Olson (1983) questioned the hypothesis that the order Piciformes is monophyletic and suggested instead that each piciform suborder is allied with a nonpiciform group. His attempt to discredit the synapomorphies joining the Galbulae and Pici is refuted by corrections of his interpretations of previous work. The Piciformes share a complex derived hindlimb morphology involving zygodactyly, a tripartite flexor hallucis longus, and Type VI flexor tendons. Olson's argument for polyphyly combines inadequate data with the inappropriate technique of equating general overall similarity with affinity. His hypothesis is therefore rejected. Problems concerning fossil taxa are also discussed. Based on current information, we believe that a monophyletic origin of the Piciformes remains the hypothesis of choice. Received 18 August 1982, accepted 18 October 1982.

Department of Biological Sciences, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 USA and Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213 USA, and 2Department of Anatomy, University of Illinois at the Medical Center, Chicago, Illinois 60680 USA and Field Museum of Natural History, Chicago, Illinois 60605 USA SWIERCZEWSKI and Raikow (1981) and Simp- son and Cracraft (1981) studied the phyloge- netic relationships of the Piciformes. Both con- cluded that the order is monophyletic and that it contains two monophyletic suborders, the Galbulae (Bucconidae and Galbulidae) and Pici (Capitonidae, Ramphastidae, Indicatoridae, and Picidae). Olson (1983) criticizes the hy- pothesis of monophyly. First, he questions the arguments supporting this view. He then pre- sents an alternative hypothesis that the Gal- bulae are most closely related to the Coracii and the Pici to the Passeriformes. Finally, he criticizes Simpson and Cracraft's discussion of certain fossil birds. After carefully examining Olson's critique, we believe that monophyly remains the more strongly supported hypoth- esis. The possible value of these discussions goes beyond the question of a single branching point in a phylogeny, as it provides an oppor- tunity to compare and contrast two profoundly different approaches to systematic analysis. PICIFORM MONOPHYLY Zygodactyly.--Olson suggests that zygodac- tyly arose independently in the Pici and Gal- bulae because Steinbacher (1935) found differ- ences in their tarsometatarsi. He does not consider the possibility that the differences arose after the two groups diverged from a zy- godactylous common ancestor. Olson consid- ers that the tarsometatarsus of the Galbulae is more primitive than that of the Pici, which is more "specialized," and therefore that mono- phyly requires the condition in the Pici to have evolved from that in the Galbulae. Here, he confuses sister-group relationship with ances- tor-descendent relationship. We did not pro- pose that the Pici evolved from the Galbulae (higher taxa cannot be ancestors). Olson asks "... why would such a transformation take place? Once a group of birds has become per- manently zygodactyl, is it possible to become more zygodactyl?" This question manifests a confusion. One group is not "more zygodac- tyl" than the other; both are described by this term. It may be expected, however, that dif- ferences will accumulate in separately evolving lineages after they have split; it is this process of evolutionary divergence or character trans- formation that gives rise to the hierarchical structure of organic diversity. We suggest that the zygodactyl conditions of the Galbulae and Pici are homologous because other characters (see below) corroborate the unity of the Pici- formes. M. flexor hallucis longus.--Swierczewski and Raikow (1981) reported that the flexor hallucis longus muscle (FHL) shows a derived condi- tion in its origin by a certain pattern of three heads. In most birds there are one or some- times two fernoral heads, but the Piciformes have as well an extensive origin from the fib- ula. Olson dissected one capitonid (Trachy- phonus; Pici) and one bucconid (Hypnelus; Gai- FHL MED INT LAT FHL' MED LAT INT FEMUR BUCCO FHL. INT LAT Fig. 1. Lateral views of the region of the knee and upper crus in three species of piciform birds to show the mode of origin of M. flexor hallucis longus (FHL). In all Piciformes the muscle arises by three separate heads, medial (MED), intermediate (INT), and lateral (LAT), all of which lie medial to the tendon of M. iliofibularis (IL. FIB.). The representative forms il- lustrated are Colaptes auratus (Picidae; Pici), Bucco tamiata (Bucconidae; Galbulae), and Galbula dea (Galbulidae; Galbulae). The drawings were made with a camera lucida and dissecting microscope and are not to scale. bulae). In Trachyphonus he confirmed the report of Swierczewski and Raikow (1981), but in Hypnelus he did not. Olson quotes Swier- czewski (1977: 57) as saying that in the Gal- bulae the heads are "somewhat difficult to sep- arate from each other." Swierczewski's full statement, however, conveys a different mean- ing: "In the Galbulidae and Bucconidae, the medial head arises semitendinous from the in- tercondyloid region of the femur, and the in- termedlate head arises fleshy from the proxi- mocaudal surface of the external femoral condyle; the heads being somewhat difficult to separate from each other." This statement re- fers to only two of the three heads; it does not refer to the fibular head, which is the signifi- cant structure. Olson dissected one specimen each of two species. In contrast, Swierczewski dissected between one and four specimens each of 45 species, and concluded unequivocally (1977: 58) that "Three heads of, origin were found in all species studied herein." Nevertheless, one of us (R.J.R.) has, in re- sponse to Olson's comments, dissected the limb of Colaptes auratus (Picidae), Bucco tamiata (Bucconidae), and Galbula dea (Galbulidae). Like Olson, Raikow confirmed in Colaptes the three heads previously described (Fig. 1). In Hypnelus, Olson "... could not detect any separate heads of origin . . ." and questioned whether "... the nature of the origin of this muscle is homologous between the Galbulae and the Pici or even that it can really be said to have three heads in the Galbulae." The dis- section of Galbula and Bucco has fully con- firmed our previous report that the three heads, including the large fibular head, are present in the pattern described (Fig. 1). This corrobo- rates the hypothesis of piciform monophyly and refutes Olson's claim that the condition is not found in both piciform suborders. Type VI flexor tendons.--Did this condition arise once in the Piciformes or separately in the Galbulae and Pici? Olson states that it is "probably a convergent similarity," but offers no evidence. He suggests that the Galbulae would be difficult to identify with their feet cut off, but fortunately all of our specimens pos- sessed feet. (Actually, the relevant structures are legs, not feet, as the FHL belly lies in the crus.) Olson feels that such characters do not justify ordinal rank, but we do not agree that taxonomic rank is related to the corporeal lo- cation of the relevant characters. Here, Olson confuses phylogeny with taxonomy; the ques- tion at issue is whether the group is mono- phyletic, not what rank it should have in a clas- sification. Perhaps the FHL should rank the group at the generic level, because its origin lies at the knee joint (Junctura genus). ANALYSIS OF OLSON'S HYPOTHESIS Inadequate data.--Olson compares characters in only some of the relevant groups, so that comprehensive comparisons cannot be made. For example, he notes that "in plumage pat- tern, the ground roller Brachypteracias lepto- somus is quite similar to certain of the Buccon- idae such as Malacoptila." But what are the patterns in the other Coracii, the other Buc- conidae, and the Galbulidae? Why are they not discussed? Convergence in plumage patterns is common, which is why comprehensive com- parisons are needed to distinguish between synapomorphies and spurious resemblances. Again, in discussing the skull Olson argues for similarities between the Galbulae and Cor- acii, but restricts his comparison almost en- tirely to the Bucconidae and Coracias when list- ing several features. "In all of these characters," he notes, "the Bucconidae are consistently dif- ferent from the Pici." But what about the Gal- bulidae and other Coracii? This pattern of selective, almost casual choice of data characterizes Olson's entire presenta- tion. It is also difficult to assess his data, be- cause they are presented in a narrative form within the text but are not tabulated in a way that would allow one to determine the state of each character in all the relevant taxa. A tab- ulation of this sort would have revealed that broad conclusions are often based on little data; e.g. Olson dissected only one barbet to deter- mine the form of the postorbital ligament in the large and diverse suborder Pici. Incorrect determination of polarity.--Although generally eschewing cladistic methods, Olson does make one attempt to show the derived nature of a condition, the complex of the post- orbital process, postorbital ligament, and ad- ductor mandibulae. He considers this derived because it "... does not occur in Archaeopter- yx, in presumably primitive land birds such as Opisthocomus and the Cuculiformes, or else- where in the higher groups of land birds." Several problems invalidate this conclusion. First, there is no reason to assume that a char- acter is primitive because it occurs in "primi- tive" taxa. All taxa, including fossil forms, are mosaics of primitive and derived characters. Second, while we concede the general primi- tiveness of Archaeopteryx, we question the ex- tent to which the ligament and muscle can be reconstructed in it. Furthermore, on what basis are the Cuculiformes and Opisthocomus "pre- sumably primitive"? What are "higher groups" of land birds? Why is the comparison limited to the nonmonophyletic nontaxon of land birds? Olson's analysis fails to show that the condi- tion is derived, or even that it is generally char- acteristic of the groups involved. Use of phenetic similarity.--The basic prob- lem with Olson's study is that he tries to de- termine phylogenetic relationships by phenet- ic similarity. He claims that we accept the idea that "... differences between taxa [are] evi- dence of nonrelationship .... "but this is in- correct. We mean only that the absence of sim- ilarities fails to corroborate a hypothesis of monophyly, not that the presence of differ- ences refutes such a hypothesis. "Nonrelation- ship" has no intrinsic meaning; we are search- ing for patterns of common ancestry identified by synapomorphy. The problem with phenetic comparisons is that characters are not analyzed so as to iden- tify the level within the phylogenetic hierarchy at which they define taxa. To illustrate this point, we will consider one example from a recent study of the relationships of Pedionomus torquatus. This species is placed in the mono- typic family Pedionomidae, and the problem is to find its closest relatives. Olson and Stead- man (1981) rejected the previous hypotheses that it is related to the Turnicidae (Gruiformes) or the Galliformes and concluded that it be- longs in the Charadriiformes. Olson and Steadman (1981: 3) note that a hallux is present in Pedionomus and most Charadriiformes but is absent in the Turnicidae. They consider (p. 21) that this supports the removal of Pediono- mus "from the vicinity of the Turnicidae and its placement in the Charadriiformes." Actual- ly, it does nothing of the kind. The hallux evolved in vertebrates as part of the transfor- mation of the pelvic fin into a limb. As such, it is a derived character at the level of the Te- trapoda. Inasmuch as birds form a tetrapod subgroup, the presence of a hallux is a primi- tive state and reveals nothing about the rela- tionships of any avian species to any other. It tells us one thing only about Pedionomus, name- ly that it is a member of the Tetrapoda. It def- initely refutes any hypothesis that Pedionomus is afish, but that is all it does. This example illustrates the principle that character comparisons convey maximum infor- mation about phylogenetic relationships only when one determines the specific points at which they are relevant within the nested sys- tem of clades forming a phylogeny. Simplistic phenetic comparisons fail to provide this in- formation. Olson's assessments of "similarity" cannot be interpreted, because we do not know which of them represent shared primitive characters and which derived for any group of species. The necessity for such analysis may be emphasized by pointing out the distinction between similarity and phylogenetic relation- ship, namely that they do not necessarily co- incide. When an evolving lineage splits, one daughter lineage may evolve faster than the other, so that a form may be less similar to a genealogically closer relative than to a more distant one. Lungfishes are more similar to goldfishes than they are to goldfinches, but they are more closely related to goldfinches than to goldfishes. Crocodiles are more similar to tur- tles than to turtle doves, but they are more closely related to turtle doves than to turtles. Such statements are based on the idea that re- lationship is defined by recency of common ancestry and are correct within the context of specific phylogenetic hypotheses, in this case those of Wiley (1979). Genealogical relation- ships are postulated by the recognition of pat- terns of monophyly, which are defined by syn- apomorphy. For extended discussions of this principle, see Eldredge and Cracraft (1980), Wiley (1981), and Raikow (1982). DISCUSSION Or Fossil TAXA The section of Olson's critique "Comments on Fossil Taxa" adds nothing new about the systematics of the taxa placed in the Primo- bucconidae and Zygodactylidae but does illus- trate the limitations of his systematic meth- odology. Simpson and Cracraft (1981) pointed out that no evidence exists to support the mon- ophyly of the Primobucconidae; Olson in con- trast, believes that this observation is "irrele- vant." But inasmuch as the "primobucconids" have played a pivotal role in the speculations of Olson and his colleagues about the history of the North American bird fauna, it does not seem "irrelevant" to us to ask whether the Pri- mobucconidae have any objective reality as a natural group. Indeed, if the primobucconids are not monophyletic, then Olson and his col- leagues are constructing evolutionary scenarios based upon an imaginary taxon. As a conse- quence, one might expect that Olson would want to demonstrate that monophyly. His cri- tique, however, is a weak attempt to defend the conclusions of Feduccia and Martin (1976) and lacks any relevant empirical evidence. Olson's attempt to shift the burden of proof onto Simpson and Cracraft is altogether spu- rious. We suggest that the burden of proof lies with workers who erect a taxon without posi- tive evidence for its existence as a geneaolog- ical unit. Present morphological evidence sug- gests that the Primobucconidae are a grade, not a clade. If so, then the family has no ontological status, and Olson should instead be defending the usefulness of discussing the evolution of a fictitious taxon. Simpson and Cracraft (1981: 491) were very specific in their criticism; none of the taxa of the Primobucconidae is known to have an en- larged sehnenhalter. For that reason, they con- cluded that there is no justification for saying that the primobucconids are piciforms as cur- rently accepted, let alone closely related to the Bucconidae. That conclusion, if true, also fal- sifies Olson's unsupported speculations about the independent origin of zygodactyly in the Galbulae. From our standpoint, the latter group was already zygodactylous when it arose, be- cause the origin of zygodactyly took place at a higher hierarchical level. This is why Simpson and Cracraft (1981: 491) suggested that one or more taxa currently included in the Primo- bucconidae might be the sister group of the Piciformes. Olson's comments about the Zygodactylidae contain further inaccuracies. Simpson and Cra~ craft (1981: 492) did provide evidence for in- cluding this fossil taxon in the Pici, namely an advanced form of the sehnenhalter, but they also stressed the very tentative nature of this placement. Furthermore, Simpson and Cracraft did not voice any major disagreement with Ballman's (1969a, b) interpretations, as implied by Olson. Their disagreement was with the placement of the Zygodactylidae in the Gal- bulae (Brodkorb 1971). Simpson and Cracraft specifically stated that Ballman may be correct in believing that Zygodactylus is not a pici- form. CONCLUSIONS Rather than reply to every individual point raised by Olson, we have instead addressed the basic differences in systematic philosophy that exemplify current controversy in biology. Olson's attempt to question the synapomor- phies linking the Galbulae and Pici stands re- futed; the Piciformes are characterized by a complex derived morphology of the hind limb involving zygodactyly, tripartite flexor hallucis longus, and Type VI tendons. Olson supports his hypothesis with a potpourri of casual phe- netic similarities analyzed by inappropriate methods, and his argument is little more than an opinion. On the basis of present under- standing, we conclude that piciform monophy- ly remains the preferred hypothesis. ACKNOWLEDGMENTS We thank Kenneth C. Parkes and D. Scott Wood, Carnegie Museum of Natural History, for providing specimens used in dissection. Parkes, Wood, and Mary C. McKitrick provided helpful criticisms of the manuscript. 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