and Mook, 1951 V Hosokawa, 1951 R Knobloch, 1951 V L Woods and Inger, 1957 R Boke, 1959 R, V Satchell, 1959 V Gans, 1960 V Emerson, 1961 V L Mlynarski and Madej, 1961 R F Stephenson, 1961 R, V Neville, 1963 V Radinsky, 1963 V F Gasc,1966 V F List, 1966 R, V A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) 298 A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) Table 1. Continued. Publication Term used for EDBS Level of strictness for R-terms Level of strictness for V-terms Gasc, 1968 R Frick and Taylor, 1968 R, V McDowell, 1969 V Kinsky, 1971 R, V L Patton and Taylor, 1971 R, V Heye,r 1972 V Patton and Taylor, 1973 R, V Taylor and Webb, 1976 R, V L Tidemann, 1976 R Greer, 1977 V L Carpenter et al., 1978 V F Lande, 1978 R, V F Webb and Taylor, 1980 R, V Brygoo, 1981 V Scadding, 1981 V L Slobodchikoff and Wismann, 1981 V Naylor, 1982 V F Scadding, 1982 V L Wilson, 1982 R, V Greer, 1985 R F Greer and Cogger, 1985 V Land, 1985 V Thomason, 1985 V van der Merwe, 1985 V F Boucher, 1986 V Stephens, 1986 V Brandoni and Brooks, 1987 V Greer and Mys, 1987 V Hancox, 1988 V Ludwig and Gibbs, 1989 V Morton and Thurston, 1988 V Carle and Whiton, 1990 R Grimaldi, 1990 R, V Heffner and Heffner, 1990 R, V Gillespie, 1991 V Mayer and Charlesworth, 1991 R, V L Renous et al., 1991 V Jouin et al., 1992 V L McFadden et al., 1994 V F Perle et al., 1994 V Triemer and Lewandowski, 1994 V Černý and Čižinauskas, 1995 V SU Fong et al., 1995 R, V L Deckel, 1996 V F Elbrächter and Schnepf, 1996 V Gilson and McFadden, 1996 V F Call and Dilcher, 1997 V F Crespi and Vanderkist, 1997 V Tague, 1997 R, V Turc and Lecour, 1997 V Bhatnagar and Meisami, 1998 R A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) 299 Table 1. Continued. Publication Term used for EDBS Level of strictness for R-terms Level of strictness for V-terms Doving and Trotier, 1981 V Doweld, 1998 R, V Murali et al., 1998 V F Argiriadi et al., 1999 V F Corley et al., 1999 V Cohn and Tickle, 1999 R F Douglas, 1999 V F Klasing, 1999 V Rodríguez-Riaño et al., 1999 V Weston et al., 1999 V F Scholtz, 2000 V Takeda, 2000 V EA Gibert et al., 2000 V Sato et al., 2000 V F Tchernov et al., 2000 R Yan et al., 2000 V Walker-Larsen and Harder, 2001 V L Beardsley and Olmstead, 2002 V Bejder and Hall, 2002 V F Grimaldi et al., 2002 V Kearney, 2002 V F Narbona et al., 2002 V Peterkova et al., 2002 R, V Roxburgh and Penshow, 2002 V Rudall et al., 2002 V Sekiguchi et al., 2002 V F Strittmatter et al., 2002 R, V Tague, 2002 R, V Ashman, 2003 V Emig, 2003 V Grimaldi, 2003 V Hall, 2003 V EA F Liman and Inman, 2003 V Zhang and Webb, 2003 V Eastman and Lannoo, 2004 V Engel and Grimaldi, 2004 V Kearney and Stuart, 2004 R Maslakova et al., 2004 V Pol and Norell, 2004 V Simões-Lopes and Gutstein, 2004 V F Streltsov et al., 2004 V Whiting et al., 2004 R Beutel and Weide, 2005 V Golonka et al., 2005 V Gotoh et al., 2005 V Kearney et al., 2005 V L Miura, 2005 V F A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) 300 A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) Table 1. Continued. Publication Term used for EDBS Level of strictness for R-terms Level of strictness for V-terms Müller et al., 2005 V Napoleão et al., 2005 V Narbona et al., 2005 V Regoes et al., 2005 V F Ronse de Craene, 2005 R, V Witter et al., 2005 V Espinasa and Jeffery, 2006 V F Franz-Odendaal and Hall, 2006 V F Garnier et al., 2006 V L Gomez and Shaw, 2006 V Grant, 2006 V F Kohlsdorf and Wagner, 2006 V Ostrovsky et al., 2006 V Prince and Johnson, 2006 V F Rehorek and Smith, 2006 V SU Rodríguez-Riaño et al., 2006 R, V F Sidell and O'Brien, 2006 V Bowsher et al., 2007 V Maxwell and Larsson, 2007 V L Mehta and Wainwright, 2007 V Tamatsu et al., 2007 V Watabe et al., 2007 V Brandley et al., 2008 V Gobin et al., 2008 V L Hunt et al., 2008 V Jonz and Nurse, 2008 V F McGowan et al., 2008 V Sherman et al., 2008 R, V F Witton and Naish, 2008 V Yoshizawa and Jeffery, 2008 V Bateman and Fleming, 2009 V Crottini et al., 2009 R Jerez and Tarazona, 2009 V Ostrovsky and Rodríguez, 2009 V Renvoisé et al., 2009 V Sauer and Hausdorf, 2009 V F Tekleva and Krassilov, 2009 R, V Wilkens and Purschke, 2009 R, V Burnham et al., 2010 V Campbell et al., 2010 V Senter, 2010a R, V F Senter, 2010b R, V F Zubidat et al., 2010 V F Barfod et al., 2011 V Buckland-Nicks et al., 2011 R, V L Chan-ard et al., 2011 R, V Gomes Rodrigues et al., 2011 V Moch and Senter, 2011 R, V F Ortega-Chávez and Stauffer, 2011 V A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) 301 Table 1. Continued. Publication Term used for EDBS Level of strictness for R-terms Level of strictness for V-terms Siler and Brown, 2011 V Xu et al., 2011 R, V L Yu et al., 2011 V L Arkhipkin et al., 2012 R, V F Bensimon-Brito et al., 2012 V EA Crole and Soley, 2012 R F Daver et al., 2012 V Hartstone-Rose et al., 2012 V Labonne et al., 2012 V F Miralles et al., 2012 R F Nweeia et al., 2012 R, V L Olympska, 2012 V Woon and Stringer, 2012 R, V Godefroit et al., 2013 V Gotoh et al., 2013 V Huang et al., 2013 V Hutson and Hutson, 2013 V Longo et al., 2013 V F Sato et al., 2014 V F Xu et al., 2014 V Table 2. . Number of articles using forms of the terms “ves- tige” and “rudiment” in specific ways in reference to evo- lutionarily degenerate biological structures in 200 twenti- eth- and twenty-first century publications in primary scientific literature. See Table 1 caption for abbreviations. 1901-2000 2001-14 1901-2014 n 100 100 200 V 87 94 181 R 36 23 59 V&R 22 17 39 EA 2 2 4 SU 1 0 1 F 19 23 42 L 10 9 19 twentieth-century articles, 36 (92%) applied R-terms to EDBS, while three (8%) advocated or applied restriction of R-terms to embryonic structures. Among the 25 relevant twenty-first- century articles, 23 (92%) applied R-terms to EDBS, while two (8%) advocated or applied restriction of R-terms to embryonic structures. The ratio of less-strict to more-strict usage of R-terms is therefore equal between the two centuries, and the prevailing usage in both centuries has been to allow application of R-terms to EDBS. Of the 200 articles, 61 were deemed relevant to strictness level for V-terms: 29 from the twentieth century and 32 from the twenty-first century. Of these 61 articles, 42 (69%) attributed a function or possible function to EDBS to which V-terms are applied, while 19 (31%) advocated restriction of V-terms to pu- tatively functionless structures. Among the 29 relevant twentieth- century articles, 19 (65.5%) attributed a function or possible function to EDBS to which V-terms are applied, while 10 (34.5%) advocated restriction of V-terms to putatively functionless struc- tures. Among the 32 relevant twenty-first-century articles, 23 (72%) attributed a function or possible function to EDBS to which V-terms are applied, while nine (28%) advocated restriction of V-terms to putatively functionless structures. In both centuries, therefore, the prevailing usage of V-terms applies them to struc- tures with a known or suspected function. A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) 302 A tla s Jo ur na l o f Bi ol og y - IS SN 2 15 8- 91 51 . P ub lis he d By A tla s Pu bl ish in g, L P (w w w .a tla s- pu bl ish in g. or g) Discussion Although terminological strictness can be conducive to com- munication in some cases, the consensus found here indicates that this is not the case with V-terms and R-terms for EDBS. This is none too surprising, because less-strict usage of both sets of terms is more logical than stricter usage in several respects. For example, strict application of V-terms only to functionless struc- tures is illogical because it is impossible to prove that a structure is functionless. A structure with no known function may have a function that is yet to be discovered (Scadding, 1981), and a structure that has lost a major function may retain a minor one (Darwin, 1860; Hall, 2003). Strict usage would therefore ulti- mately render V-terms inapplicable to any structure. It is more logical to apply the term “vestigial” to a structure that is demon- strably a vestige (a remnant of an ancestrally greater structure) than to define V-terms so strictly as to render them obsolete. A term is pointless if its definition is so strict as to prohibit its usage. In addition, rendering V-terms obsolete would provide ammu- nition to anti-evolution authors who employ the strict usage of V-terms to cast doubt upon biological evolution by claiming that vestigial structures do not exist because functionlessness cannot be demonstrated (Bergman and Howe, 1990; Sarfati, 2002). It would be incongruous for evolutionary biologists to deliberately provide support for the anti-evolution movement. Less-strict usage of R-terms for EDBS is logical from an evo- devo perspective. The evolutionary process that produces EDBS often includes the arresting of the structure’s development at an early stage (Brues, 1903; Bejder and Hall, 2002; Espinasa and Jeffery, 2006; Rehorek and Smith, 2006), and biologists have long used R-terms for structures in early developmental stages. Because EDBS can therefore be considered to be persistently rudimentary, R-terms are appropriate for EDBS. A recent call for terminological strictness recommended that a given EDBS be called a “rudiment” in the embryo and a “vestige” in the adult (Hall, 2003), but such terminological differentiation is unneces- sary, because a structure with immature morphology is morpho- logically still a rudiment, even if it is in a mature body. To insist that a persistent rudiment not be called a rudiment merely be- cause it persisted as one, is no more logical than to insist that a town not be called a town because it has never grown into a city. Also, biologists frequently use the term “rudimentation” for the evolutionary process that produces EDBS (Berger Dell’Mour, 1985; Tague, 2002; Maxwell and Larrson, 2007). It is linguisti- cally logical to call the product of rudimentation a rudiment. Stricter usage is not only illogical but is also in opposition to the consensus. As shown here, prevailing usage in primary scien- tific literature applies R-terms to EDBS and V-terms to structures that retain a function. The few calls for greater terminological strictness have therefore been met with an implicit veto by the rest of the scientific community. It is therefore recommended here that R-terms not be restricted to embryonic structures and that V-terms not be restricted to putatively functionless structures. Acknowledgments I would like to thank the following individuals for contributing to the search for articles that formed the sample used here: Zenis Ambrocio, Julia B. Andrade, Katanya K. Foust, Jasmine E. Gaston, Ryshonda P. 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