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Taxonomic Aspects of Avian Hybridization

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Taxonomic Aspects of Avian Hybridization TAXONOMIC ASPECTS OF AVIAN HYBRIDIZATION LESTER L. SHORT INSTANCESof natural hybridizationfrequently pose difficult taxonomic problems.A decisionregarding the taxonomicstatus of hybridizingforms shouldbe basedupon an evaluationof all availablebiological information concerningtheir relationship.The paucity of availableinformation usually hamperstaxonomists treating hybridizationamong the higher vertebrates. Also it is difficult or impossibleto subjectsuch animals, and particularly animal populations,to experimentalstudy. Neverthelesstaxonomists must renderdecisions that reflect current availableknowledge. I offer the follow- ing discussionand definitionsin the hopeof facilitating suchdecisions by taxonomists,especially those working with terrestrial vertebrates. Ex- amplesforming the frameworkfor the discussionare drawn from the litera- ture of avian hybridization,including some work of my own. The widely divergent treatment of hybridizing forms evident in the recent ornithological literature demonstratesthe need for some guide- lines. At one extremethis divergenttreatment appearsto reflect a hold- over of typologicalthinking (e.g. Godfrey, 1966; Sutton, 1967), while the oppositeextreme (e.g. Phillips et al., 1964; West, 1962) resultsfrom an overly strict interpretationof the biologicalspecies definition in that different speciesdo not interbreed,and henceinterbreeding forms must be conspecific.The resultsof such divergenttreatment are taxonomicover- splittingin the formercase, and overlumpingin the latter. I definehybridization as the interbreedingof individualsof morphologi- cally and presumablygenetically distinct populations,regardless of the taxonomicstatus of such'populations (seen Anderson, 1949: 61; Short, 1965: 360). This definition of hybridization,essentially that of Mayr (1963: 110), does not differ fundamentally from its broader genetic meaning,in which it can even includeinterbreeding of genotypicallydif- ferent individuals; I have simply restrictedits usage. Included are both sympatricand allopatrichybridization; the latter includessecondary inter- gradation,but not primary intergradation (see Mayr, 1963: 368 369). Secondary intergradation is distinguished,occasionally with difficulty, from primary intergradation by the increasedvariability of populations within comparedwith thoseoutside the area of intergradation,contrasted with the normal variability of populationswithin and outsidean area of primary intergradation. I define "hybrids" as the productsof sympatric and allopatric hybridization (including secondaryintergradation), and "intergrades"as the productsof primary intergradation. The practice of defining hybridization on the basis of the taxa involved should be dis- 84 The Auk, 86: 84-105. January, 1969 Jan., 1969] Taxonomic Aspectsof Avian Hybridization 85 couraged,because it requiresa taxonomicdecision before "hybridization" can even be employed (see Mayr, 1963: 111; Bigelow, 1965), and it unnecessarilylimits the use of this word. I cannot see the need for en- cumberingtaxonomically a term that is fundamentallynontaxonomic. How distinct must populationsbe to warrant consideringtheir inter- breedingas "hybridization"? The populationsought to be sufficiently distinct to permit recognitionof a hybrid as such. I suggestthat their distinctnessought to approach,equal, or exceedthe levelof distinctnessof subspecies.Criteria for subspeciesare a purely taxonomicmatter, ir- relevantto this point, the ideabeing that the populationsare about as dis- tinct, or more so, than subspecies.For example,designation of an area of primary intergradationdepends on the distinctnessof the populations involved.These populations are usuallyconsidered as subspecies,but that dependson the criteria of the taxonomistviewing the situation. I have designatedas intergradesabove the productsof interbreedingthrough an area of primary intergradation,but this does not preclude the possibility of true hybridizationoccurring between individuals of the sametwo popu- lations. Wanderingbirds. from one populationoccasionally may traverse the area of intergradationand mate with birds of the other population, producingtrue hybrids. Of courseone might obtain individualsof the two populationsand mate them under captive conditionsto produceoffspring that are F • hybrids,not intergrades. While the framework for the considerationof hybridization presented herein is novel, most of the basic ideas are drawn, some insensibly,from the vast literature on speciationand populationgenetics. Especially help- ful have been the important works of Mayr (1942a, 1963), Anderson (1949, 1953), Dobzhansky(1951), Stebbins(1959), and Sibley (1957, 1959, 1961). The biologicalinformation forming the basis for a taxonomicdecision in casesof hybridizationshould ideally include: 1) knowledgeof the oc- currenceand/or lack of occurrenceand the place of occurrenceof hybrids; 2) knowledgeof the distribution and habitats of the parental forms and hybrids; 3) knowledgeof the relative frequenciesof hybrid and parental phenotypesin the area of hybridization;4) knowledgeof the type of cross- ing (F •, F 2, backcross)that is occurring;5) knowledgeof the occurrence and the extent of any introgression(introgression is gene flow resulting from hybridization;for discussionand definitionsee Short, 1965: 360); and 6) knowledgeof the populationdynamics (especiallyof the spatial distributionof individualsand the populationstructure) of the forms in- volved. Althoughfull data are often unavailablefor someof theseitems, theremay existinferential bases for their considerationpermitting a tenta- 86 LESTERL. SaORT [Auk, Vol. 86 tive taxonomicevaluation. Other (behaviorial,genetic, ecological) data are of coursedesirable; their availability greatly aids in the taxonomic evaluation,and strengthensthe decisionthat is ultimatelyreached. Hybrids producedunder captive conditionsare not discussedin this paper. I agree with Mayr (1963: 112) that "the mere possibilityof hybridizationin captivity provesnothing as far as speciesstatus is con- cerned." Artificially induced hybridization proves only the existenceof considerablegenetic similarity and compatibility. Of coursestudies of hybridizationunder captive experimental conditions are a usefuladjunct to the study of natural hybridizationin thoseanimals that can be maintained and bred successfully,and in sufficient numbers. The severalcategories of naturalhybridization are discussedin the order from rare hybridizationto frequenthybridization and morecomplex inter- actions. INFREQUENT AND RARE HYBRXDIZATION Singleinstances of hybridizationare generallyof limited taxonomicuse- fulness,although they may provide clues to relationship. Many inter- generic arian hybrids involving taxonomicallyvalid genera have been recorded.Other purportedcases involve species of overlysplit generathat shouldbe congeneric,for examplehybrids among species of North Amer- icanhummingbird "genera" (Short and Phillips, 1966). Recentlyreported unique intergenerichybrids include a flycatcherContopus sordidulus X Empidonaxtraillii (Short and Burleigh,1965), a woodwarbler Dendroica strfataX Seiurusnoveboracensis (Short and Robbins,1967) and a hum- mingbird Cynanthus latirostrisX Euge'nesfulgens (Short and Phillips, 1966). In the first two of thesecases the occurrenceof the hybrid tends to corroborategenerally accepted generic relationships and the current adjacentplacement (A.O.U., 1957) of the generainvolved. Study of the third caseled to the suggestedclose relationship between Cynanthus and Eugenes,which previously had not beenconsidered closely related. Thus, lone hybrids betweenspecies representing valid genera may corroborate relationshipsdetermined by other studies,or they may suggestcomparisons not previouslyconsidered. The occurrenceof severalunique intergeneric hybridsinvolving different species of the sametwo generais morestrongly suggestiveof genericrelationship. I have elsewhere(Short and Phillips, 1966) noted as inconsistentwith modernphylogenetic and evolutionary conceptsthe suggestion(Banks and Johnson,1961: 26) that intergeneric hybridizationmight occur more frequently than intragenerichybridization. Aside from serving a taxonomicfunction, these hybrids may help to elucidatethe pattern of evolutionof a group,or they may aid in establish- ing the patternsof evolutionof a trait or traits within a group.. Jan., 1969] Taxonomic Aspectso! Avian Hybridization 87 Unique intragenerichybrids vary in their taxonomicsignificance. Gen- erally they are of little significancein groupsin which hybridsare un- common. Miller (1955) suggestedthe evolutionaryhistory of certain North Americanwoodpeckers of the genusDendrocopos mainly on the basisof a singlehybrid D. scalarisx D. villosus.Recently, I have found (Short, MS) numbersof hybrids of D. scalarisx D. nuttallii, and two hybridsof D. nuttalliix D. pubescens.The newevidence from these speci- mens necessitates considerable modification of Miller's views. Hence a loneintrageneric hybrid is of limited useas a basisfor inferringrelation- ships. The amountof hybridizationmay suggestor corroboraterelation- shipsin generasuch as the duck genusAnas, that exhibit, for reasons elucidatedby Sibley (1957), a great deal of hybridization(Johnsgard, 1960). Important here is the rare occurrenceor absenceof hybridsbe- tween certain species,contrasted with more frequently occurring wild hybridsamong other speciesof a genus. One must, of course,evaluate theseoccurrences on th'ebasis of the opportunityof the variousspecies to hybridize,i.e. the degreeof
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