Regular cognates: A new term for homology relations in linguistics

The identification of homologous words between genealogically related languages is one of the crucial tasks in historical linguistics. In contrast to biology where, especially at the level of genetic sequences, we find a rather rich terminology contrasting different types of homology among genes and gene sequences, linguistic terminology is still not very precise. Most scholars seem to be content if they can claim that they have identified words that are cognate, which means that they are homologous but have not been borrowed throughout their history.

On various occasions in the past, I have tried to work on a more precise terminology for linguistic frameworks (see for example List 2014 and List 2016, or this earlier blogpost on homology in linguistics). In this context, I have often tried to emphasize that we need to be specifically more careful with the problem of partial cognacy in linguistics, since many words across related languages are not fully homologous, but show homology only in specific parts (List et al. 2016).

Thanks to an increase in accurately annotated linguistic data, resulting specifically from my very productive collaboration with Nathan W. Hill (SOAS, London) on the Burmish languages (see Hill and List 2017), my view has now again changed a bit, and I thought it would be useful to share it here.

Cognacy and homology

The starting point for my earlier proposals to refine the notion of cognacy in linguistics was the rather refined distinction between orthologs, paralogs, and xenologs in molecular biology (Fitch 2000). To account for the distinction between directly inherited (orthologs), duplicated (paralogs), and laterally transferred genes (xenologs), I proposed the terms direct cognates, indirect cognates (inspired by the term oblique cognates by Trask 2000), and indirectly etymologically related words or morphemes (word parts).

While the first and last term are more or less straightforward with respect to linguistic processes, the notion of indirect cognates, however, turned out to be insufficient, given that it is not clear which processes lead to indirect cognacy. Originally, I thought of morphological processes, that is, processes of word formation, by which a word is slightly modified to account for a slightly derived meaning (usually involving processes like suffixation or compounding). My idea was that words that have "experienced" these processes would behave similarly to genes that have been duplicated in biological evolution, and that it would be sufficient to just assign them to a common sub-class of cognates.

However, the research with Nathan W. Hill recently revealed that these terms are insufficient to capture the processes underlying lexical change in historical linguistics.

In order to understand this idea, it is useful to get back to the biological terms and have a closer look at how they distinguish the underlying processes. As far as I understand it, a directaly inherited gene sequence may differ from its ancestral sequence due to processes of random mutation, by which the original gene sequence becomes modified throughout its history. In cases of paralogy, the original gene sequence is duplicated and both copies are subsequently inherited. The copies may, during this process, become more different from each other than would be expected when assuming direct inheritance and random mutation. Similarly, in cases of lateral transfer of genetic material, the changes may again be different from the ones introduced by "normal" random mutation.

If we adopt the view of "normal change", as it is employed in the biological processes, we find a counterpart in the process of sound change in linguistics. As I have mentioned earlier, sound change is a systemic process by which certain sounds in certain environments change regularly across all words in the lexicon of a given language. This process is definitely not comparable with random mutation in sequence evolution, since the process involves a class of "letters" in the sound system of a language that are systematically turned into another sound. However, regarding the crucial role that sound change plays in language evolution, it seems that it is in some sense comparable with random mutation resulting in orthologous genes. Sound change is somewhat the baseline of what happens if languages change, and we have the means to identify its traces by searching for regular sound correspondence patterns across related languages (see my earlier blogpost on this matter).

That sound change is the default which can be handled with some confidence, while other processes, like word formation, semantic change, or the notorious process of analogical leveling, by which not only complex paradigms are transformed to reduce complexity, but other complexities can emerge (compare the German irregular plural of Morgen-de "mornings", which is built on the template of "evenings" Abend-e), is also the reason why Gévaudan (2007) does not include it into the major processes of lexical change. If we take sound change as the default process of language change and as our key evidence for homologous word relations, however, this means that we can no longer make the distinction between direct and indirect cognates following my earlier proposal, since indirect cognates do not necessarily reflect instances of irregular sound change.

This is in fact easy to illustrate. If we follow the former definition of indirect cognacy, the comparison of German Handschuh "glove" (lit. hand-shoe) with English hand would reflect indirect cognacy, since the German word is a compound of Hand "hand" and Schuh "shoe", and thus a derived word form. The morpheme Hand in this example, however, is phonetically identical with German Hand, and the sound correspondences between the English word and the first element of the German compound are still regular by all means. In fact, only a small amount of word formation processes in language evolution also impact on the pronunciation of the base forms.

This means, in turn, that any distinction of cognate word forms (and word parts, i.e., morphemes) into direct and indirect ones that is based on the absence or presence of morphological (= word formation) processes, does not tell us much about the degree to which the sound change affecting these word forms was regular. We could state that direct cognates should always reflect regular sound change, since any irregularity would have to be accounted for by alternative explanations (eg. shortening of a given word due to frequent use, assimilation of sounds serving the ease of pronunciation, etc.).

I wonder whether this would be useful for the initial idea behind the concept of direct cognacy. If we find direct cognates, that is, words that we assume were used by a couple of languages without further modification, apart from regular sound change and potentially sporadic sound changes, it seems still useful to assume that these reflect vertical language history better than cognate sets with residues that were exposed to various morphological processes. Thus, when coding direct cognacy in linguistic datasets, sporadic sound change (if it can be illustrated properly) should not serve as an argument against direct cognacy.

The only way around this problem seems to be to establish a further shade of cognacy, which describes the relations among words and morphemes that have been only affected by sound change, in contrast to words whose history reflects various morphological derivations that impact directly on pronunciation, or processes of irregular sound change due to analogical leveling or assimilation. While I first thought that the biological term ortholog would be useful to describe these specific word relations in linguistics, I realized later that, judging from the Ancient Greek meaning of ortholog (ortho "straight, direct" + logos "relation"), the fact that differences are due to regular sound change is not that neatly reflected.

For now, I think that it should be sufficient to use the term regular cognates for those words or word parts for which we can demonstrate that their change was following the regular "laws" of sound change. Regular cognates are thus defined as words or word parts that have been affected only by sound change during their history. This notion deliberately excludes differences in meaning, frequency of use, or whether the word forms are only reflected in compounds or derived word forms. In fact, for some cases, we could even propose that only parts of a word form that no longer bear any meaning of their own (eg. the first two sounds of a word form) are regular cognates, as long as we can propose good arguments for the regularity of the correspondences.

Note that our tools for alignment analyses in historical linguistics already account for this property. The EDICTOR (http://edictor.digling.org, List 2017), a web-based tool for editing, analyzing, and publishing etymological dictionaries, allows users to exclude those parts from an alignment that are assumed to be irregular, as can be seen in the following illustrative alignment of Proto-Germanic *bakanan "to bake". Scholars who want to be explicit about what parts of an alignment they consider to be regular can use this annotation framework to provide more refined analyses.

EDICTOR alignment of regular cognates for Proto-Germanic *bakanan "to bake"

A crucial consequence of using only regularity in the sound correspondences as the criterion to distinguish regular from irregular cognates is that regular cognacy may also be found to hold for borrowings, since borrowings can, as well, be shown to be regular, especially when the language contact between languages was intensive. Identifying regular cognates is furthermore the first and most important step of the classical comparative method (Weiss 2015) for historical language comparison, since (unless we have written evidence for the true relations between languages) regular cognates (as proven by readily aligned cognate sets) are the fundament upon which we build all our hypotheses regarding the external history of languages.

References

Fitch, W. (2000) Homology: s personal view on some of the problems. Trends in Genetics 16.5: 227-231.

Hill, N. and J.-M. List (2017) Challenges of annotation and analysis in computer-assisted language comparison: a case study on Burmish languages. Yearbook of the Poznań Linguistic Meeting 3.1: 47–76.

List, J.-M. (2014) Sequence Comparison in Historical Linguistics. Düsseldorf University Press: Düsseldorf.

List, J.-M. (2016) Beyond cognacy: Historical relations between words and their implication for phylogenetic reconstruction. Journal of Language Evolution 1.2: 119-136.

List, J.-M., P. Lopez, and E. Bapteste (2016) Using sequence similarity networks to identify partial cognates in multilingual wordlists. In: Proceedings of the Association of Computational Linguistics 2016 (Volume 2: Short Papers). Association of Computational Linguistics, pp. 599-605.

List, J.-M. (2017) A web-based interactive tool for creating, inspecting, editing, and publishing etymological datasets. In: Proceedings of the 15th Conference of the European Chapter of the Association for Computational Linguistics. System Demonstrations, pp. 9-12.

Trask, R. (2000) The Dictionary of Historical and Comparative Linguistics. Edinburgh University Press: Edinburgh.

Weiss, M. (2015) The comparative method. In: Bowern, C. and N. Evans (eds.) The Routledge Handbook of Historical Linguistics. Routledge: New York, pp. 127-145.

Alignments and phylogenetic reconstruction in linguistics and biology

In a very interesting article from 2009 (Morrison 2009), David discusses the question of why phylogeneticists would "ignore computerized sequence alignment". This article was really interesting to me for two reasons: First, the article provides some interesting statistics regarding the degree to which biologists manually adjust the alignments that were automatically produced by software. Second, the article points to the seemingly strange situation in biology in which tree-building is considered to be a task that can be entirely carried out by machines, while the majority of scholars would not trust their final sequence alignments to a computer (Morrison 2009: 150).

This situation finds a direct analogon in historical linguistics. Phylogenetic reconstruction is gaining more and more ground, with many scholars applying (mostly Bayesian) phylogenetic tools to analyze their data (Indo-European: Bouckaert et al. 2012, Tupí-Guaraní (South America): Michael et al. 2015, Japonic: Lee and Hasegawa 2011, Pama-Nguyan (Australian): Bowern and Atkinson 2012, Semitic: Kitchen et al. 2009, Bantu: Grollemund et al. 2015, etc.). Fully automated workflows involving automatic sequence comparison are also practiced (Holman et al. 2011, Jäger 2015, Wheeler 2015), but many linguists remain sceptical regarding their results.

One major difference between biology and linguistics is the selection of comparanda. Biological methods usually derive phylogenetic trees from multiply aligned sequences. Linguistic methods derive trees from sets of homologous (cognate) words (cognate sets) distributed across languages whose evolution is modeled as a process of word-gain and word loss (similar to gene-family gain-loss-studies in biology). While biologists fiddle with their alignments, linguists fiddle with their cognate sets. Cognate identification is exclusively done manually at the moment, and scholars use all kinds of information about word relations that they can get, be it etymological dictionaries, which have been published for more than 200 years, or the intuition of the expert who is annotating the data for cognacy.

Identification of cognate sets in linguistics is essentially a task of sequence comparison (List 2014), and algorithmic as well as manual procedures involve the multiple and the pairwise alignment of words (even if it is done only implicitly by human experts). Compared to biology, sequence comparison in historical linguistics is exacerbated by two factors:

• alphabets (phoneme systems) in linguistics are themselves mutable (Geisler and List 2013), so that when aligning two words we need to find both a mapping between the two alphabets, translating one alphabet into the other, plus a scoring function by which we can score the alignment,
• regular sound change (the process by which the phoneme system is changed) and sporadic sound change (the process by which a sound is sporadically assimilated, lost, or added) are not the only processes that contribute to change of words in the lexicon, and morphological change (by which whole blocks of meaningful parts of a word are re-arranged, exchanged, lost, or added) yields patterns that are essentially unalignable.

The problem of finding the correct mapping between two alphabets in linguistics is further exacerbated by language contact: If languages exchange words on a large scale, then this may have a huge impact on the system of the languages, and it may even introduce new sounds to a language that were not there before (thanks to English, German has now the sound [dʒ], as in journalist or job). If borrowing is frequent enough, it may get close to impossible to judge from comparing the words alone, whether two words in different languages have been transferred directly (vertically) from an ancestral language, or laterally.

As a result, it is probably understandable why linguists often refuse to carry out full alignments of the words in their data. An alignment itself does not necessarily tell us much, compared to all of those processes that an expert infers when comparing language data, which are not alignable.

As an example, let us consider the word for "sun" in six Indo-European languages. Since "sun" is a very basic concept, probably fundamental for all human cultures, experts assume that this word was present as *séh₂u̯el- in Indo-European (an asterisk indicates that the word is not reflected in written sources), and that it was retained as Russian солнце [sɔnʦə], Polish słońce [swɔnjʦɛ], French soleil [sɔlɛj], Italian sole [sole], German Sonne [sɔnə], and Swedish sol [suːl] (Wodtko et al. 2008). An obvious alignment, reflecting the surface similarity between all of these words, would be the following one (taken from List 2014: 135):

Alignment based on sequence similarity.

This alignment, however, is by no means correct. Russian [sɔnʦə] and Polish [swɔnʲʦɛ], for example, share a common suffix, which is reflected as [nʦə] in Russian and as [nʲʦɛ] in Polish, and which was innovated in the the common ancestor of Russian and Polish, but is not present in either of the four other languages. So the [n] in German [sɔnə] is essentially not homologous with the [n] in Russian or the [nʲ] in Polish. The same applies to the [ɛj] in French [sɔlɛj] which reflects a diminutive suffix in Latin sol-iculus "small sun", the regular ancestor form of French soleil. Furthermore, the [w] in the Polish word regularly corresponds to the [l] in French, Italian, and Swedish, but it reflects a swap (metathesis) in the order of the vowel and the consonant in Polish — [sɔl] became [slɔ] which became [swɔ]).

Taking all (and more) of this into account, we need to modify our alignment to account more closely for the processes that experts have inferred from intensive language comparison, as shown in the next figure below (taken from List 2014: 135). In this alignment, the swap in Polish is reflected by the white font of the sounds involved, and gray-shaded columns are supposed to reflect the oldest layer of homology.

Historically informed alignment.

However, even this alignment is essentially misleading. The Indo-European word for "sun" supposedly had a complex paradigm in which the word's stem was alternating in the nominative (and accusative) case and the other cases (oblique cases). So, nominative and accusative used the stem *sóh₂u̯el-, while the other cases used the stem *sh₂én-. The Russian, Polish, French, Italian, and the Swedish form go back to the former, while the German form goes back to the latter, since it is further assumed (or it can be assumed) that the alternation was still preserved in the ancestor of Swedish and German.

This means, however, that our alignment above shrinks to an alignment in which only the first letter, the s, is still reflected in all languages! The following graphic (taken from List 2016) illustrates the processes that led to the current situation for four of our six languages:

Morphological processes of lexical change.

What does this example tell us? On the one hand, it gives some explanation for why linguists do not really want to align words (although the first alignments go back to the early 20th centur, cf. Dixon and Kroeber 1919). It also explains, why classical linguists have a very sceptical attitude towards the computerization of word comparisons, based on the (partially justified) assumption that computers could not handle the complex patterns that are so characteristic of language change.

On the other hand, comparing the situation with biology as reported in Morrison (2009), we can find an interesting parallel between the two disciplines: both linguists and biologists do not really trust machines for comparing their sequences (albeit at different levels of analysis), but they do not seem to have many problems in trusting machines to reconstruct their trees.

However, especially this last point, the fact that we trust machines to grow our trees, while we distrust them to prepare the seeds, should ring an alarm bell. First, we seem to lack clear guidelines (at least in linguistics) regarding the way the manual adjustment (of alignments in biology and cognate sets in linguistics) should be carried out, which has a clear impact on repeatability. Second, if we have processes in both fields that yield essentially unalignable patterns, such as duplications and other molecular processes in biology (Morrison 2009: 156), and morphological processes in linguistics, how can we assume that a phylogenetic tree analysis can sufficiently cope with them, even if we manually adjust everything?

References

• Bouckaert, R., P. Lemey, M. Dunn, S. Greenhill, A. Alekseyenko, A. Drummond, R. Gray, M. Suchard, and Q. Atkinson (2012): Mapping the origins and expansion of the Indo-European language family. Science 337.6097. 957-960.
• Bowern, C. and Q. Atkinson (2012): Computational phylogenetics of the internal structure of Pama-Nguyan. Language 88. 817-845.
• Dixon, R. and A. Kroeber (1919): Linguistic families of California. University of California Press: Berkeley.
• Geisler, H. and J.-M. List (2013): Do languages grow on trees? The tree metaphor in the history of linguistics. In: Fangerau, H., H. Geisler, T. Halling, and W. Martin (eds.): Classification and evolution in biology, linguistics and the history of science. Concepts – methods – visualization. Franz Steiner Verlag: Stuttgart. 111-124.
• Grollemund, R., S. Branford, K. Bostoen, A. Meade, C. Venditti, and M. Pagel (2015): Bantu expansion shows that habitat alters the route and pace of human dispersals. Proceedings of the National Academy of Sciences 112.43. 13296–13301.
• Holman, E., C. Brown, S. Wichmann, A. Müller, V. Velupillai, H. Hammarström, S. Sauppe, H. Jung, D. Bakker, P. Brown, O. Belyaev, M. Urban, R. Mailhammer, J.-M. List, and D. Egorov (2011): Automated dating of the world’s language families based on lexical similarity. Curr. Anthropol. 52.6. 841-875.
• Jäger, G. (2015): Support for linguistic macrofamilies from weighted alignment. Proceedings of the National Academy of Sciences 112.41. 12752–12757.
• Kitchen, A., C. Ehret, S. Assefa, and C. Mulligan (2009): Bayesian phylogenetic analysis of Semitic languages identifies an Early Bronze Age origin of Semitic in the Near East. Proc. R. Soc. London, Ser. B 276.1668. 2703-2710.
• Lee, S. and T. Hasegawa (2011): Bayesian phylogenetic analysis supports an agricultural origin of Japonic languages. Proc. R. Soc. London, Ser. B 278.1725. 3662-3669.
• List, J.-M. (2014): Sequence comparison in historical linguistics. Düsseldorf University Press: Düsseldorf.
• List, J.-M. (2016): Beyond cognacy: Historical relations between words and their implication for phylogenetic reconstruction. Journal of Language Evolution 1. DOI: 10.1093/jole/lzw006.
• Michael, L., N. Chousou-Polydouri, K. Bartolomei, E. Donnelly, V. Wauters, S. Meira, and Z. O’Hagan (2015): A Bayesian phylogenetic classification of Tupí-Guaraní. LIAMES 15.2. 193-221.
• Morrison, D. (2009): Why would phylogeneticists ignore computerized sequence alignment? Syst. Biol. 58.1. 150-158.
• Wheeler, W. and P. Whiteley (2015): Historical linguistics as a sequence optimization problem: the evolution and biogeography of Uto-Aztecan languages. Cladistics 31.2. 113-125.
• Wodtko, D., B. Irslinger, and C. Schneider (2008): Nomina im Indogermanischen Lexikon [Nouns in the Indo-European lexicon]. Winter: Heidelberg.

Lexicostatistics: the predecessor of phylogenetic analyses in historical linguistics

Phylogenetic approaches in historical linguistics are extremely common nowadays. Especially, probabilistic models that model lexical change as a birth-death process of cognate sets evolving along a phylogenetic tree (Pagel 2009) are very popular (Lee and Hasegawa 2011, Kitchen et al. 2009, Bowern and Atkinson 2012), but also splits networks are frequently used (Ben Hamed 2005, Heggarty et al. 2010).

However, the standard procedure to produce a family tree or network with phylogenetic software in linguistics goes back to the method of lexicostatistics, which was developed in the 1950s by Morris Swadesh (1909-1967) in a series of papers (Swadesh 1950, 1952, 1955). Lexicostatistics was discarded by the linguistic community not long after it was proposed (Hoijer 1956, Bergsland and Vogt 1962). Since then, lexicostatistics is considered a methodus non gratus in classical circles of historical linguistics, and using it openly may drastically downgrade one's perceived credibility in certain parts of the community.

To avoid the conflicts, most linguists practicing modern phylogenetic approaches emphasize the fundamental differences between early lexicostatistics and modern phylogenetics. These differences, however, apply only to the way the data is analysed. The basic assumptions underlying the selection and preparation of data have not changed since the 1950s, and it is important to keep this in mind, especially when searching for appropriate phylogenetic models to analyse the data.

The Theory of Basic Vocabulary

Swadesh's basic idea was that in the lexicon of every human language there are words that are culturally neutral and functionally universal; and he used the term "basic vocabulary" to refer to these words. Culturally neutral hereby means that the meanings expressed by the words are independently used across different cultures. Functional universality means that the meanings are expressed by all human languages independent of the time and place where they are spoken. The idea is that these meanings are so important for the functioning of a language as a tool of communication, that every language needs to express them.

Cultural neutrality and functional universality guarantee two important aspects of basic words: their stability and their resistance to borrowing. Stability means that words expressing a basic concept are less likely to change their meaning or to be replaced by another word. An argument for this claim is the functional importance of the words — if the words are important for the functioning of a language, it would not make much sense to change them too quickly. Humans are good at changing the meanings of words, as we can see from daily conversations in the media, where new words tend to pop up seemingly on a daily basis, and old words often drastically change their meanings. But changing words that express basic meanings like "head", "stone", "foot", or "mountain" too often might give rise to confusion in communication. As a result, one can assume that words change at a different pace, depending on the meaning they express, and this is one of the core claims of lexicostatistics.

Resistance to borrowing follows also from stability, since the replacement of words expressing basic meanings may again have an impact on our daily communication, and we may thus assume that speakers avoid borrowing these words too quickly. Cultural neutrality of concepts is another important point to guarantee resistance to borrowing. Words expressing concepts which play an important cultural role may easily be transferred from one language to another along with the culture. Thus, although it seems likely that every language has a word for "god" or "spirit" and the like (so the concept is to a certain degree functionally universal), the lack of cultural independency makes words expressing religious terms very likely candidates for borrowing, and it is probably no coincidence that words expressing religion and belief rank first in the scale of borrowability (Tadmor 2009: 232).

Lexical Replacement, Data Preparation, and Divergence Time Estimation

Swadesh had further ideas regarding the importance of basic vocabulary. He assumed that the process of lexical replacement follows universal rates as far as the basic vocabulary is concerned, and that this would allow us to date the divergence of languages, provided we are able to identify the shared cognates. In lexical replacement, a word w₁ expressing a given meaning x in a language is replaced by a word w₂ which then expresses the meaning x, while w₁ either shifts to express another meaning, or completely disappears from the language. For example, older thou did in English was replaced by the plural form you, which now also expresses the singular. In order to search for cognates and determine the time when two languages diverged, Swadesh proposed a straightforward procedure, consisting of very concrete steps (compare Dyen et al. 1992):

• Compile a list of basic concepts (concepts that you think are culturally neutral and functionally universal; see here for a comparative collection of different lists that have been proposed and used in the past)
• translate these concepts into the different languages you want to analyse
• search for cognates between the languages in each meaning slot; if words in two languages are not cognate for a given meaning, then this points to former processes of lexical replacement in at least one of the languages since their divergence
• count the number of shared cognates, and use some mathematics to calculate the divergence time (which has been independently calibrated using some test cases of known divergence times).

As an example for such a wordlist with cognate judgments, compare the table in the first figure, where I have entered just a few basic concepts from Swadesh's standard concept list and translated them into four languages. Cognacy is assigned with help of IDs in the column at the right of each language column, but also further highlighted with different colors.

Classical cognate coding in lexicostatistics

Phylogenetic Approaches in Historical Linguistics

Modern phylogenetic approaches in historical linguistics basically follow the same workflow that Swadesh propagated for lexicostatistics, the only difference being the last step of the working procedure. Instead of Swadesh's formula, which compared lexical replacement with radioactive decay and was based on aggregated distances in its core, character-based methods are used to infer phylogenetic trees. Characters are retrieved from the data by extracting each cognate from a lexicostatistical wordlist and annotating the presence or absence of each cognate set in each language.

Thus, while Swadesh's lexicostatistical data model would state that the words for "hand" in German and English were cognate, and also in Italian and French, but not in Germanic and Romance, the binary presence-absence coding states that the cognate set formed by words like English hand and German Hand is not present in Romance languages, and that the cognate set formed by words like Italian mano and French main is absent in Germanic languages. This is illustrated in the table below, where the same IDs and colors are used to mark the cognate sets as in the table shown above.

Presence-absence cognate coding for modern phylogenetic analyses

The new way of cognate coding along with the use of phylogenetic software methods has brought, without doubt, many improvements compared to Swadesh's idea of dating divergence times by counting percentages of shared cognates. A couple of problems, however, remain, and one should not forget them when applying computational methods to originally lexicostatistic datasets.

First, we could ask whether the main assumptions of functional universality and cultural neutrality really hold. It seems to be true that words can be remarkably stable throughout the history of a language family. It is, however, also true that the most stable words are not necessarily the same across all language families. Ever since Swadesh established the idea of basic vocabulary, scholars have tried to improve the list of basic vocabulary items. Swadesh himself started from a list of 215 concepts (Swadesh 1950), which he then reduced to 200 concepts (1952) and then later to 100 concepts (1952). Other scholars went further, like Dolgopolsky (1964 [1986]) who reduced the list to 16 concepts. The Concepticon is a resource that links many of the concept lists that have been proposed in the past. When comparing these lists, which all represent what some scholars would label "basic vocabulary items", it becomes obvious that the number of items that all scholars agree upon sinks drastically, while the number of concepts that have been claimed to be basic increases.

An even greater problem than the question of universality and neutrality of basic vocabulary, however, is the underlying model of cognacy in combination with the proposed process of change. Swadesh's model of cognacy controls for meaning. While this model of cognacy is consistent with Swadesh's idea of lexical replacement as a basic process of lexical change, it is by no means consistent with birth-death models of cognate gain and cognate loss if they are created from lexicostatistical data. In biology, birth-death models are usually used to model the evolution of homologous gene families distributed across whole genomes. If we use the traditional view according to which words can be cognate regardless of meaning, the analogy holds, and birth-death processes seem to be adequate in order to analyze datasets that are based on these root cognates (Starostin 1989) or etymological cognates (Starostin 2013). But if we control for meaning in the cognate judgments, we do not necessarily capture processes of gain and loss in our data. Instead, we capture processes in which links between word forms and concepts are shifted, and we investigate these shifts through the very narrow "windows" of pre-defined slots of basic concepts, as I have tried to depict in the following graphic.

Looking at kexical replacement through the small windows of basic vocabulary

Conclusion

As David has mentioned before: We do not necessarily need realistic models in phylogenetic research to infer meaningful processes. The same can probably be said about the discrepancy between our lexicostatistical datasets (Swadesh's heritage, which we keep using for practical reasons) and the birth-death models we now use to analyse the data. Nevertheless, I cannot avoid an uncomfortable feeling when thinking that an algorithm is modeling gain and loss of characters in a dataset that was not produced for this purpose. In order to model the traditional lexicostatistical data consistently, we would either (i) need explicit multistate-models in which concepts are a character and the forms represent the states (Ringe et al. 2002, Ben Hamed and Wang 2006), or (ii) we should directly turn to "root-cognate" methods. These methods have been discussed for some time now (Starostin 1989, Holm 2000), but there is only one recent approach by Michael et al. (forthcoming) in which this is consistently tested.

References

• Bergsland, K. and H. Vogt (1962): On the validity of glottochronology. Curr. Anthropol. 3.2. 115-153.
• Bowern, C. and Q. Atkinson (2012): Computational phylogenetics of the internal structure of Pama-Nguyan. Language 88. 817-845.
• Dolgopolsky, A. (1964): Gipoteza drevnejšego rodstva jazykovych semej Severnoj Evrazii s verojatnostej točky zrenija [A probabilistic hypothesis concering the oldest relationships among the language families of Northern Eurasia]. Voprosy Jazykoznanija 2. 53-63.
• Dyen, I., J. Kruskal, and P. Black (1992): An Indoeuropean classification. A lexicostatistical experiment. T. Am. Philos. Soc. 82.5. iii-132.
• Ben Hamed, M. and F. Wang (2006): Stuck in the forest: Trees, networks and Chinese dialects. Diachronica 23. 29-60.
• Hoijer, H. (1956): Lexicostatistics. A critique. Language 32.1. 49-60.
• Holm, H. (2000): Genealogy of the main Indo-European branches applying the separation base method. J. Quant. Linguist. 7.2. 73-95.
• Kitchen, A., C. Ehret, S. Assefa, and C. Mulligan (2009): Bayesian phylogenetic analysis of Semitic languages identifies an Early Bronze Age origin of Semitic in the Near East. Proc. R. Soc. London, Ser. B 276.1668. 2703-2710.
• Lee, S. and T. Hasegawa (2011): Bayesian phylogenetic analysis supports an agricultural origin of Japonic languages. Proc. R. Soc. London, Ser. B 278.1725. 3662-3669.
• Pagel, M. (2009): Human language as a culturally transmitted replicator. Nature Reviews. Genetics 10. 405-415.
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Homology and cognacy: fundamental historical relations between words

This is a guest blog post, following on from his previous post, by:

Johann-Mattis List

Centre des Recherches Linguistiques sur l'Asie Orientale, Paris, France

Introduction

All languages constantly change. Words are lost when speakers cease to use them, new words are gained when new concepts evolve, and even the pronunciation of the words changes slightly over time. Slight modifications that can barely be noticed during a person's lifetime sum up to great changes in the system of a language over centuries. When the speakers of a language diverge, their speech keeps on changing independently in the two communities, and at a certain point of time the independent changes are so great that they can no longer communicate with each other — what was one language has become two.

Demonstrating that two languages once were one is one of the major tasks of historical linguistics. If no written documents of the ancestral language exist, one has to rely on specific techniques for linguistic reconstruction (see the examples in this previous post). These techniques require us to first identify those words in the descendant languages that presumably go back to a common word form in the ancestral language. In identifying these words, we infer historical relations between them. The most fundamental historical relation between words is the relation of common descent. However, similarly to evolutionary biology, where homology can be further subdivided into the more specific relations of orthology, paralogy, and xenology, more specific fundamental historical relations between words can be defined for historical linguistics, depending on the underlying evolutionary scenario.

Homology and Cognacy in Linguistics and Biology

In evolutionary biology there is a rather rich terminological framework describing fundamental historical relations between genes and morphological characters. Discussions regarding the epistemological and ontological aspects of these relations are still ongoing (see the overview in Koonin 2005, but also this recent post by David). Linguists, in contrast, have rarely addressed these questions directly. They rather assumed that the fundamental historical relations between words are more or less self-evident, with only few counter-examples, which were largely ignored in the literature (Arapov and Xerc 1974; Holzer 1996; Katičić 1966). As a result, our traditional terminology to describe the fundamental historical relations between words is very imprecise and often leads to confusion, especially when it comes to computational applications that are based on software originally developed for applications in evolutionary biology.

As an example, consider the fundamental concept of homology in evolutionary biology. According to Koonin (2005: 311), it "designates a relationship of common descent between any entities, without further specification of the evolutionary scenario". The terms orthology, paralogy, and xenology are used to address more specific relations. Orthology refers to "genes related via speciation" (Koonin 2005: 311); that is, genes related via direct descent. Paralogy refers to "genes related via duplication" (ibid.); that is, genes related via indirect descent. Xenology, a notion which was introduced by Gray and Fitch (1983), refers to genes "whose history, since their common ancestor, involves an interspecies (horizontal) transfer of the genetic material for at least one of those characters" (Fitch 2000: 229); i.e. to genes related via descent involving lateral transfer.

In historical linguistics, the only relation that is explicitly defined is cognacy (also called cognation). Cognacy usually refers to words related via “descent from a common ancestor” (Trask 2000: 63), and it is strictly distinguished from descent involving lateral transfer (borrowing). The term cognacy itself, however, covers both direct and indirect descent. Hence, traditionally, German Zahn 'tooth' is cognate with English tooth, and German selig 'blessed' with English silly, and German Geburt 'birth' with English birth, although the historical processes that shaped the present appearance of these three word pairs are quite different. Apart from the sound shape, Zahn and tooth have regularly developed from Proto-Germanic *tanθ-; selig and silly both go back to Proto-Germanic *sæli- 'happy', but the meaning of the English word has changed greatly; Geburt and birth stem from Proto-Germanic *ga-burdi-, but the English word has lost the prefix as a result of specific morphological processes during the development of the English language (all examples follow Kluge and Seebold 2002, with modifications for the pronunciation of Proto-Germanic). Thus, of the three examples of cognate words given, only the first would qualify as having evolved by direct inheritance, while the inheritance of the latter two could be labelled as indirect, involving processes which are largely language-specific and irregular, such as meaning shift and morpheme loss. Trask (2000: 234) suggests the term oblique cognacy to label these cases of indirect inheritance, but this term seems to be rarely used in historical linguistics; and at least in the mainstream literature of historical linguistics I could not find even a single instance where the term was employed (apart from the passage by Trask).

In the table above (with modifications taken from List 2014: 39), I have tried to contrast the terminology used in evolutionary biology and historical linguistics by comparing to which degree they reflect fundamental historical relations between words or genes. Here, common descent is treated as a basic relation which can be further subdivided into relations of direct common descent, indirect common descent, and common descent involving lateral transfer. As one can easily see, historical linguistics lacks proper terms for at least half of the relations, offering no exact counterparts for homology, orthology, and xenology in evolutionary biology.

Cognacy in historical linguistics is often deemed to be identical with homology in evolutionary biology, but this is only true if one ignores common descent involving lateral transfer. One may argue that the notion of xenology is not unknown to linguists, since the borrowing of words is a very common phenomenon in language history. However, the specific relation which is termed xenology in biology has no direct counterpart in historical linguistics: the term borrowing refers to a distinct process, not a relation resulting from the process. There is no common term in historical linguistics which addresses the specific relation between such words as German kurz 'short' and English short. These words are not cognate, since the German word has been borrowed from Latin cŭrtus 'mutilated' (Kluge and Seebold 2002). They share, however, a common history, since Latin cŭrtus and English short both (may) go back to Proto-Indo-European *(s)ker- 'cut off' (Vaan 2008: 158). The specific history behind these relations is illustrated in the following figure.

A specific advantage of the biological notion of homology as a basic relation covering any kind of historical relatedness, compared to the linguistic notion of cognacy as a basic relation covering direct and indirect common descent, is that the former is much more realistic regarding the epistemological limits of historical research. Up to a certain point, it can be fairly reliably demonstrated that the basic entities in the respective disciplines (words, genes, or morphological characters) share a common history. Demonstrating that more detailed relations hold, however, is often much harder. The strict notion of cognacy has forced linguists to set goals for their discipline which may often be far too ambitious to achieve. We need to adjust our terminology accordingly and bring our goals into balance with the epistemological limits of our discipline. In order to do so, I have proposed to refine our current terminology in historical linguistics to the schema shown in the table below (with modifications taken from List 2014: 44):

Fifty Shades of Cognacy

In a recent blog post, David pointed to the relative character of homology in evolutionary biology in emphasizing that it "only applies locally, to any one level of the hierarchy of character generalization". Recalling his example of bat wings compared to bird wings, which are homologous when comparing them as forelimbs but who are analogous when comparing them as wings, we can find similar examples in historical linguistics.

If we consider words for 'to give' in the four Romance languages Portuguese, Spanish, Provencal and French, then we can state that both Portuguese dar and Spanish dar are homologous, as are Provencal douna and French donner. The former pair go back to the Latin word dare 'to give', and the latter pair go back to the Latin word donare 'to gift (give as a present)'. In those times when Latin was commonly spoken, both dare and donare were clearly separated words denoting clearly separated contexts and being used in clearly separated contexts. The verb donare itself was derived from Latin donum 'present, gift'. Similarly to English where nouns can be easily used as verbs, Latin allowed for specific morphological processes. In contrast to English, however, these processes required that the form of the noun was modified (compare English gift vs. to gift with Latin donum vs. donare).

What the ancient Romans (who spoke Latin as their native tongue) were not aware of is that Latin donum 'gift' and Latin dare 'to give' themselve go back to a common word form. This was no longer evident in Latin, but it was in Proto-Indo-European, the ancestor of the Latin language. Thus, Latin dare goes back to Proto-Indo-European *deh₃- 'to give', and Latin donum goes back to Proto-Indo-European *deh₃-no- 'that which is given (the gift)' (Meiser 1999; what is written as *h₃ in this context was probably pronounced as [x] or [h]). The word form *deh₃-no- is a regular derivation from *deh₃-, so at the Indo-European level both forms are homologous, since one is derived from the other. That means, in turn, that Latin dare and donum are also homologs, since they are the residual forms of the two homologous words in Proto-Indo-European. And since Latin donare is a regular derivation of donum, this means, again, that Latin dare and donare are also homologous, as are the words in the four descendant languages, Portuguese dar, Spanish dar, Provencal douna, and French donner. Depending on the time depth we apply, we will arrive at different homology decisions. I have tried to depict the complex history of the words in the following figure:

Judging from the treatment in linguistic databases, many scholars do not regard these different "shades of homology" as a real problem. In most cases, scholars use a "lumping approach" and label as cognates all words that go back to a common root, no matter how far that root goes back in time (compare, for example, the cognate labeling for reflexes of Proto-Indo-European *deh₃- in the IELex).

Importantly, this labeling practice, however, may be contrary to the models that are used to analyze the data afterwards. All computational analyses model language evolution as a process of word gain and word loss. The words for the analyses are sampled from an initial set of concepts (such as 'give', 'hand', 'foot', 'stone', etc.) which are translated into the languages under investigation. If we did not know about the deeper history of Latin dare and donare, we would assume a regular process of language evolution here: at some point, the speakers of Gallo-Romance would cease to use the word dare to express the meaning 'to give' and use the word donare instead, while the speakers of Ibero-Romance would keep on using the word dare. This well-known process of lexical replacement (illustrated in the graphic below), which may provide strong phylogenetic signals, is lost in the current encoding practice where all four words are treated as homologs. Our current practice of cognate coding masks vital processes of language change.

Outlook

Historical linguistics needs a more serious analysis of the fundamental processes of language change and the fundamental historical relations resulting from these processes. In the last two decades a large arsenal of quantitative methods has been introduced in historical linguistics. The majority of these methods come from evolutionary biology. While we have quickly learned to adapt and apply these methods to address questions of language classification and language evolution, we have forgotten to ask whether the processes these methods are supposed to model actually coincide with the fundamental processes of language evolution. Apart from adapting only the methods from evolutionary biology, we should consider also adapting the habit of having deeper discussions regarding the very basics of our methodology.

References

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Fitch WM (2000) Homology: a personal view on some of the problems. Trends in Genetics 16.5, 227-231.

Gray GS, Fitch WM (1983) Evolution of antibiotic resistance genes: the DNA sequence of a kanamycin resistance gene from Staphylococcus aureus. Molecular Biology and Evolution 1.1, 57-66.

Holzer G (1996) Das Erschließen unbelegter Sprachen. Zu den theoretischen Grundlagen der genetischen Linguistik. Frankfurt am Main: Lang

Katičić R (1966) Modellbegriffe in der vergleichenden Sprachwissenschaft. Kratylos 11, 49-67.

Kluge F, Seebold E (2002) Etymologisches Wörterbuch der deutschen Sprache. 24th ed. Berlin: de Gruyter.

List J-M (2014) Sequence Comparison in Historical Linguistics. Düsseldorf: Düsseldorf University Press.

Meiser G (1999) Historische Laut- und Formenlehre der lateinischen Sprache. Wissenschaftliche Buchgesellschaft: Darmstadt.

Trask RL (2000) The Dictionary of Historical and Comparative Linguistics. Edinburgh: Edinburgh University Press.

Vaan M (2008) Etymological Dictionary of Latin and the Other Italic Languages. Leiden and Boston: Brill.