Angiosperms Taxonomy Capitulo 4

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Principles ofANGIOSPERMTAXONOMY

P. H, DAVIS, PH.D., D.Se.University oj Edinburgh

V. H. HEYWOOD, PH.D., D.Se.University o f Liverpool

1991

KRI EGER PUBLISHING COMPANY Malabar, Florida, USA

Exclusive North American Distributor


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Chapter 4

THE CONCEPT OF CHARACTERS"A knowledge of the relative importance of characters can onlybe acquired by lcng study; and without a due appreciation oftheir value no natural group can be defined."J. D. HOOKER, . 1855

INTRODUCTIONTaxonomy, in so far as it is concerned with variation in organisms and theirclassification, deals with characters. Characters provide the taxonomicevidence for natural classification in the Adansonian sense and a large partof the evidence for evolutionary ("phylogenetic") classification. As Cain hasrecently stressed, the basis for a natural classification is entirely the charactersavailable.* I t is therefore surprising that the problems of characters as suchshould have received so little consideration by taxonomists, since the successof most taxonomic studies must depend to a very large extent on whatattributes are selected and considered as characters, how they are tested andtreated, and whether they are relevant for the purpose in hand. It should bepointed out straightaway that the concept of characters as recognisable,separate entities is a product of man's necessity to communicate and there:fore to describe. This explains why characters are so difficult to define. Thisaspect is returned to later.Although it is organisms (or life cycles of organisms) which are classjfied,tit is their characters which provide the evidence used in classification.Ideally the whole organism (i.e. all its attributes) should be employed, butsince each individual possesses thousands of potential characters, practicallimitations impose a restriction on the numbers used. As Muyr r e m a ~ k s , thenumber is limited by the patience of the investigator. This leads on to theproblems of selection. Lam (1959) believes that it is probably not possible tomake a fully relevant choice if ultimate. goal is a completely naturalclassification!' Moreover, different selections of attributes will produce

Throughout this chapter, natural classification is understood as meaningphenetic.t More strictly speaking, it is our knowledge of them at . any time that isclassified.


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INTRODUCTION I I Idifferent classifkations or different versions of the "same" classification. Theselection of features made will, in turn, depend on a number of factors such asthe purpose of the classification (an aspect often ignored. cf. CrawshayWilliams, 1961), the general convenience of the scientist, the availability ofcharacters, and traditional usage.

Most classification hitherto has employed morphological and anatomicalcharacters as its major source of evidence, but as is shown later in thischapter other kinds of features are now being increasingly empl0yed, and it isby no means certain that the results will be acceptable for general purposes.It has, for example, so far been tacitly assumed that natural classificationswill be morphologically expressible and comprise non-overlapping groups.Both these assumptions are open to question.

All classification, as Lam has put it, must be a compromise because of theparticular selection of characters and the degree of precision and unity withwhich they can be defined. Human limitations are in fact the main limitingfactor.

For general purposes it is widely, although by no means universally,agreed that a maximum-attribute classification is the most useful: it is theclassification which allows the maximum number of generalisations to bemade from it (Gilmour & Turrill, 1941). As Bremekamp (1939) has explained,it is the most serviceable because it conveys the maximum possible amou!}t ofinformation about the groups it contains.

Such classifications are, in theory at least, undertaken following theprecepts established by Michel Adanson (p. 23) and are frequently calledAdansonian. The two essential postulates are: (1) in constructing the classification each attribute selected is given equal weight; (2) taxa are based oncorrelations between these attributes. Further, the taxonomic groups derivedby these means are of the class of concepts which have been termed polythetic(Sneath, 1962) "in which no single att ribute is in theory sufficient and necessaryfor membership in the group so long as the members share ahigh proportionof characters" (Snc.ath & Sokal, 1962).

Many taxonomists find great difficuJty in accepting these premises,largely because they regard them as n o n - e ~ o l t i t i o n a r y and therefore unacceptable in this day and age; but failure to accept them is to distort classification by the introduction of phyletic, logical or other sorts of weightingprinciples. Such principles may be justified or are indeed necessary for theconstruction of special classifications (e.g. phyletic), but are not compatiblewith the aims of a general classification. In other words, they serve a differentpurpose. By giving greater weight to particular evidence, a classificationceases to be a general one.These Adansonian principles for the construction of phenetic classifications have received great support from the almost spectacular developmentin the past five or six years of what Sneath and Sokal have called "NumericalTaxonomy"-"the numerical evaluation of the affinity or similarity between


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112 TH E CONCEPT OF CHARACTERS [Ch.4taxonomic units and the ordering of these units into taxa on the basis of theiraffitiities"-which is essentially an extension of Adansonian classificationusing mathematical procedures with th e primary aims of repeatability andobjectivity: The methods and applicability of numerical taxonomy are discussed in a later section (p. 131).

The groups recognised in natural classifications are obviously based onoverall similarity. Mathematical tecluiiques, often employing computers, arenow being used for making the character correlations (see p. 135). Thegroups so delimited are not, however, to be interpreted as necessarily phyletic,due to a number of factors such as convergence, functional correlation anddifficulties of ascertaining homologies. In considering the taxonomic .valueof the kinds of characters which are discussed in this chapter, a clear distinction must be made between their role in maximum-attribute (natural)classifications and special (phylogenetic) classifications (phenetic and phyleticrespectively, as A. J. Cain (1959d) has recently termed them). The conversionof a phenetic classification into a phyletic one involves a further series ofproblems of interpreting characters and weighting them (see p. 127). I t isimportant not to mix the two approaches, as has frequently been done inthe past. The separation of phenetic and phylogenetic classifications is notgenerally accepted today due to the intense conviction that classificationshould be evolutionary and that it is the only sOft of classification whichshould occupy the serious attention of scientific taxonomists. We mustinsist, however, that it is not inconsistent with evolutionary thinking toencourage this separation, and advocate great concentration on the primarytask of producing general classifications based on maximum co-variation ofattributes/or general purposes. In flowering plants the possibilities of achievinga phyletic arrangement or fully evolutionary classification as it is normallyunderstood today are limited for reasons which are discussed below. This isnot to deny, however, that one of the major activities of taxonomy is theevolutionary interpretation of evidence, and part of this chapter is devoted toconsidering how this may be done.

DEFINITION OF CHARACTERSFrom an evolutionary point of view characters should probably not beconsidered in isolation but in combination, since it is on character-combinations that natural selection operates. This is particularly true i f one is considering the possible selective or functional value of characters, but forgeneral taxonomic purposes it is necessary to deal with unit characters whichcan then be treated quantitatively. As is explained elsewhere (Chapter 2),the taxonomist' s assessment of similarity is made to a large extent intuitively,and only those characters which appear to be differential between organismsor groups of them are set down on paper. The current tendency in taxonomy


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DEFINITION OF CHARACTERS ll3is to analyse these intuitive processes and rationalise them by distinguishing the individual characters which, the taxonomist assesses subconsdously together. TlUs raises the problem of the definition of unit characters. It seems unlikely, however, that strict or objective definition can be made, due to the difficulties of deciding in many cases whether any particular attribute . of a plant should be regarded as one or more c h a r a ~ t e r s for the purposes of making comparisons. Many of the difficulties arise from a lack of knowledge of the anatomy and morphology of the various organs and features of plants. A general definition of a character is: any attribute (or descriptive phrase)referring to form, structure or behaviour whiCh the taxonomist separates fromthe whole organism for a particular purpose such as comparison or interpretation.Cain & Harrison (1958) describe a single character as "anything that canbe considered as a 'variable independent of any other thing considered at thesame time".Characters as such are strictly speaking abstract entities: it is theirexpressions or states that taxonomists deal with. "Sepal length" for instancemay be considered as a character; but "sepal length 12mm," is an expressionof that character. For purposes of comparison, a character must be divisibleinto two or more expressions or states (including here presence or absencein qualitative features).For practical purposes a character may be defined as any feature whoseexpression can be measured, counted or otherwjse assessed (cf. Hedberg,1957). Examples are: seed wdgfit, ratIo of corolla to calyx length, number ofteeth on a leaf, presence or absence of tubercles . n leafaxils, number oftransverse septa in anthers, to


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114 THE CONCEPT OF CHARACTERS [Ch.4objective comparisons. In other words, although the comparison of charactersmay be objective, the selection of what is to be regarded as a character mustnecessarily be largely subjective.Let us consider an actual example: how many characters are involved in aleaf shape, for instance? The expression "obovate" may be controlled bydistinct genes, some of which influence length, others of which influencewidth, and all of which probably interact. The concept "obovate" may bedivided into components dealing with length, maximum breadth, position ofmaximum breadth, breadth at various intervals, length/breadth ratio, etc.-the shape being the product of differential growth rates, themselves controlle4by genetically determined hormone systems. In many cases it may besufficient to take the general outline, i.e. obovate, as a single character;in other cases it may have to be broken down and analysed biometrically.Leaf dentation and the form of the apex may be treated as different characterssince they may vary independently of general outline. In compound leavesthe problems are correspondingly complicated.

I t follows from what we have just said that no precise general answercan be given to the question "what is a character?" This can only be consideredin individual cases and what we treat as a character will depend on what wewant to use it for. Even apparent absence of differential characters or expres-sions between individuals or groups need not indicate that they are identical:differences may well come to light after detailed study. I t has been remarkedthat one will always find characters for separation if one tries hard eno\lgh-and, one might add, find that characters used for separation do not hold whenmore material is examined!Characters may be considered from two viewpoints corresponding to twomain activities of taxonomy:

(a) Identification, characterisation and delimitation of species.(6) Grouping (classification) of these species into higher taxa (naturalgroups), i.e. into a system that as far as possible expresses their naturalrelationships.Corresponding with these activities, characters may be regarded asanalytic or synthetic: in each procedure the actual characters used areusually different in each, particular instance, although there is nothing

inherently different in them; it is rather that different kinds of character areselected. In identification and characterisation the taxonomist seeks to employdiagnostic or key characters, i.e. those of limited ou.:urrence, selected so thattheir use alQl1e is sufficient for reaching a correct diagnosis. In arriving at adecision about the delimitation of species numerous characters may be used;many of them of a highly specialised nature which do not lend themselves toeasy description or recognition, e.g. chemical, cytological, physiologicalcharacters or those which show only a statistically significant difference.These characters are not normally given in descriptions (in Floras, revisions,etc.) which often have to be short and precise. Descriptions are really


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DEFINITION OF CHARACTERS 115abstracts for practical convenience-they give some, at least, of the characterswhich are easily recognisable and comparable, not all those features whichmay have been used in making a decision about status. In this activity,taxonomists not only gather information (i.e. characters) together,butpresent them predigested. This is often not realised, especially by nontaxonomists who tend to assume that a particular classification stands orfalls by the evidence quoted. It does raise the problem, however, of the amountof data that should be presented in any ,articular context, and clearly th epurpose of the publication is a major consideration.

The purpose of a Flora is still today primarily to indicate what plantsoccur in a particular area, how they may be identified (by keys and descriptions) and what scientific names refer to them (correct nomenclature andsynonymy). Certain subsidiary information is normally included, such asecological and distributional data and chromosome counts (cf. discussion inHeywood, 1960a: 153-154). Diagnostic characters make the process ofidentification in such Floras easier. Discussion about them and the othercharacters which mayor may not be given in the description is ou t of placeand hinders rather than hastens the purposes of identification. Such information may be published elsewhere. as it often is in detailed monographicstudies.1. Analytic and Synthetic Characters

As noted above, characters used in identification, characterisation anddelimitation may be termed analytic, following Just (1946). In the classification of the results of these activities, ,synthetic characters are used, i.e.characters of a constant nature which unlike diagnostic characters are of wide

O C C ~ I r r e n c e . The analytic or diagnostic characters which serve to recognise agroup are seldom of use in synthesising the group along with others into ahigher group. Fpr this purpose one needs'characters whose constancy ofoccurrence increases the higher the position of the group in the hierarchy.Those characters used to synthesise lower groups into higher groups serve atthe same time to distinguish the higher groups from others of the same iank.This was recognised by Darwin who noted that the classificatory valueof a character which has a wide occurrence (in many forms), as opposedto one which has restricted occurrence, was in terms of hierarchical value.In other words, characters helped to determine different ranks. Characterswhich serve to separate genera in one group are of no value for such a purposein other groups (cf. Darwin, Origin, Ch. XllI, discussion of Robert Brownand characters of the Connaraceae).

As already said, there is no inheren t di fTerence between analytic and synthetic characters: the difference is more one of the particular use made ofthem in a particular case. Charactcrs which differ belween taxa are usuallysimilar in kind to those that arc found varying within species populationsas Simpson (l953a) has noted. He comments that "one needs only to take the

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116 THE CONCEPT OF CHARACTERS [Ch.4diagnosis of virtually any taxonomic group to find that the same diagnosticcharacters are variable in other and related taxonomic ~ o u p s of the sameor lower hierarchic rank and that eventually they can be traced down tovariation in populations" (1953a:' 1(0).

On the other hand, certain features which characterise higher groupings inthe Angiosperms, such as. superior versus inferior ovary, are seldom todayfound to vary within populations. These are the so-called "constitutive"characters discussed on p; 122.

The distinction between analysis and synthesis in taxonomy is not alwaysappreciated. Genera, for example, are syntheses of species, but havinginherited most of our genera from previous generations of taxonomists, wefind that they have, over th e years, been enlarged by accretion and not bydeliberate resynthesis from species. When 'a periodical review of genera isundertaken, this resynthesis often leads to an alteration in generic limits andconsequent change in status. As an example of the use of analysis when.synthesis was required, we may cite Williams's revision of Arenaria in whichhe divided the greater part of the genus into 3 subgenera based on supposeddifferences in structure of staminal glands, thereby disregarding the a c k n o w ~ledged affinities of the species as judged by overall resemblances. This is anexample of analysis by a r i ~ r i weighting of characters and contrasts with thesynthetic treatment of the same group by Fenzl (1840) and McNeill (1962). I tshould be pointed out that staminal gland structure serves as a useful diagnostic (analytic) character to separate species ( e . g ~ Arenaria armeniaca andA. zargariana) within the genus after it has been revised by synthetic procedures.2. Qualitative ,and Quantitative Characters

A distinction is often made between qualitative (including presence orabsence) character s and quantitative characters for descriptive purposes.-Thusfeatures assessed by size, length, etc. are quantitative while those relatingto form, for example, can be considered as qualitative. Such characters asopposite/alternate leaves and placentation types are clearly qualitative.Often, however, the distinction is more apparent than real: many qualitativecharacters may also be expressed quantitatively (cf. leaf shape discussedabove), and it is perhaps more useful to consider characters according towhether they may be assessed directly or not (Heslop-Harrisop, 1952b).

Characters may be a s s ~ s s e d directly by counting (by number), by simp1emeasurements of line


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117EFINITION OF CHARACTEBSleaves, flower parts) are COmIilonly employed in taxonomic descriptions.The term meristicvariation is applied to such cases where the variate can onlyassume discrete values. Seed number when of low value is assessed in thisway too, but when the values are high, assessment by weight of numericalunits (e.g. 100 seeds) is often employed. By their very nature the recordingof such numerical characters seldom presents particular problems.

(b) Size. Linear dimensions, too, are commonly given in descriptions,often with a conventional indication of range and extremes, e.g. petals(3-) 5-7 (-9) mm. Overlapping values between two taa in such characters(especially vegetative) are frequent and they are c o ~ s e q u e n t l y not the mostuseful for taxonomic separation. Apparent differences may be due to inadequate sampling, especially when dealing with conventional herbariumcollections. On the other hand, the differences may be on quite a smallscale (such as anther length in Centaurium spp.) and afford a reliablecriteriqn. Such small-scale differences are frequently neglected since they arenot evident without precise measurement under strong magnification. Thenthere is the psychological problem, too, that small-scale characters tend tobe considered unimportant, while immediately obvious ones are regarded asvaluable. On the other hand, many botanists have considered characters ofprimary importance just because they are difficult to observe] Neither claimis valid-the importance of a character is to be judged by other criteria, as isdiscussed below. I t is also possible that above certain dimensions differencesbetween organs may be ignored, particularly when other species in the genushave relatively small organs.In many instances differences in linear dimensions (between popUlations)may only be statistically significant and consequently of little value asprimary recognition characters.

(c) Shape. As"we have already seen, d i f f e r e n c e ~ in shape present considerable problems although they are often more valuable for diagnostic purposeSthan size differences. They often contribute more to differences in faciesand are instrumental in causing taxa to be recognised as distinct, but theirdescription is correspondingly difficult. The techniques involved in the detailedspecification of leaf shape by numerical indices are discussed later, but somemore general problems have to be considered at this point. Simple leafshapes, such as ovate, lanceolate, linear, etc., are bedevilled by problems ofterminology arid different usages. It soon,becomes evident, even to relativelyinexperienced taxonomists, that there is a lack of precision in the use of the 'various leaf-shape terms and inconsistency of usage between differentFloras, etc. Nothing, of course, can be done to alter past usage although it isoften possible to learn what conventions have been employed in particularcountries or by particular authors. I t is strongly to be recommended thattoday precise systems for designating conventional shapes should be followed,such as those given by Stearn (1956). There are signs that the matter isreceiving inte rnational consideration. (See p. 271).


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118 THE CONCEPT OF CHARACTERS [Ch.4Many characters may not be susceptible to direct numerical assessment,

such as colour, texture, indumentum, smell; taste, patterning, etc. They arenone the less widely used in taxonomy, as is d i s c ~ s s e d in the next c h a p t e r ~ andclearly precise means of describing them are necessary.

Colour can be treated in several ways: the use of international standardssuch as the Ridgway and Royal Horticultural Sociery colour charts, wherebycolour is divided into a number of arbitrary named classes and specimens arematched against them, is common especially in horticulture. In these casesthe number of classes is higher than is normally to be advocated for otherarbitrary class divisions. I t may be found, however, that the range of variationin population samples is so great as not to repay the labour involved, unlesscolour differences appear to show some significant relationship with, say,environmental factors . Other methods of colorometric analysis maybeemployed, such as measurement of amounts of pigment extracted per unit .area, but as Heslop-Harrison (l952b) points out, the amount of pigmentpresent may not bear a linear relationship to the visual effect it produces. Theinterpretation of colour will depend on what is known about the way inwhich it is geneticaIly controlled, ontogenetically and environmentallymodified.

Indumentum can similarly be divided into arbitrary classes or comparedagainst certain standards. Here again problems of terminology arise if it isdivided into named classes such as pubescent, hirsute, sericeous, etc., sincethey are often used imprecisely and are in faot to some extent overlapping andinterchangeable. Detailed analysis of indumentum often reveals differencesin hair length, shape, number of cells, presence or absence of glandulartrichomes, posture, etc. (see p. 154). .

Sculpturing (as in seeds and fruits) and patterning (as for example themaculation in Digitalis purpurea corollas) are more difficult to assess. Theycan, given time and patience, be specified by elaborate geometrical analysis.The problems involved here are discussed by Heslop-Harrison (1952) andexemplified by his series of studies on Dactylorchis populations (1948, 1951,1952a, 1953a). . .Division of numerically assessable characters into arbitrary classes mayalso be a useful procedure . This method has the advantage that it is a greatdeal more rapid than very precise scoring and 'is probably as valuable,although care must be taken, through adequate sampling, to see that thearbitrary grades are not absurd.

Quantitative characters appear to be more frequent iIi the lower taxonomiccategories, at specific and infra-specific levels. Thus species may be separatedby leaf size, corolla length, seed weight, etc., characters which are quantitatively assessed and seldom of use at higher levels in the hierarchy._Mostmorphological differences between ecotypes (and subspecies), for example,hll.Ve been found to be of quantitative nature and may depend on polygenicor multiple-gene inheritance; they may in fact only be appreciated after


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DEFINITION OF CHARACTERS 119statistical analysis. On the other hand, family characters tend to bequalitative.3. "Good" and "Bad" Characters

All characters arc theoretically of use in classification but experience hasshown that their value or use varies according to a number of factors andaccording to the type of taxonomic activity envisaged. 'Great reliance isplaced on "good" characters, at least in practical taxonomy. Good charactersare those tl\at (1) are not subject to wide variation within the samples beingconsidered; (2) do not have a high intrinsic genetic variability; (3) are noteasily susceptible to environmental modification; (4) show consistency,i.e. agree with the correlations of characters existing in a natural system 'ofclassification which was constructed without their use.

We will consider this last point first. The consistency of a characterwill help us to make a decision between two alternative classifications: it canhelp us to resolve differences. In constructing a classification the taxonomistis frequently faced with conflicting evidence. In these circumstances aguiding principle would appear to beto select for the purpose of group-making 'that character or group of characters which agrees with the overall facies of th econsequent groups. This is what we have referred to in Chapter 2 as correla- ,lion or a posteriori weighting of characters,. It is essentially an intuitive orneural use of Adansonian methods of makmg classes according to maximumco-variation of attributes, except that facies is substituted for analysis ofattributes. Ther.:: are two aspects of this use of characters to be distinguished:firstly, there is the value of the character as regards making a choice between 'possible alternative groupings, and secondly, the use of the character as adiagnostic, i.e. for its predictive value. I t is in the latter sense that it is moreuseful to regard a character as reliable since its use will allow us to make a largenum'ber of deductions about the other attributes of the groups it s e p a r ~ t e s off.In b0th senses we are dealing with characters which have already been selectedfor taxonomic use and which are being then considered for a particularpurpose.It should be pointed out that there is a stroni element of circular argumentin this procedure of relying upon a character because it agrees with an existingclassification and then saying that the classification must be a good one, sinceone may be doing no more than bolster up .the errors of that classification.Ideally the whole classification should be broken down and the new characterinduded along with the others for the purposes of completely estimating thecorrelations anew. ' '

The other aspects of good characters, which we may loosely term theirfixity, is concerned with the selection of characters for general usc. Whetherfor diagnosis or synthesis, a good charactcr must obviously be one that islittle influenced by environmental ' changes. The range of expression of a

c h a r a c t e ~ is genetically determined and the actual expression will depend on


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120 THE CONCEPT OF CHARACTERS [Ch.4the environment-genotype reaction within that range. Good characters willhave ltllarrOW range of expression and will be easily recognizable.* The inherent genetic variability of characters is a subject about which weoften have very inadequate information. As a general rule we would agree 'with Rollins (1958) that "we wish to rely on those characters as indicators of

relationship (or lack of it) that are so deeply seated in the genetic constitutionof the species that they cannot be easily obliterated or greatly modified bythe direct effects of any given, simply segregating factor or combination ofsegregating factors". The effects of simple gene differences on morphologicalfeatures will be considered in more detail in Chapter 11. It IS sufficient tomention ' here that many of the characters regarded by . taxonomists asfundamental may be found to vary within populations and be inherited bysimple genetic mechanisms. A well-known example is described in Layiaspp. (Compositae subtrib. Madiinae) where 1- or 2-factorial inheritancegoverns the presence or absence of ray flowers and r e c e p t a c u l ~ r paleae,characters which are extensively treated by taxonomists as having tribalsignificance (Clausen, 1951; Clausen & Hiesey, 1958a). Other features whichmay be more important from a genetical and evolutionary viewpoint mayhave been ignored by taxonomists.It is, however, important to distinguish between those characters whichcan and do vary by simple genetical mechanisms in a population and thosewhich are shown to be simply based genetically but which are no t in facteasily changed.Thus the glabrous-fruited condition in Dithyrea wislezenii (Cruciferae),'which was the main basis for describing D. griffithsii as a distinct species, isa simply inherited character under single gene control. The pubescent plantsstudied by Rollins (1958) were all homozygous and recessive for the pubescentfruit character and he concluded from his studies that presence or absence ofpubescence on the fruits of D. wislezenii is of no taxonomic significance.Here the character in question is no't only simply inherited but shows ahigh likelihood of change.t

It would appea( to follow from this discussion that we should rely onthose characters that depend on a multiplicity of genes or gene combinationsfor their ultimate expression, but great caution should be exercised in applyingthis generalisation since it is not just the genetic basis of a character that hasto be assessed but its possibility of change in practice and the effects of suchchanges on the continued integrity of taxa. In other words, a taxonomiccharacter is only as good as its constancy, no matter what its genetic basis.

Plastic characters may be used to differentiate species if their ranges do notoverlap or i f the overlap is not too extensive.t In other Crucifers glabrous and hairy-fruited variants can form pure populations or ' grow mixed together-presumably depending on whether the initialcoloniser was homozygous or heterozygous for the character in question (e.g. /saris,Sisymbrium). The same often applies to albino variants in many families.


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FUNCTION AND TAXONOMIC CHARACTERS 121Other asptCts of genetic control of characters in assessing their value,including pleiotropism causing mUltiple effects, are discussed in Chapter 11.As we have already mentioned in Chapter 1, there-is a tendency to rely oncertain kinds of characters, such as those pertaining to the flower, in constructing classifications. In terms of consistency and lack of plasticity floralcharacters probably still justify their primary role, but this is not to deny that

other kinds of characters, notably vegetative ones, mlty not be equallyimportant in many cases. More fundamental considerations of floral featuresare concerned with the concepts of constitutive and biological characterswhich are discussed in the next section.FUNCTION AND TAXONOMIC CHARACTERS

For many purposes today the plant taxonomist is seldom concerned withthe functional value of the characters he deals with in delimiting, describingand classifying. In practice his concern is more with the constancy, reliabilityand lack of variation or characters, as we have just seen, than with theirpossible adaptive or functional significance (Heywood, 1959a). It was, however,a characteristic of most pre-Darwinian taxonomy that the characters chosento construct classifi


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122 T H ~ CONCEPT OF CHARACTERS [Ch.4presence or absence of sepals and/or petals ; polypetaly versus sympetaly;actinomorphy versus zygomorphy; numerous stamens versus few stamens;apocarpy versus syncarpy; numerous Qvules versus few ovules; axile versusparietal placentation; superior versus inferior ovary. Only 860f the possible256 combinations of these characters were represented in the 438 families and part-families studied. Nearly half of the total families (or part-families)considered fell into one of twelve combinations indicating a s trong deviationfrom random representation. In other words, there is little scope for diversification with such limited themes and it is precisely such features thatWernham had in mind when discussing biological characters. These selfsamecharacters were those distinguished by Diels as constitutive an d by otherauthors as organisational characters; they are constant throughout a widerange of individuals, species, genera, etc., that is, throughout a higher taxon.Their taxonomic value is high and, as Sporne (1956) says, they are characteristic of a large area of ilJfinity. They are contrasted with non-constitutivefeatures which are less constant and are subject to environmental modification.

As we have noted, the number of "basic" characters in Angiosperm flowersis limited and they evolved more than once in separate lines. Epigyny,sympetaly, etc. may therefore be considered as adaptive convergences in theAngiosperms. This has serious taxonomic implications, as van der Pijl haspointed out. The occurrence of such features in flowers otherwise consideredunspecialised (e.g. epigynyor perigyny in Vict.orU:, C.alycanthus; syn:Petaly.insome Annonaceae) should not be taken as ah mdlcatlOn of taxonomic affimtyand used as a motive for the displacement of taxa with such features to o therparts of the taxonomic system. In other words, such features should not beweighted. It seems in fact probable that many Angiosperm families arebetter regarded as grades as a result of parallel and convergent evolution.Whether they are, as a result, unnatural is a question requiring furtherresearch.Although constitutive, organisational and biological characters are oftenregarded as synonymous (cf. Spbrne, 1956) and the particular exampleschosen to illustrate them in fact coincide, they differ in the functional valueascribed to them. As we have seen, Wernham's biological characters implieda functional relationship with the environment, i.e. they were adaptive;constitutive characters on the other hand are regarded (e.g. by Lam, 1959) asnon-adaptive. The explanation perhaps lies in the confusion that surroundsthe use of the term adaptation. An adaptation is generally defined as afeature of an organism that confers some advantage on it, while adaptationis the acquisition of such a feature by organisms. The words therefore referboth to a process and to the result of that process; as far as thc organism isconcerned, adaptation is active or passive (Mayr, 1942)-it is happeningnow or has happened in the past and is being exploited now. Function andfunctional are used in essentially the same sense. As Simpson (1953a) notes, it isa relationship between organism and environment that makes a charaCter

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http:///reader/full/syn:Petaly.inhttp:///reader/full/syn:Petaly.in


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FUNCTION 'AND TAXONOMIC CHA RACTERS 123advantageuus. It IS difficult to ' define adaptation more strictly withoutfinding that the definition hinges almost entirely on the degree to which wecan recognise whitt may be considered an ) ldaptive relationship. In otherwords, while conspicuous relationship betwcen organism and environment(as for example in p rostrate maritime races of inland species, such as Cytisusscoparius subsp .maritimus) present no problem, in the majority of cases theacceptance of the adaptive nature of particular h a r a c t e r s of,il plant dependson the ingenuity, perception, skill and indeed imagination of the botanist(cf. Barber, 1956).

For most of this century a negative and somewhat extremist attitude tofunction and taxonomic characters has been adopted, largely for historkalreasons (cf. Straw, 1956; van der Pijl, 1 9 6 0 ) But it would appear to stemmore from ignorance than positive knowledge. Barber (1955) goes so far asto say that "the question of the a d ~ p t i v e value of the morphological characteristics which the taxonomist uses to separate species populations is stillone of the crucial problems of evolutionary biology". He further comments,with some j u s t i f i c a t i o ~ , that the museum or herbarium taxonomist oftenregards diagnostic characters as having no selective value, if he considers theproblem at all. He is more concerned with the constancy of the features he haschosen for discrimination between_groups (Heywood, 1959a). Quite apart fromthe problems of whether such characters may be interpreted in terms offunction, some botanists doubt the value, for taxonomic separation, ofcharacters which can be shown to be adaptive (Borril1, 1961c).* From anevolutionary viewpoint, Barber considers there are four possible explanationsconcerning the nature of Jiagnostic characters:

(1) They are the result of genetic drift causing random fixation of genesand may therefore have no adaptive value.

(2) They are the byproducts of the activity of other genes (pleiotropicgene-action). Selection for one character may be due ' to the pleiotropiceffect of the ge'ne controlling another.

(3) They are the result of developm-ental correlations such as allometricgrowth. I f increase in size, for example, arjses through selection, any featurecorrelated developmentally with size will also be subject to change; and ifthese corresponding changes are selectively neutral they will persist.

(4) They have, or have had at some time during the history of the species,some selective value and arose througlLnatur.al-"'S election acting on randommutations and recombinations.

A fifth possibility should be added. The diagnostic character may formpart of a complex of integrated adaptive characters. In other words, it maybelong to an adaptive complex based on an equilibrium due to the interactionof many genes, often held together by linkage.

Very careful investigation is needed to determine which of the above A viewpoint stemming from de Candolle's rejection of physiolbgical charactersfor taxonomic comparison (cf. Cain, 1959a).


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124 . THE CONCEPT OF CHARACTERS [Ch.4mechanisms may have been involved. Adaptation shoUld be considered interms of ability to survive in a particular habitat and will therefore concernsuch features as leaf size, internode developmerit, etc., features which aregreatly modified by the environment and therefore the least reliable. Adaptation depends, however, not only on survival of the individual buton thepropagation of the race. Reproductive structures will therefore be importantand the relative constancy of these structures may be promoted by naturalselection.

In the grass Melica mutica an apparently trivial taxonomic character-aclub-shaped rudiment at the apex'of the spikelets-hasa high selective valueas an effective dispersal mechanism since it is attractive to ants. Similarly inthe Hordeeae tribe of the Gramineae some of the most conspicuous of thereproductive features used in the generic and specific taxonomy of the group(shown by the glumes, lemmas and awns) consist of a series of different butefficient seed-dispersal mechanisms. In the Compositae similar features withsimilar selective values are used for taxonomic separation of genera. Stebbins(1950) quotes examples from the tribe Cichorieae, where the floral struc ture ofthe various genera is essentially similar and genera and species are diagnosedby differences in the involucral bracts and in the mature fruits, especially inthe pappus of the cypselas ("achenes"). These are largely related to seedprotection and seed dispersal mechanisms: In the Chrysantheminae dimorphism of the cypsela is related to different dispersal mechanisms, ,this featurebeing one of the characters separating Chrysanthemum from related generasllch as Leucanthemum. Other examples are known in Hypochaeris, Crepis,etc. in the same family. (See Salisbury, 1942, and Dansereau & Lems, 1957,for a fuller consideration.)

In such groups as the Compositae and the Gramineae with reducedflowers crowded into inflorescences, the whole inflorescence often evolvesas a unit adapted to the methods of pollination or dispersal, or both. Zohary(1950) has outlined trends in the evolution of the Composite capitUlum inwhich the involucre often opens like a capsule to release the cypselas (e.g.Senecio vulgaris) or retains them and is itself the unit of dispersal (as inEchinops and Centaurea squarrosa). In some cases (e.g. Crepis aspera) themarginal cypselas are retained by the indurated involucral bracts while theothers are freely dispersed-thereby achieving different degrees of dispersal.A similar result is obtained by the production of heteromorphic cypselas,the outer being epappose and the inner pappose (Galinsoga parvij/ora), or thepappus of the outer cypselas being caducous (Senecio jacobaea-cf. Burtt,1961 a). L o ; ~ of pappus is not to be regarded as a retrograde loss of an efficientmechanism but is often a specialisation. Although the pappus must haveplayed an important part in achieving the vast distribution of the Compositae,under particular conditions it is disadvantageous (Burtt, 1961a).

One of the objections that has been raised to the hypothesis that elaborateform and colour has evolved in response to pollinating agents is that many


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125UNCTION AND TAXONOMIC CHARACTERSsuch flowers are either selfing or apomictic. They almost certainly evolved,however, from cross-pollinating ancestors-the change in breeding systembeing brought about by changes in selective pressure.Adaptive specialisation appears more clearly at the specific and particularly infra-specific level: in populations showing adaptive response toenvirortmental conditions (Heywood, 1959a). This is discussed in detail inChapter 12. On the other hand, Simpson is of the opinion that the adaptivesignificance of diagnostic characters becomes more apparent, at least inanimal groups, the higher the taxonomic category. Thus higher categoriesshould be distinguished by features of evident adaptive nature. This relationship is not, however, so obviously apparent in flowering plants, and zoologistsoften express surprise that floral characters which, as we have mentioned,'play such a marked role in higher-plant taxonomy have been seldom studiedfrom a functional or adaptational viewpoint.As mentioned above, many people have denied the adaptive value of theso-called biological (constitutive, organisational) characters, such as zygomorphy, sympetaly, complex pollination mechanisms, etc. A survey of theviews of such anti-selectionists as Goebel, Troll, Good, and Nelsson is givenby van der Pijl (1960), who discusses their interpretation of the flower in termsof "Gestalten", automatic autogenesis or physiological necessity. During thelast few decades there has been a renewal of research into floral biology,particularly in tropical countries, using the tools of ecology, genetics, zoological physiology, and it has been possible to demonstrate by experimentthat the intricacies of form, colour, odour and pattern in flowers have animportant role in attracting and orientating pollinating insects.Van der Pijl points out that a major stumbling-block in considering theflower from a functional point of view was that studies were made ontemperate floras where bees and butterflies were normal pollinators, andwhere nectaries are considered a fundamental floral attraction. In other wordsthe climate was wrong both for understanding the flower and for interpretingthe problems. If we look at the earliest known fossil Angiosperms we findthat they show such features as we have previously been considering constitutive or organisational, e.g. epigyny, sympetaly, syncarpy in flowerswhich are otherwise "unspecialised" or simple. It is difficult to see in whatway such features could be adaptive if we consider them in relation to theirpresent day pollinators. We now know, however, from palaeontologicalresearch that in the period when the early A n g i o ~ p e r m s were developing(Lower Cretaceous and probably Upper Jurassic), beetles (Coleoptera) werewell differentiated and abundantly available as pollinators and that someflies '(Diptera) were present. The Hymenoptera and Lepidoptera were not,on the other hand, sufficiently well evolved to be of much significance as thepollinators of early angiosperms. A good summary is given by Barnard (1961).Parallelism of form in flowers known to be pollinated by similar agentsmay in itself be regarded as evidence of an adaptive relationship. For example


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126 TH E CONCEPT OF CHARACTERS [Ch. 4the flowers of Orchidaceae and Zingiberaceae, Polygalaceae and Pap ilion-oideae may be regarded as convergent adaptations in response to similarmethods of pollination. Bradshaw (1958) makes the same point about paralleland repeated correlations between habitat and characters at the genecologicallevel as suggesting probable adaptation. On the other hand, characters whichare adaptive in a broad basic sense (e.g. the very features which define theAngiosperms or the birds) are more likely to remain conservative for longerthan characters that have been evolved in relation to special conditions. Forexample, those floral characters related to specialised pollinators (as in Salvia)or to particular agents of dispersal (as in the subgenera of Trifolium) can befairly safely accepted as adaptive characters that have played little part in thebasic evolution of the higher groups and are associated more often withgeneric or specific differentiation. The cushion habit and the evolution ofberries from capsules by the continued growth of the pericarp wall after fertilisation are other characters than one can reasonably accept as "superficial"specialisations that have had little to do with the basic evolution of families.Each case must be judged on its own merits .

In summary, we may agree with Cain (1959c) that the taxonomic value ofcharacters should not be judged, for purposes of comparison , mainly interms of their function, following the early a priori taxonomists such asLinnaeus and Caesalpinus; nor should we deny taxonomic value to characterswhich can be demonstrated as functional and rely on those which have noapparen,t functional value, as has been proposed by several more recentbotanists such as Sachs (1906), Wilmott (1950), and Borrill (196Ic).As regards the constant characters of taxonomic groups, it is mostprobable that this very constancy is maintained through the forces of naturalselection acting on the populations containing the characters, so that theyare more likely to have a selective value than not. That we cannot comprehendin what way such characters may be functional does not mean they have(or had) no adaptive significance.

For the purposes of phenetic comparisons, knowledge of the adaptivenature of characters has little value except in so far as it stops us countingthe same character more than once through the functional correlation ofapparently separate characters. This question of the co-variation of characterswhich are necessarily correlated is discussed below. The role of function inthe evolutionary interpretation of comparative data is, however, considerable.

CHARACTERS AND THE DEFINITION OF GROUPSAs mentioned above when referring to species, the characters chosen todescribe a taxonomic group form an abstract for p Iactical convenience.They do not in fact define the group. Similarly, as Simpson (l953a) says, thediagnosis of a higher taxon (i.e. above the species level) is not a descriptionof any ancestor of the group. In other words, by abstracting from a highertaxon those cnar&cters which are common to all its members, one may get


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EVOLUTIONARY INTERPRETATION OF DATA 127what is often referred to as a common ground-plan-some sort of generalbasic pial). on which the group was modelled. This idealistic approach wascharacteristic of certain schools of archetype seekers or typologists, particularlythe Germanic school. But as Cain & Harrison (1960) point out, thisprocedure may in fact give us less and less information about the presumedancestor of the group, since the rank of the group from which the charactersare abstracted is higher and contains a much greater diversity than anyancestral taxon. It is also quite possible that the commoll ancestor did notitself possess these characters in common. Features it did possess may havebeen lost in the course of evolution; or the common ancestor may have hadthe characters shown by an existing member of the group which havesurvived by stasigenetic persistence. It is of course possible, too, that thegroup may not have had a common ancestor at all if the resemblances arepartly the result of convergence (cf. Sporne, 1956).

Apart from considerations of common ancestors, it is by considering theabstracted characters as in some way representing and defining the groupas a whole that we fall into serious typological error. A genus or highertaxon is not defined by possessing a particular combination of floral andvegetative features: it can be differentiated or recognised by these. Thesecharacters are the ones which the component species share in common andit is useful for the practical purposes of characterisation, identification andkeying that they be abstracted. (I t should, however, be pointed out that sincephenetically determined groups are based on overall affinity, it is possiblethat in some cases particular features may be modified or even lost in someof the constituent members, with the result that the group cannot be diagnosed(cf. Cain, 1954b; discussion in Simpson, 1961, pp. 93-103) but only described.A number of currently accepted genera probably come under this heading.)They are not, however, the only characters the group has in common, norindeed the only diagnostic characters since others may remain unrecognisedor be too difficult for practical use.

EVOLUTIONARY INTERPRETATION OF COMPARATIVE DATASo far we have been concerned with the arrangement of comparative dataderived from all possible sources so as to produce a phenetic classificationbased on overall similarity. The groups in this classification are recognisedby their resemblances without any weighting. This is an essential first stepin the production of an evolutionary or "phylet ic" classification, although itwill not by itself produce one. Further interpretation of the phenetic featuresof the groups and additional procedures are necessary if a phylogeneticclassification is aimed at. It may not be possible in some groups to go beyonda phenetic arrangement, but usually it is possible to derive some phyleticinformation from' the comparative data. The two most important kinds ofinformation that can be sought are those referring to convergence andprimitive characters. A further aspect to be considered is the necessary


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128 TH E CONCEPT OF CHARACTERS [Ch.4correlation of characters as already mentioned; and the genetic evaluationof characters and its bearing on the likelihood of their alteration or modification is also an increasingly fruitful source of information. Primitive charactershave already been discussed in Chapter 2 in detail and are not consideredfurther here.1. Convergence

Convergent and parallel evolution has already been defined in Chapter 2.The effects of convergence on the interpretation of natural classifications havebeen underestimated in the past (Philipson, 1961) and, in flowering plants at;east, the extent to which it has been unconsciously accepted in currentclassifications is still not yet appreciated. If evolutionary divergence onlyoccurred in groups, then, as Cain (l959b) points out, a natural and phyleticarrangement of it would virtually coincide. However, if convergence hasoccurred, it might well result in forms being grouped together on the basisof their convergent features although in respect of other features they wouldbe separated. In the absence of a fossil record it may be very difficult to tellwhether a natural arrangement is also phyletic, since the extent to which itcontains convergent forms cannot be determined. The assumption thatforms which are most like each other will be phyletically related in the sensethat they have a common ancestor is basically valid, but it does not allowfor the distortion of similarity by convergence which may not even be suspected in the group in question.

The Adansonian method of classification, when thoroughly applied, islikely to overcome the effects of convergence in that the more thoroughlythe comparisons are made, using as many kinds of data as possible, the lesslikely are convergent forms to affect the result. In other words, the largenumbers of different features used will probably compensate for any convergent similarities. Groups which on the basis of their general affinities arewidely separated will show resemblances in only the few convergent charactersBy accepting these convergent characters as indicative of phyletic relationshipwe would clearly be weighting them, and this is of course contrary to theprinciples of general Adansonian classification. Unfor tunately this weightingprocedure is nowadays frequently used, especially when newer kinds ofevidence are employed. As a result phenetic arrangements become distortedand pseudo-phylogenetic classifications are proposed. The normal justificationof this procedure is that the new line of evidence (such as biochemical,cytological, etc.) is regarded as more important, fundamental or at leastmore trustworthY" than the traditional characters. There are two objectionsto this procedure: firstly, as already mentioned, in phenetic comparisonscharacters are not weighted, largely because we have no valid criteria forjUdging the relative importance of different kinds of comparative data;secondly, there is no reason to believe that these newer features are not alsosusceptible to J : : o n v e r g ~ n c e , deviation and even reversal, so that reliance on

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Angiosperms Taxonomy Capitulo 4