Wang William S-Y.

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Distances and Trees in Linguistics'William S-Y. Wang2IntroductionThis talk is divided in two parts. In Part I, I will discuss the various uses thatcomputers have in linguistics. I will briefly survey how they have been used in the past, andspeculate a bit on future applications. In Part II, I will report on some of my own recentresearch, using computers to study the evolution of languages, especially the languages ofChina.Part II will begin with some observations on the term "computational linguistics". In thelast decade or two, we see this term more and more in a variety of contexts, in the names ofconferences, associations, and even university courses. It is a convenient label, and there isno harm in using it as long as we are clear about what we are labeling.I think it is important to keep in mind that the term "computational linguistics" is notparallel in content with many other terms in linguistics, such as "anthropological linguistics"or "psycholinguistics", or "phonology" or "pragmatics". These terms all refer to contentaspects of language; these are, respectively, the role of language in various cultures, therelations between language and psychological mechanisms, how sounds pattern in language,and the use of language in different social contexts. In other words, they all deal withdelimited, intrinsic aspects of language on which we are accumulating knowledge.On the other hand, computational linguistics does not deal with any delimitedsegment of language per se. Rather, computers can be used with profit in all aspects oflinguistic scholarship. In fact it is often a measure of the maturity of any aspect of linguistics'Synopsis of paper presented at the 10th Pacific Asia Conference on Language,Information and Computation.2 Professor of Language Engineering at the City University of Hong Kong, andProfessor of Graduate School of the University of California at Berkeley.E-mail address : WSYW@VIOLET . BERKELEY . EDU, EEWSYW@CITYU EDU HK1to see how much computational power it can harness. The more it can harness this power,the more likely it can achieve results which are statistically significant and of cumulativevalue.We use tape recorders in our fieldwork to preserve the speech sounds of variouslanguages. The linguist who uses this tool has a tremendous advantage over others who relyon pencil and paper alone. For one thing, he has accumulated a database which can beanalyzed over and over again, by himself later, as well as by others which wish to verify orbuild upon his work, and thus make the scholarship cumulative. The support of appropriatetools in all sectors of human endeavor, including, of course, linguistic research. There is aChinese saying from the Lunyu:i,tiff:Ian14*-ZMMigRoughly translated, this means that to do our work well, we must make sure we have thebest tools. And computers are the best tools par excellence for many intellectual tasks.I will dwell on these points at some length partly as a reaction against the excesses ofabstract theorizing that have pervaded linguistics these past decades. Linguistics has losttoo much of its intellectual resources chasing unicorns such as "ideal speaker-hearers" and"homogeneous speech communities." Anytime we study language in the real world,whether synchronically or diachronically, we find the data inevitably very rich and highlycomplex.Part 11A language is a collection of elements, which are organized at various levels. In thephonological hierarchy, for instance, we build from distinctive features, to segments, tosyllables, and so on. In the grammatical hierarchy, we build from morphemes, to words, andto constructions of various sizes.A classic set of questions in linguistics has to do with how these elements are relatedto each other. These questions may be posed syntagmatically, along the axis of time. Thusto the question of how the words of a sequence are related to each other grammatically, wehave the ubiquitous constituent structure tree. To take a phrase made famous by the lateProfessor Y. R. Chao,If these four characters are analyzed as (1,(2,3),4), then the phrase means: cows withno lung disease. If they are analyzed as ((1,2),(3,4)), then meaning would be: sick cowswithout lungs. The difference lies in the scope which the first character modifies, whetherthe scope is just the 2nd character, or the compound built from characters 2 and 3. Theambiguity derives from whether or not to treat 2 and 3 as a constituent. In such trees, the2central idea is that a sequence of elements is a constituent if and only if they derive from asingle node. In other contexts, this condition of deriving exhaustively and exclusively froma single node is called monophyletic.Relations among linguistic elements may be studied paradigmatically as well, that is,how they are related to each other within a set. As an example from phonology, we mayconsider the trees built from distinctive features. These trees are the bases of defining animportant concept in phonological theory - the natural class. This concept underlies almostall formulations of rules and discussions of sound change. Again, natural classes aresegments which are monophyletic on the distinctive feature tree.A paradigmatic example from grammar uses semantic features instead of distinctivefeatures. Such was the method used in an early paper by Katz and Fodor, in analyzing theword "bachelor." Similar methods have been used more recently in constructing semanticnetworks; for example by Winograd.The earliest use of trees was in diachronic linguistics; it dates back to the middle ofthe 19th century, with August Schleicher. Here the elements were individual Indo-European languages, which Schleicher arranged in terms of their phylogenetic relations.Charles Darwin had used trees to represent a few years earlier to represent phylogeneticrelations among organisms. Since Schleicher was deeply impressed with Darwin's ideas, hemight have been influenced by the latter in the use of this method as well.Glottochronology was a method used in diachronic linguistics. It was inspired bycarbon-14 dating, especially as it was developed in archeology. The limitation of theoriginal formulation was that the method was applicable to languages only one pair at a time,thus missing out information which bear upon relations of groups of three or larger.Given this diversity of uses to which trees have been put, it is necessary to definethese various trees in terms of the following four [4] properties.We begin by noting that a tree is a graph consisting of nodes, branches, and tips.Tips are distinct from nodes in that they are each connected to a single branch. We may [1]distinguish an unrooted tree from a rooted one. The branches in an unrooted tree have nodirectionality. Trees which represent semantic networks typically have no directionality.We may [2] distinguish trees in which each of the nodes, not tips, is connected toexactly three branches. These we call binary trees, as opposed to nonbinary trees. Theexception that needs to be made is that the root of a binary tree is connected to exactly twobranches. A tree displaying phonological features, where each distinctive feature is either +or -, e.g., voicing or nasality, would be a rooted binary tree.We may [3] distinguish trees the lengths of whose branches are significant, asopposed to those whose branch lengths are not significant. In a tree designed to show thesyntactic constituents of a sentence, the lengths of the branches have no meaning per se.3Lastly, among trees which are rooted and whose branch lengths are significant, wemay [4] further distinguish the trees whose tips are all equidistant from the root, from thetrees whose tips may not be all equidistant from the root. In a tree showing the historicalderivation of a family of languages, we may wish to require that all the languages haveevolved exactly the same amount from their common ancestor. Alternatively, we may wishto make no such assumption, and the branch lengths reflect the differences, if any, in theamounts to which each language has changed from the common ancestor.I hope to provide examples for all the properties discussed above, and point up thestrengths and weaknesses of each type of tree. Syntactic trees, for instance, have difficultyin displaying discontinuous constituents. Phylogenetic trees are an effective representationof common inheritance; but do not portray easily results due to horizontal transmission.I will also discuss the magnitude of the computation in working with trees. As thenumber of tips increases, the number of trees grow extremely fast, so the task of evaluatingall trees exhaustively is highly demanding of computing power. I will discuss the standardmethods of tree construction, as well report on some on-going work using matrix solutions.Lastly, I will present some linguistic results in the use of trees in analyzing the historicalrelations among the major Chinese dialects.ReferencesFitch, W.M. and E.Margoliash. 1967. Construction of phylogenetic trees. Science155.279-284.Felsenstein, J. PHYLIP: Phylogeny Inference Package. Department of Genetics,University of Washington.Greenberg, J.H. 1957. Essays in Linguistics. University of Chicago Press.Halle, M. 1959. The Sound Pattern of Russian. The Hague: Mouton.Hennig, W. 1966. Phylogenetic Systematics. University of Illinois Press.Hoenigswald, H. and L.Wiener, eds. 1987. Biological Metaphor and CladisticClassification. University of Pennsylvania Press.Katz, J. and J. Fodor. 1964. The Structure of a Semantic Theory. In Fodor, J. and J. Katz,eds. 1964. The Structure of Language: Readings in the Philosophy of Language.Englewood Cliffs, NJ: Prentice Hall.Meyers, R. and W. S-Y. Wang. 1963. Tree representations in linguistics. Ohio StateUniversity: Project on Linguistic Analysis Report #3.Qiao, S.Z. and W.S-Y. Wang. Matrix solutions for additive trees. In preparation.Saitou, N. and M.Nei. 1987. The neighbor-joining method: a new method forreconstructing phylogenetic trees. Mol.Biol.Evol. 4.406-425.Sneath, P.H.A. and R.R.Sokal. 1973. Numerical Taxonomy. New York: W.H.Freeman.4Wang, W.S-Y. 1988. Diannao zai yuyanxue li de yunyong. (The use of computers inlinguistics.) Proceedings of ROC Computational Linguistics Workshop I, pp.257-287.Taiwan. [CAtirili4-4..T.Rig.ffi]Wang,W.S-Y. and Z.W.Shen. X. if-+ 1992. Fangyan guanxide jiliang biaoshu.(Quantitative description of dialect relationship) Zhongguo Yuwen 227.81-92. [ 	 gWang, W. S-Y. Glottochronology, Lexicostatistics, and other Numerical Methods.Pp.1445-1450. Encyclopedia of Language and Linguistics. Pergamon Press.Winograd, T. 1986. "Computer Software for Working in Language". In Wang, W. S-Y. ed.1986. Language, Writing and the Computer. New York: W.H. Freeman, pp.61-72.Xu, Tongqiang. &VI' 1991. ji]	 A-41	 : rai*Eft Act [Historical Linguistics.]Zhong, Yang et al. VI3fh 4 1994. 3--)---)-30101-iit_ifiA Itr; : fl-*-±,*1±[Theory and Method in Classification.]56

Distances and Trees in Linguistics

Wang, William S-Y.

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Distances and Trees in Linguistics

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