homoplasy in the commelinaceae: a comparison of different classes of morphological characters

547

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M O N O C O T SI I

HOMOPLASY IN THE COMMELINACEAE : A COMPARISON OF DIFFERENT

CLASSES OF MORPHOLOGICAL CHARACTERS

Timothy M. Evans1 and Robert B. Faden2

1Department of Biology, Hope College, 35 East 12th Street, Holland, MI 49423-9000, USA.2Department of Botany, NHB 166, National Museum of Natural History, Smithsonian Institution, Washington,

DC 20560-0166, USA.

Abstract

Recent cladistic analyses by Evans of morphological and molecular (rbcL sequence) data in the plant family Commelinaceae revealed substantial

incongruence between the two data sets. While the rbcL-based phylogeny closely resembled the current taxonomy for the family, the morpholog-

ical data set produced discordant topologies. The goals of this study were to: 1) determine the degree of conflict between the molecular and mor-

phological data sets using consensus and statistical (partition homogeneity test) techniques; 2) evaluate which morphological characters contribute

to the conflict; and 3) examine selective pressures that may lead to character incongruence. Both the consensus approach and the partition homo-

geneity test indicated that the two data sets do, in fact, contain different phylogenetic structure. When the morphological characters were mapped

onto each phylogeny, it was found that the types of character that provide support for the two different analyses, as well as the amount of support

provided, differed. Specifically, anatomical characters provided significantly greater support for the rbcL phylogeny than did any other class of char-

acter (as indicated by consistency index values). The strong selective pressures relating to pollination biology of nectarless flowers, as well as a puta-

tively simple genetic regulation of gross inflorescence morphology, are attributed to the high degree of homoplasy that is apparent in the

morphological data set.

Keywords: character evolution; homoplasy; data congruence; Com-

melinaceae; combining data sets; phylogeny; monocots

INTRODUCTION

The family Commelinaceae exhibits a broad range of diversity,particularly in its floral and inflorescence morphology. The flow-ers range from actinomorphic to strongly zygomorphic, with sev-eral intermediate forms. Each whorl of floral parts may exhibitmodifications. The androecium, for example, may consist of sixequal fertile stamens in two whorls (as in Tradescantia L.), or oneto several stamens may be absent or modified; staminal hairs maybe present or absent; the filaments may be free or epipetalous; theandroecium may be actinomorphic, zygomorphic, or asymmetricat anthesis. Likewise, the inflorescence spans a broad spectrum oftypes. The basic unit of the inflorescence is a helicoid cyme(cincinnus). The arrangement and number of cincinni varywithin the family, from a single terminal cincinnus to numerous

cincinni that are either terminal or axillary. The cincinni may be

arranged along a single axis (e.g. Palisota Reichb.) or compressed

into a globose inflorescence (e.g. Amischotolype Hassk.).

This variation in morphology has made interpretation of rela-

tionships among genera difficult, and several conflicting taxo-

nomic treatments for the family have been proposed (e.g.

Woodson 1942; Pichon 1946; Brenan 1966; Faden and Hunt

1991). The most recent classification for the family was by Faden

and Hunt (1991). They relied heavily upon anatomical charac-

ters to define the major groups, but a wide range of morphologi-

cal characters were considered as well.

To test the competing classifications, intergeneric relationships

within the family were recently evaluated using cladistic analyses

of morphological (Evans 1995; Evans et al., in review) and rbcL

sequence data (Evans 1995; Evans et al., unpublished). These

Timothy M. Evans et al.

548

analyses yielded results that were apparently discordant with each

other, in that the molecular phylogeny closely resembled theclassification of Faden and Hunt (1991) while the morphologicaltopology did not (nor did it closely resemble any of the othercompeting classifications). Specifically, the two studies differed

in the basal relationships within the family, as well as the compo-sition of the major clades. The morphological study placedDichorisandra (tribe Tradescantieae) as the sister genus to the restof the family, whereas the rbcL study placed Cartonema in the

basal position (Figs. 1 and 2). Additionally, the composition ofthe major lineages differed substantially, with the rbcL phylogenyclosely resembling the tribal and subtribal classification proposed

by Faden and Hunt.

The discrepancies between the phylogenies were attributed to ahigh amount of homoplasy in the morphological characters,resulting from numerous cases of convergent and/or parallel evo-lution. When the two data sets were combined in a ‘total data’

approach, however, a phylogeny was produced that closelyresembled the rbcL tree, but which contained novel clades notproduced by either data set alone, indicating that in spite of theconflict between them, both data sets contained useful phyloge-

netic information (Evans 1995).

While the two different types of data clearly yielded differinghypotheses for the evolutionary relationships among genera ofCommelinaceae, the degree of incongruence, as well as the pre-cise nature of the conflict (i.e. specifically what types of charac-

ters contributed to the conflict), were not examined. The goals ofthis study were to: 1) determine whether the morphological andrbcL data sets are, in fact, significantly incongruent, and if so, toevaluate the degree of conflict between them; 2) determine

which morphological characters are contributing to disagree-ment between the two data sets; and 3) examine possible evolu-tionary or selective factors that might cause these morphological

characters to conflict with other data.

MATERIALS AND METHODS

The morphological analysis of relationships among genera ofCommelinaceae was based on 49 unordered characters (Table 1;see Evans et al. (in review) for discussion of character coding).The analysis included an ingroup of 40 Commelinaceae genera

(Fig. 1; the recently described genus Tapheocarpa (Conran 1994)was not included due to insufficient data) and two outgroup gen-era, Pontederia (Pontederiaceae) and Anigozanthos (Haemodora-ceae). When possible, a single exemplar species was scored for

each genus. In four genera (Porandra, Pseudoparis, Rhopalephora,and undescribed genus), data for individual species were notavailable, so the characters were scored to represent the entiregenus (i.e. a character was scored as polymorphic for a genus if it

possessed two different states in the genus as a whole).

The molecular study (Evans 1995; unpublished) included rbcLsequences for 36 species representing 29 genera of Commel-inaceae. Additionally, representatives from five different families

were included in the outgroup (Fig. 2): Pontederia cordata(Pontederiaceae), Philydrum lanuginosum (Philydraceae), Anigo-zanthos flavidus (Haemodoraceae), Zingiber gramineum (Zingib-eraceae), and Hanguana malayana (Hanguanaceae). Taxon

collections and accession details and results from the molecularanalysis will be published elsewhere (Evans, in prep.).

For each study (morphological and molecular), cladistic analyseswere performed using PAUP version 3.1.1 (Swofford 1993) orPAUP* version 4.0 (Swofford 1999). All character state recon-structions were conducted with MacClade version 3.05 (Maddi-son and Maddison 1992) using the accelerated transformation(ACCTRAN) optimization.

Evaluation of congruence between morphological and molecular data sets

Evans (1995) used two approaches to determine that the mor-phological and molecular data sets were incongruent. First, taxo-nomic incongruence was determined by producing a strictconsensus of all trees produced in each analysis (Swofford 1991).The highly unresolved consensus tree (not shown) was inter-preted as evidence that the two different data sets were leading tosubstantially different phylogenetic interpretations.

Second, character congruence (agreement between the data sets,regardless of the phylogenies produced by them independently)was examined. Two congruence indices, IMF (Mickevich and

Fig. 1. A single representative of the 376 most parsimonious trees

found in a cladistic analysis of 49 morphological characters in Commel-

inaceae (modified from Evans et al., in review). This topology was used

for all character state reconstructions onto the morphology-based phyl-

ogeny. * Indicates outgroup taxa.

HOMOPLASY IN COMMELINACEAE

549

Farris 1981) and IM (Miyamoto 1985; Swofford 1991), were cal-

culated to determine how much character conflict was attributa-

ble to disagreement between as opposed to within data sets. These

indices suggested that 5.5–33% of the disagreement could be

attributed to conflict between data sets (IMF = 0.55, IM = 33.4).

While both of these procedures (consensus techniques and con-

gruence indices) provide information regarding the agreement

between multiple data sets, neither provides statistical support

nor any method of estimating confidence. As a statistical meas-

ure of conflict between the two data sets, the partition homoge-

neity test (Farris et al. 1994, 1995) was employed using PAUP*

version 4.0 (Swofford 1999). This test compares the total length

of the trees produced by the original data sets with the distribu-

tion of tree-lengths produced by a set of randomly partitioned

data sets (i.e. the original two data sets are randomly shuffled

into two new data sets that are the same size as the originals). By

producing a distribution of tree-lengths from randomly re-sam-

pled data, the partition homogeneity test provides a statistical

estimate of the probability that the two data sets contain differ-

ent phylogenetic signals. One hundred replicates were performed

Fig. 2. A single representative of the 148 most parsimonious trees

found in a cladistic analysis of rbcL sequences in Commelinaceae (modi-

fied from Evans 1995). This topology was used for all character state

reconstructions onto the molecular-based phylogeny. * Indicates out-

group taxa.

Table 1. Characters and character coding used in

morphological cladistic analysis (data from Evans, in review).

Characters are grouped into categories as discussed

in text.

Inflorescence characters

1. Inflorescence type

0 = thyrsiform (i.e. central axis; more than 2 cincinni)

1 = capituliform

2 = subumbelliform or corymbiform, or paired (short or no central

axis; 2 or more equal cincinni; not fused)

3 = 1–2 (unequal) cincinni; not fused back-to-back

4 = 2 cincinni; fused back-to-back

2. Inflorescence position

0 = terminal (and terminal & axillary)

1 = axillary, non-perforating

2 = axillary, perforating

3 = basal

4 = leaf opposed and terminal

3. Inflorescence peduncle development

0 = well developed

1 = absent or nearly so

4. Cincinnus bract development

0 = small & persistent

1 = small & cauducous or absent

2 = all large & foliaceous or spathaceous

3 = some large & foliaceous or spathaceous and some small or

lacking

4 = grading from large basal to small upper

5. Cincinnus peduncle development

0 = well developed

1 = absent or nearly so

6. Bracteole development

0 = persistent, not perfoliate

1 = persistent, perfoliate

2 = caducous or absent

7. Flowers/cincinnus

0 = sometimes more than one

1 = always one

Floral characters (excluding androecial)

8. Flower types present

0 = all perfect

1 = perfect and staminate (distribution random)

2 = perfect and staminate (types largely separate)

9. Flower stalk

0 = flowers pedicellate

1 = flowers sessile or nearly so

10. Sepal coloring

0 = sepaline (although sometimes colored)

1 = petaline (colored like the petals)

11. Sepal width

0 = sepals narrower than inner petals

1 = sepals equal in width to inner petals

2 = sepals wider than inner petals

12. Petal size and form

0 = all equal or subequal

1 = one strongly differentiated from the other two

13. Petal fusion

0 = free

1 = fused at least basally

14. Petal form

0 = none clawed

1 = at least one clawed

Timothy M. Evans et al.

550

15. Petal fringing

0 = margins not fringed

1 = margins fringed

16. Style curvature

0 = always central

1 = irregularly curved

2 = regularly curved (flower typically enantostylous)

Androecial characters

17. Outer antesepalous stamen (OAS)

0 = present

1 = absent

18. Inner antesepalous stamens (IAS)

0 = present

1 = absent

19. Outer antepetalous stamen (OAP)

0 = present

1 = absent

20. Inner antepetalous stamens (IAP)

0 = present

1 = absent

21. OAS fertility

0 = fertile

1 = sterile

22. IAS fertility

0 = fertile

1 = sterile

23. OAP fertility

0 = fertile

1 = sterile

24. IAP fertility

0 = fertile

1 = sterile

25. OAS filament bearding

0 = glabrous

1 = bearded

26. IAS filament bearding

0 = glabrous

1 = bearded

27. OAP filament bearding

0 = glabrous

1 = bearded

28. IAP filament bearding

0 = glabrous

1 = bearded

29. OAS & IAS filament length

0 = equal

1 = outer long; inner short

2 = outer short; inner long

30. OAP & IAP filament length

0 = equal

1 = outer long; inner short

2 = outer short; inner long

31. OAS & IAP (top) filament length

0 = equal

1 = antesepalous long; antepetalous short

2 = antesepalous short; antepetalous long

Table 1. Characters and character coding used in

morphological cladistic analysis (data from Evans, in review).

Characters are grouped into categories as discussed

in text. (Continued)

32. IAS & OAP (bottom) filament length

0 = equal

1 = antepetalous long; antesepalous short

2 = antepetalous short; antesepalous long

33. Filament adnation to corolla

0 = filaments free

1 = filaments epipetalous

34. Androecial symmetry

0 = androecium actinomorphic at anthesis

1 = androecium zygomorphic at anthesis

2 = androecium asymmetric at anthesis

35. Anther dehiscence

0 = longitudinal

1 = poricidal by terminal pores/slits

2 = poricidal basally

36. Connective width

0 = narrower than the anther sacs

1 = broader than the anther sacs

Fruit/seed characters

37. Fruit locules

0 = three equal

1 = three unequal (occasionally two)

2 = two

38. Seed arrangement

0 = uniseriate

1 = biseriate

39. Seed number/dorsal locule

0 = more than two

1 = two

2 = one

3 = zero

40. Seed number/ventral locule

0 = more than two

1 = two

2 = one

3 = zero

41. Embryotega position

0 = dorsal

1 = semi-dorsal to semi-lateral

2 = lateral

3 = terminal

42. Hilum

0 = punctiform to elliptic

1 = oblong to linear

43. Aril

0 = present

1 = absent

Vegetative/anatomical characters

44. Ptyxis

0 = convolute or conduplicate

1 = involute

45. Stomatal structure

0 = 2-celled

1 = 4-celled

2 = 6-celled; terminal cells large

3 = 6-celled; termianl cells small

Table 1. Characters and character coding used in

morphological cladistic analysis (data from Evans, in review).

Characters are grouped into categories as discussed

in text. (Continued)

HOMOPLASY IN COMMELINACEAE

551

using the TBR branch swapping algorithm (simple addition

sequence, steepest descent). Because of the large amount of com-puter time required to complete the partition homogeneity test,the maximum number of trees retained for each replication waslimited to 100. Although setting the maximum number of trees

to 100 may reduce the chance of finding the most parsimonioustopologies, Farris et al. (1994) note that even a single passthrough the data should be sufficient, as the exact tree-lengths

are not critical to the test.

Comparison of conflict among different classes of morphological characters

To gain a clearer understanding of what types of characters pro-vide the most reliable phylogenetic information in Commel-inaceae, the consistency index (CI) of each morphological

character was determined as mapped onto the morphology- andthe rbcL-based trees. Additionally, the characters were groupedinto six different classes (Table 2). The mean CI values for the sixclasses of characters were compared using the Bonferroni test of

pairwise mean comparison with the Systat 5.2 computer statisti-cal software package. The Bonferroni method allows for multiplecomparisons amongst pairs of means while guaranteeing that no

single mean differs from the others by chance alone (see Mooreand McCabe 1989).

RESULTS

The cladistic analysis of morphological data produced 376equally most parsimonious trees, with a consistency index (CI) of0.43 and retention index (RI) of 0.61 (without autapomorphies;

Evans et al., in review). A representative tree (Fig. 1) was arbitrar-

ily chosen for character state reconstructions.

The rbcL study (Evans 1995), consisting of 257 potentially

informative characters, yielded 148 equally most parsimonioustrees, with a CI of 0.52 and RI of 0.60 (excluding autapomor-

phies). Again, a single representative tree was arbitrarily chosen

for character state reconstructions (Fig. 2).

The partition homogeneity test indicated that the molecular andmorphological data sets have significantly different phylogenetic

structure. During the 100 replicates (100 analyses of randomly

partitioned data), the lengths of the combined trees varied from2032 steps (in the original, non-randomized data sets) to 2057

steps (Fig. 3). The null hypothesis that the two data sets are

homogeneous was rejected (P = 0.01).

Mean CI values for different classes of morphological charactersmapped onto the morphology-based tree ranged from 0.37 (ana-

tomical characters) to 0.55 (inflorescence characters). Although

the mean CI for anatomical characters was lower than for theother types of characters, the Bonferroni test of pairwise mean

comparisons found no statistical difference among any of the cat-

egories (Fig. 4). When the characters were mapped onto therbcL-based phylogeny, however, the anatomical characters per-

formed significantly better than did the other characters (Fig. 5;

Table 2). While the other five categories of morphological char-

acters yielded CI values of 0.26–0.47, anatomical charactersstrongly supported the rbcL-based tree, with a mean CI of 0.78.

Anatomical characters were not significantly different from the

inflorescence or the combined androecial/floral characters (P =0.44 and 0.059 respectively; Table 3).

DISCUSSION

While it is not the aim of this study to discuss whether incongru-

ent data sets should be combined or analysed separately (for var-

ious perspectives on that debate, see Kluge 1989; Barrett et al.1991; Swofford 1991; Donoghue and Sanderson 1992; Bull et

al. 1993; de Queiroz 1993; Miyamoto and Fitch 1995), knowl-

edge of the degree of conflict between two data sets may lead toinsights regarding the evolution of the different types of charac-

ters. Evans (1995; unpublished) found substantial topological

differences between the morphology- and rbcL-based phyloge-nies for genera in Commelinaceae, but it was not clear how

much the two data sets were actually in conflict (i.e. whether a

relatively small number of characters was responsible for majordifferences in the topologies, or if many characters contributed

46. Hook hairs

0 = absent

1 = present

47. Glandular microhairs

0 = absent

1 = present

48. Raphide canals

0 = absent

1 = present, next to vein

2 = present, not next to vein

49. Leaf sheath

0 = absent

1 = present (defines Commelinaceae)

Table 1. Characters and character coding used in

morphological cladistic analysis (data from Evans, in review).

Characters are grouped into categories as discussed

in text. (Continued)

Table 2. Mean CI values for each class of characters as mapped onto the morphological or rbcL phylogeny.

Class Characters included a Mean CI (morphology Mean CI (molecular)

Anatomical 44–49 0.37 ± 0.1 0.78 ± 0.11

Androecial 17–36 0.53 ± 0.05 0.28 ± 0.05

Floral (including androecial) 8–36 0.43 ± 0.8 0.31 ± 0.05

Floral (excluding androecial) 8–16 0.50 ± 0.04 0.38 ± 0.08

Fruit/seed 37–43 0.40 ± 0.09 0.26 ± 0.09

Inflorescence 1–7 0.55 ± 0.09 0.47 ± 0.09

a Numbers indicate characters as listed in Table 1.

Timothy M. Evans et al.

552

to the disagreement). Based on the results of the partition homo-geneity test, however, the two data sets do, in fact, provide signif-

icantly different interpretations of the evolutionary history of the

family. The total length of the trees produced by either data setalone is significantly less than the lengths of the trees produced

when characters from the two data sets are randomly recombined

(Fig. 3). In other words, a given character from one data set tendsto agree more closely with other members of the same data set

than with characters from the other data set.

This difference in phylogenetic structure may be a result of the

fact that the two types of data are under different selective pres-sures. Specifically, the morphological characters exhibit a broad

range of variation, which perhaps serves as an explanation for thevariety of taxonomic classifications that have been proposed for

the family in the past (Meisner 1842; Hasskarl 1870; Brückner

1926, 1930; Woodson 1942; Rohweder 1956; Pichon 1946;Brenan 1966; Faden and Hunt 1991). Depending upon which

morphological characters an author chooses to emphasize in a

taxonomy, any of a number of substantially different hypothesesof relationships could be (and have been) derived. Both Meisner

(1842) and Hasskarl (1870), for example, divided the family intotwo tribes based solely upon the number of fertile and sterile sta-

mens. The circumscription and taxonomic ranks of these groups

were altered by later authors to reflect the greater or lesser impor-tance of different morphological characters (i.e. Brückner (1926,

1930) emphasized floral symmetry; Woodson (1942) and

Rohweder (1956) each emphasized inflorescence features; andFaden and Hunt (1991) used a suite of morphological and ana-

tomical characters). The molecular characters most certainly arenot under the same types of selectional constraints, so they may

provide phylogenetic interpretations that differ from the mor-

phology. Indeed, many examples are available in which the mor-phology and molecule provide different interpretations of

relationships (e.g. Hamby and Zimmer 1992; Mishler et al.1994; Olmstead and Sweere 1994), presumably due at least inpart to differing selective regimes.

Comparison of different classes of morphological characters

The partition homogeneity test indicates that the two data setsare significantly incongruent, but it does not provide informa-tion as to which types of characters contribute to the conflict(either within or between the two data sets). Likewise, the con-gruency indices, IMF and IM, determined by Evans (1995) indi-cated that as much as 33% of the conflict among characters maybe attributed to disagreement between the data sets (as opposedto disagreement within either data set). These two indices donot, however, provide information about which types of charac-ter (i.e. anatomical, androecial, etc.) support or conflict with theother data set. Assuming that CI provides an indication of acharacter’s performance on a given topology, then a comparisonof CI values may provide information about the relative per-formance of the characters (comparison of retention indices (RI;data not shown) yielded similar results). When the performanceof the six classes of morphological characters on the morphologi-cal phylogeny were compared, no statistical difference betweenthe mean CI values for any two classes of characters was found(Fig. 4). The agreement (or at least the lack of disagreement) ofall classes of characters may be attributed to either of two factors.First, it may indicate that all of the characters do, in fact, reflectthe same evolutionary history. In the absence of homoplasy, withthe positive identification of homologous structures across therange of genera in the family, the morphological data should pro-duce a close approximation to the correct phylogeny. The factthat the morphological phylogeny is in disagreement with themolecular phylogeny, however, provides reason to question thephylogenetic utility of one of the two data sets.

Fig. 3. Distribution of tree lengths found from the partition homogeneity test of morphological and molecular data sets in Commelinaceae. * Indicates

the length of the original trees representing the null hypothesis.

HOMOPLASY IN COMMELINACEAE

553

A second explanation for the lack of significant disagreement

among character classes is that the they are homoplastic enough

to mask the true phylogenetic signal. The large error bars in Fig.

4, indicating a wide range of CI in each category, suggest that

this may be the case (although error may be the result of rela-

tively low numbers of characters in some categories).

When the characters were mapped onto the rbcL tree, anatomi-

cal characters performed significantly better than any other class

(Fig. 5; Table 3), and anatomical characters possessed the lowest

mean CI for the morphological tree (Fig. 4). The question

arises, then, as to why anatomical characters would show strong

support for the rbcL tree while providing relatively weak support

for the morphology-based tree they helped to produce. One

possible explanation is that the majority of non-vegetative,

reproductive characters are under strong selective pressure, caus-

ing a high degree of convergent and/or parallel evolution (see

below for an example of convergence of a suite of morphological

characters in Commelinaceae). If vegetative anatomical charac-

ters are operating under less severe selective constraints, then

convergent evolution would be expected to be less common and

the characters would be less homoplasious. Such characters,

being less homoplasious, would therefore provide a greateramount of phylogenetic utility.

Pollination strategies and convergent evolution in Commelinaceae

The high degree of incongruence between the morphological

and molecular phylogenies of Evans (1995) and Evans et al. (inreview) has been attributed in this paper to the high degree of

convergent and/or parallel evolution in several morphologicalcharacters. The adaptations relating to pollination strategies in

Commelinaceae provide an ideal example in which suites of

characters may evolve in concert as a result of strong selection.

Faden (1992) emphasized two major features of flowers in theCommelinaceae that affect their reproductive biology: the lack of

nectar as a reward, and the short flowering time. Because the

flowers do not produce nectar, they must utilize alternativemechanisms for attracting and rewarding insect pollinators. The

Fig. 4. Graph showing the mean consistency index (CI) for the six cate-

gories of morphological characters mapped onto a representative mor-

phology-based cladogram. No statistical difference was found between

any two character classes.

Fig. 5. Graph showing the mean consistency index (CI) for the six cate-

gories of morphological characters mapped onto a representative molec-

ular-based cladogram. Anatomical characters showed statistically greater

support for the cladogram than any other character class (P = 0.002).

Table 3. Bonferroni adjusted probabilities for pairwise comparisons of mean CI value of morphological characters mapped onto the

rbcL phylogeny. Values indicate the probability (P) that the mean CI values for each class of characters are statistically the same.

Anatomical Androecial Floral (all)

Floral (minus

androecium) Fruit/seed Inflorescence

Anatomical 1.000

Androecial 0.002 1.000

Floral (all) 0.002 1.00 1.000

Floral (minus androecium) 0.059 1.00 1.00 1.000

Fruit/seed 0.006 1.00 1.00 1.00 1.000

Inflorescence 0.440 1.00 1.00 1.00 1.00 1.000

Timothy M. Evans et al.

554

short flowering time limits the types of pollination mechanismsthat the flower may utilize.

In the absence of nectar, several features of the androecium haveevolved to attract insect pollinators. These structures (includingshowy staminal hairs; yellow antherodes (staminode anthers);and broad, yellow connectives) give the appearance of largequantities of pollen, and are involved in a deceptive pollinatorattraction system (Faden 1992; Vogel 1978). Indeed, the fertilepollen-bearing anthers are often nondescript compared to the

showy antherodes or staminal hairs, effectively diverting atten-

tion away from the fertile pollen (Faden 1992, this volume). The

combination of a generalist insect pollination strategy, along

with the lack of a nectar reward and a short flowering time, leads

to a limited range of mechanisms for pollination. As all members

of the Commelinaceae must accomplish pollination using these

same limited strategies, features relating to pollination (particu-

larly androecial characters) have arisen numerous times inde-

pendently within the family. Bearded staminal filaments, for

example, have been gained and then lost several times within the

family (Fig. 6). Likewise, broad anther connectives have arisen atleast three times (Fig. 7). It is impossible to determine a priori

whether different species share these character states due to their

common ancestry or because of the strong selection acting upon

them. Thus, characters associated with pollination mechanisms

tend to have poor phylogenetic utility.

Characters relating to the inflorescence may be subject to similar

degrees of convergence. While the genetic basis of inflorescence

structure is not known for the Commelinaceae, evidence is

accumulating that the inflorescence, as well as many plant

organs in general, may be under relatively simple genetic control

(i.e. one or two regulatory genes may be responsible for varia-

tions in inflorescence type; e.g. Dorweiler et al. 1993; Doebley

et al. 1995; Dorweiler and Doebley 1997; Irish 1997). Thecombination of a simple genetic basis coupled with the strong

selective pressures associated with the presentation and arrange-

ment of the flowers may contribute to high levels of homoplasy

in the inflorescence.

In conclusion, Commelinaceae provides an ideal example in

which multiple, independent data sets may be used to evaluate

the evolutionary history of specific morphological features. The

biological characteristics of the family, especially those related to

reproductive biology, have led to a high degree of convergent and

parallel evolution. As evidenced by the numerous conflicting

classifications for the family, however, it has been impossible to

determine which characters accurately reflect evolutionary his-

tory and which ones are more susceptible to convergence due to

selection. Emphasis upon one type of character (e.g. andr-oecium) will tend to produce a classification that differs from

one based on another type of character (e.g. inflorescence).

Faden and Hunt’s classification (1991) appears to resemble the

actual evolutionary history of the family largely because they

relied upon anatomical characters, which are not subject to the

same severe selective pressures found in the reproductive charac-

ters. Without the integration of data sets operating under differ-

ent selection regimes, it is impossible to untangle the effects of

convergence and evolutionary descent. Careful comparison of

character-state distribution patterns in the molecular and mor-

phological data sets has provided insight into which specific

characters or suites of characters are homoplasious and whichones have a greater potential for phylogenetic utility.

ACKNOWLEDGEMENTS

The authors wish to thank Ken Cameron for many helpful com-

ments on the manuscript. Additionally, Ken Sytsma provided

guidance on phylogenetic analyses and the comparison of classes

.Fig. 6. Distribution of staminal filament bearding on the three antese-

palous filaments as mapped onto a representative tree from the rbcL-

based phylogeny of Commelinaceae (some branches have been pruned

from the tree to include only a single species for each genus). Bearded fil-

aments have been gained or lost several times within the family.

HOMOPLASY IN COMMELINACEAE

555

of characters, and Tom Givnish provided assistance with dataanalysis and the partition homogeneity test.

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