The Auk 110(1):117-126, 1993

GENETIC DIVERGENCE AMONG POPULATIONS OF A TROPICAL , THE STREAKED (SALTATOR ALBICOLLIS)

GILLES$EUTIN, 1 JEFFREYBRAWN, TM ROBERT E. RICKLEFS,2 AND ELDREDGE BERMINGHAM L3 •SmithsonianTropical Research Institute, Apartado 2072, Balboa,Panamd; and 2Departmentof Biology,University of Pennsylvania,Philadelphia, Pennsylvania 19104, USA

ABSTRACT.--Weanalyzed mitochondrial-DNA (mtDNA) restriction-sitevariation in pop- ulations (subspecies)of the (Saltatoralbicollis) in , , and the LesserAntilles. Genetic differentiation between populations (Panama vs. Peru, 0.035; Lesser Antilles vs. Panama/Peru, 0.063) greatly exceededvalues reported for populationsor sub- specieswithin North Americanbird species(0.0028-0.0086), and was near the high end of the range reported for congenericspecies of passerinebirds (0.016-0.073). Nucleotide di- versity within populationswas similar to that reported for other speciesof passerinesand did not differ markedly between mainland and island populations.Thus, founder effectsand population bottlenecksassociated with island colonizationappear to have causedlittle, if any, lossof mtDNA nucleotidediversity. No significantmtDNA differentiationwas observed between populationsof named subspecieson mainland Panama(S. a. isthmicus)and the Pearl Islands(speratus), or between two subspeciesin the LesserAntilles (albicollisand guadelupensis). Saltatoralbicollis exhibits marked geographicalgenetic differentiation, as assayedby mtDNA polymorphism,but this bearslittle relationshipto subspeciesdistinctions. Finally, the genetic data suggestthat Antillean populations of Streaked Saltatorsshould be distinguished at specificrank (S. albicollis),with continental forms referred to S. striatipectus.Received 7 May 1992, accepted14 August1992.

MITOCHONDRIAL-DNA(mtDNA) sequencedi- serines. Additionally, Escalante-Pliego(1991) vergencebased on restriction-siteanalysis (Avise has documented substantial isozyme differ- et al. 1987, Dowling et al. 1990) has provided ences among disjunct populations of yellow- informative measuresof geneticdifferentiation throats (Geothlypis)in Central and South Amer- within avian speciesand genera (Mack et al. ica, and Hackett and Rosenberg (1990) have 1986, Avise and Zink 1988, Ball et al. 1988, Zink demonstrated pronounced geographical sub- and Avise 1990, Quinn et al. 1991, Zink 1991, division and differentiation in Neotropical ant- Zink and Dittmann 1991, Zink et al. 1991a, b, wrens (Formicariidae). Berminghamet al. 1992). With regard to North Appraisals of genetic differentiation among American passerinebirds, for example, mtDNA populations of tropical would address divergence between speciesis almost an order questions of population structure and incipient of magnitude greater than differences among speciesformation in regions of high species populations within species.These findings re- richness.Tropical speciesdiversity is thought fer exclusively to temperate taxa. At present, to result from high ratesof speciesproduction, there is little information available on mtDNA low rates of extinction, or great age of tropical divergenceamong populations of tropicalbirds, regions. Speciesproduction presumably varies although allozyme surveys (Capparella 1988, in relation to rate of evolutionary changewith- Petersonet al. 1992) have revealed marked ge- in populations and degree of population sub- netic differentiation over short distances within division. Estimatesof mtDNA divergence di- severalgroups of Amazonian and Mexican pas- rectly address the issues of population subdivisionand the relative genetic differen- 3 Mailing address:Smithsonian Tropical Research tiation of taxa of specificrank. It has been sug- Institute, Unit 0948, APO, AA 34002-0948, USA. gested that the constant temperaturesand di- 4 Present address: Illinois Natural History Survey, minished seasonal change in climate in the 607 East PeabodyDrive, Champaign, Illinois 61820, tropics have led to increased ecological spe- USA. cialization, increasedfragmentation of species

117 118 Ssu'r•NST AI,. [Auk,Vol. 110

of Panama; and 7 to continental Central and A½•GuadeloupeDomlnloa South America. A• Mad•n•que ß • St. Lucia In this study, we characterizemtDNA restric- tion-site polymorphism in saltator populations from central Panama, the Pearl Islands in the

Venezuela Bay of Panama, Peru, and three islands in the Pearl LesserAntilles (Fig. 1). Although our sample Islands coversonly a small part of the geographicrange Colombia of the Streaked Saltator, it includes geograph- ically distant populations and five named sub- species.Furthermore, the StreakedSaltator pro- [] Distributionof vides an excellent opportunity to investigate genetic diversity and differentiation of popu- Saltator albicollis lations on islands of different age. ß collection localities The Pearl Islands are land-bridge islands thought to have had a land connectionto main- land Panamaas recently as 9,500to 11,000years ago (Bartlett and Barghoorn 1973, Fairbanks 1989). In contrast, the Lesser Antilles are oce- anic islandswhose modern geographyprobably dates from the Miocene (Pregill 1981, Sykeset Fig. 1. Distribution of Saltatoralbicollis, with col- al. 1982, Rosen 1985, Burke 1988). The four An- lection localities indicated. tillean islands inhabited by the Streaked Sal- tator have never been connectedby land either to each other or to the mainland of South Amer- distributions, and reduced migration of indi- ica. Saltatorsundoubtedly colonized these is- viduals between populations(Haffer 1969,1974). lands by long-distancedispersal and, therefore, Under these circumstances,one would expect might be expected to exhibit founder effectsin to find more subpopulationsper speciesand the form of reducedgenetic diversity. Further- greater genetic differentiation among them in more, considering the potentially greater age the tropics comparedto temperate latitudes. We of oceanicisland populations, they may exhibit contribute to the analysis of genetic differen- greatergenetic divergencefrom mainland forms tiation among tropical New World than do populations on land-bridge islands. Fi- by reporting mtDNA sequencedivergence for nally, we assessthe relationship between sub- populationsof the StreakedSaltator (Saltator al- specificdistinctions and mtDNA differentiation bicollis;Cardinalinae, Emberizidae). in this species. This medium-sized saltator (35-45 g; Wet- more et al. 1984) is typically found in resident, MATERIALS AND METHODS sedentary populations without pronounced Fieldmethods and tissuesamples.--We obtained blood seasonal movements. Saltator albicollis is distrib- or pectoral muscle samples from Streaked uted from southwestern Costa Rica, through from: three Antillean islands; Atlantic coastal and in- Panama,south to Peru and eastthrough Colom- land localities in central Panama; the Pearl Islands in bia to northern Venezuela and Trinidad (AOU the Bay of Panama;and coastalPeru (Table 1, Fig. 1). 1983). The speciesalso resideson several land- Each sample consistedof tissuescollected primarily bridge islands along the Pacificcoast of Panama from adult birds during a single season.Judging from (Coiba, Taboga, Pearl Islands), and on the is- the presence or absence of a cloacal protuberance, lands of St. Lucia, Martinique, Dominica, and both sexeswere representedin most samples.In most Guadeloupe in the LesserAntilles. Several au- localesour permit provisionsprevented us from sac- thors have referred the Antillean forms and rificingbirds for voucherspecimens. All sampleswere collectedand importedunder appropriatepermits and continental forms to different species(S. albi- licenses. collisVieillot 1817 and S. striatipectusLafresnaye We collected only blood (50 to 150 tA) from most 1947, respectively). Paynter and Storer (1970) Panamanian birds through wing-vein puncture. The recognized 12 subspecies:2 referable to the sampleswere diluted in 1 x TE buffer (10 mM Tris, 1 Lesser Antilles; 3 to the Pacific coastal islands mM EDTA, pH 7.5) and stored frozen in liquid hi- January1993] GeneticDivergence in Saltator albicollis 119 trogen in the field, and in an ultracold freezer in the TABLE1. Streaked Saltator samplesused in mtDNA laboratory. In representativePanamanian localities, analysis. we sacrificeda few individuals, from which we pu- rified mtDNA from pectoral muscle to establishre- Locationa Code Subspeciesb n striction patterns without ambiguity and to use as Lesser Antilles probe. Peruvian birds were collectedby a team of the Dominica DOM guadelupensis 9 Museum of Natural Sciences, Louisiana State Uni- Martinique MAR albicollis 8 versity; saltator specimenswere stored in the field in St. Lucia STL albicollis 10 liquid nitrogen, and later transferredto an ultracold Central Panama freezer. Antillean sampleswere collectedin July and Atlantic coast PAC isthmicus 10 August 1991. Birds were caught with mist nets, and Inland PIN isthmicus 10 0.05 to 0.1 g of pectoral muscle was biopsied and Las Perlas islands PPI speratus 31 preserved in a salt-DMSO solution (Seutin et al. 1991). Peru The biopsy procedurefollowed Baker (1981), except that we excised a triangular piece of the lower part Lambayeque PER immaculatus 3 of the breast muscle; birds were released within an Precise locations available from authors. hour of the procedure. Pickled tissue samples were Subspecificdesignations follow Paynter and Storer(1970). kept in the field (one to five weeks) at ambient tem- perature; upon arrival at the laboratory, they were transferred to 4øC.More than 98% of the birds cap- from tissue, approximately 2 •g of DNA (1-2 •1 of tured were releasedalive whether we sampledblood sample)were digestedwith 5 to 10 units of enzyme. or biopsiedtissue. However, our experiencewith both We used 2 enzymes recognizing a 5.3-basesequence blood and biopsiedpectoral muscle as sourcesof total (Aval,and HincII; seeNei 1987),as well as11 enzymes genomicDNA for subsequentfilter hybridizationwith recognizingsix bases(BamHI, BclI, BglI, BglII, DraI, purified mtDNA strongly suggestsusing the biopsy EcoRI, EcoRV, HindIII, PvuII, Sacl, and XbaI). Restric- technique. tion fragmentswere separatedelectrophoretically in DNA extraction,restriction digests, electrophoresis, and 0.9 to 1.2%agarose gels. After in-gel denaturation(see Southernblotting.--We extractedtotal cellular DNA Seutin et al. 1991), the size-fractionatedDNA samples from each sample. For Panamanian specimens,we were Southern blotted overnight onto ZetaBind digested 50 to 75 •1 of whole blood with proteinase membranes. SCP buffer (10x: 1 M NaC1, 0.3 M K (20 •1, 10 mg/ml), in the presenceof 0.5% SDS. Na2HPO4, 10 mM EDTA, pH 7.0-7.5) was used for Sampleswere digested overnight at 65øCand then transfers and post-transfer washing of the mem- extractedtwo times with an equal volume of phenol- branes. chloroform-isoamylalcohol solution (PCI; 25:24:1), Probing.--We used a Streaked Saltator mtDNA and oncewith a chloroform-isoamylalcohol solution preparationpurified on CsC1-EtdBrdensity gradients (CI; 24:1).The DNAs were recoveredby conventional asthe probe.Because analyses of DNA extractedfrom isopropanol precipitations and redissolved in 200 •1 blood required extremelypure probes,the samples of 1 x TE. One-half of the Caribbean sampleswere were purified twice on density gradients and traces extracted by the tissue-preparation technique de- of nuclear DNA were removed by running EcoRI- scribedby Seutin et al. (1991).The remaining Carib- digestedsamples in 1.0%low-melting-point agarose bean samples,and thosefrom Peru, were first ground gels and extracting the mtDNA fragmentswith the in 500 •1 of 2 x CTAB buffer (Murray and Thompson GeneCleanprocedure (Bio101, LaJolla, California). In 1980) and incubated at 50 to 55øC for 1 to 16 h with random priming reactions,a few nanogramsof probe constantagitation in presenceof 6 to 12 units of Pro- were radioactively labelled with [alpha-32p]dCTPto teinase K. The homogenate was then extracted with very high specificactivity (108-109dpm/•g). an equal volume of CI solution,followed by PCI so- Transfermembranes carrying blood-extracted DNA lution, and finally with CI. The DNA was recovered were prehybridized for 1 to 3 h at 65øCin rotating by cold-ethanolprecipitation and redissolvedin 200 canistersspinning at approximately 5 rpm. Three to •1 of 1 x TE, and then wasdialyzed overnightagainst five blotswere prehybridizedand hybridized at once lx TE with three changesof buffer. In addition, in 25 to 40 ml of prehybridization solution(7.4% dex- purified mtDNA was obtained from mainland Pana- tran sulfate, 4.4 x SCP, 0.74% N-lauryl-sarcosine,0.4 manian and Pearl Islands samplesby ultracentrifu- mg/ml heparin). Hybridizations (12 to 48 h) were gation in cesium chloride-ethidium bromide (CsCI- followedby three 15-minwashes of increasingstrin- EtdBr) density gradients,followed by dialysis (see gency at 65øC(2x SCP, 1% SDS; 0.2x SCP, 0.1% SDS; Lansman et al. 1981). 0.1 x SCP,0.05% SDS). Complete details on the tech- Approximately 1 •g of total genomicDNA extract- niques for analyzing blood-extractedDNA are pre- ed from blood (<10 •1 of sample)was digestedwith sented by Brawn et al. (unpubl. manuscript).Mem- 20 units of restrictionenzyme following the manu- braneswith tissue-extractedDNAs were prehybridized facturer's recommendations. For samples extracted and hybridized in the conditionsdescribed above, but 120 SEv• Et AL. [Auk,Vol. 110

Invariant sites

LesserAntilles .? •, .$ ,? ,•. ? .? ?? . ?. . . ? . .

Fig. 2. mtDNA restriction-sitemaps for Saltatoralbicollis. Invariant-sites map showsrestriction sites shared by all individuals surveyedacross all three populations.HincII and HindIII not mappedbecause each enzyme producedlarge numbers of smallfragments. Sites indicated as variable (O) or invariant(0) within a population. Two variable BamHI sitesand one variable BclI site observedin Panamasamples were not mapped. Tic marks spacedat 1-kilobaseintervals. with a solution containing 10%dextran sulfate, 0.5 M RESULTS NaC1and 1%SDS, and they were washedas described by Seutin et al. (1991). Scorablebands on autoradio- Restriction-siteanalysis.--We used 13 restric- graphs were obtained in 20 to 120 h on Kodak XAR tion enzymes to analyze the mitochondrial ge- film at -70øC usingone or two intensifyingscreens. nomes of 81 Streaked Saltators collected from No attempt was made to scorefragments smaller than populations in the Lesser Antilles, mainland 300 base pairs (bp). Panama (and some nearshore Pacific islands), Restriction-site data analysis.--Each distinctive and coastal Peru (Fig. 1). The most common mtDNA fragment pattern for an enzyme was given an alphabeticlabel; an increasein the letter sequence mtDNA haplotyperecovered from eachgeneral indicates a site gain, and a decreaseindicates a site region (LesserAntilles, Panama, and Peru) was loss. For most of the enzymes,all the fragment pat- mapped (Fig. 2); accordingto thesemaps, which terns could be related to each other by assumingthe were based on all enzymes used except HindIII gain or loss of one or more sites. Nevertheless, to and HincII, the size of the saltator mtDNA is establishfragment homologiesacross patterns with approximately 16.5 kilobasepairs (kbp). Includ- certainty, we used double digestionsto physically ing sitesinferred from the restriction-fragment map the restrictionsites for mostof the enzymes(Fig. data for HindIII and HincII, an average of 42 2). For HinclI and HindlII, we could not fully map restriction siteswere assayedfor each . Of sitesdelimiting many smallfragments, but homology the 62 restriction sitesanalyzed in total, 25 were of large fragments found in representativesamples from the West Indies, Panama, and Peru was verified shared by all 81 saltators. through double digests.Once hornology was estab- Cluster analysis (UPGMA; Fig. 3) of the 14 lished,sites were inferred for theseenzymes; all frag- unique mtDNA haplotypes, based on the ge- ment patterns could be related to each other without netic distance matrix presented in Table 2, re- ambiguity. vealed three genotypic groupings that corre- Statisticalanalyses were basedon the presenceand sponded to the geographic sources of the absence of restriction sites. Data were summarized samples(Fig. 4). One of the clusterscontained using the programsREAP (McElroy et al. 1992)and only the five S. albicollismtDNA haplotypes(I- DXY (H. Lessiosunpubl. manuscript)to calculatein- V) identified from the islands of St. Lucia, Dom- dicesof nucleotidediversity (•r) and divergence(Dxy) inica, and Martinique in the LesserAntilles. We describedby Nei and coworkers(Nei and Tajima1983, found that 78% of the Antillean birds carried Nei 1987, and Nei and Miller 1990). UPGMA clus- tering of haplotypeswas performed using the NTSYS mtDNA haplotype I, and this haplotype pre- package(Rohlf 1990),and phylogeneticnetworks and dominated on the three islands. Three of the a consensustree of haplotypeswere producedusing four rarer haplotypes (II, III, V) were obtained PAUP (Swofford 1990) and MacClade (Maddison and only once; the fourth mtDNA haplotype (IV) Maddison 1987). occurred only on Martinique, where it was rel- January1993] GeneticDivergence in Saltator albicollis 121

mtDNAsequence divergence, dxy thus, the nucleotide diversity for this popula- 0.08 0.06 0.04 0.02 0.00 tion is not informative. Our analysis of 51 sal- tators from mainland Panama and nearshore Pacific islands in the Pearl Archipelago re- 11 Antilles vealed a relatively high level of intrapopulation variability (•r = 0.0018, S. a. isthmicus;0.0024, S. a. speratus).The two most-commonPanamanian saltator mtDNA haplotypes (VIII and XI) are widely distributed acrossmainland and island locales.Among Panamaniansamples, the max- imum Dxy-value(0.013) separatedtwo unique mtDNA haplotypesrecovered from individuals collected in the Pearl Islands. The level of Fig. 3. UPGMA clusteranalysis of Saltatoralbicollis mtDNA sequence divergence between Pana- mtDNA haplotypesbased on geneticdistance matrix in Table 2. manian mainland and Pearl Island populations of saltators(Dxy = 0.0034) was similar to the levels observedwithin each of thosetwo pop- ulations. atively common (37%). Thus, the measure of Wagner-parsimonyanalysis revealed 52 rival, mtDNA nucleotide diversity among the Lesser minimum-length trees of 46 steps and a con- Antillean birds surveyed on each island was sistencyindex of 0.80. All trees support the dis- low to moderate (•r = 0.0000, 0.0008, 0.0016). tinctivenessof the three geographic groupings The greatestmtDNA sequencedivergence (Dxy) of mtDNA haplotypes discussedabove; this between any pair of Antillean saltators was finding was further supported by bootstrap 0.0060 (Table 2). This level of intrapopulational analysis(Fig. 4). These treesidentify Panaman- mtDNA sequencedivergence strongly contrasts ian and Peruvian saltators as sister mtDNA taxa with the meanDxy-value of 0.063(range 0.054- with the Antillean saltatorsas the outgroup. A 0.076) observed between the Antillean saltators minimum of 10 restriction-sitechanges distin- and the clade that includes all Panama and Peru guish the two Peruvian mtDNA haplotypesfrom birds. the seven haplotypes observed in Panamanian Saltator mtDNAs from Panama and Peru, al- saltators; a minimum of 20 site changes dif- though less differentiated than Antillean and ferentiate the five Lesser Antillean mtDNA continental haplotypes,fall into geographically haplotypesfrom all others.There is not strong congruent clustersseparated by a genetic dis- bootstrap support for mtDNA haplotype rela- tance (mean Dxy)of 0.035 (range 0.030-0.040). tionships within the geographic locales of the Only three birds from Peru were analyzed and, LesserAntilles, Panama, and Peru (Fig. 4).

T^I•LE2. Geneticdistances (lower left) and their standarderrors (SE; upper right) betweenmtDNA haplotypes of Saltator albicollis.

I II III IV V VI VII VIII IX X XI XII XIII XIV

I .0027 .0039 .0027 .0026 .0120 .0121 .0102 .0104 .0104 .0103 .0103 .0102 .0125 II .0020 .0028 .0039 .0038 .0119 .0120 .0104 .0106 .0106 .0105 .0105 .0104 .0129 III .0041 .0020 .0028 .0048 .0123 .0124 .0106 .0107 .0108 .0106 .0107 .0106 .0132 IV .0019 .0041 .0020 .0038 .0124 .0125 .0104 .0106 .0106 .0105 .0105 .0104 .0129 V .0019 .0040 .0061 .0040 .0117 .0118 .0100 .0102 .0103 .0101 .0102 .0100 .0122 VI .0596 .0574 .0614 .0636 .0557 .0026 .0069 .0071 .0069 .0069 .0070 .0071 .0094 VII .0616 .0596 .0636 .0657 .0578 .0020 .0070 .0072 .0070 .0070 .0071 .0072 .0096 VIII .0607 .0649 .0693 .0649 .0576 .0311 .0332 .0028 .0039 .0052 .0060 .0062 .0054 IX .0649 .0693 .0741 .0693 .0607 .0341 .0363 .0020 .0028 .0042 .0052 .0054 .0050 X .0671 .0716 .0764 .0716 .0628 .0311 .0332 .0041 .0020 .0052 .0060 .0062 .0054 XI .0605 .0648 .0695 .0648 .0563 .0299 .0320 .0064 .0043 .0064 .0030 .0031 .0051 XII .0627 .0671 .0718 .0671 .0585 .0320 .0341 .0085 .0064 .0085 .0021 .0044 .0055 XIII .0582 .0625 .0671 .0625 .0541 .0330 .0351 .0087 .0066 .0087 .0022 .0044 .0047 XIV .0634 .0679 .0726 .0679 .0593 .0378 .0400 .0131 .0110 .0131 .0113 .0135 .0091 122 SEUT•NET An. [Auk, Vol. 110

I 9 4

63 II L•er Ill Antilles 100 IV

v

lOO vI 1 Peru vii 2

IX 1 59 viii 13 7

x 1

90 xI 5 21 xII 1

xIII 1

xIv 1

Fig. 4. Consensusof unrootedminimum-length Wagner-parsimony trees (length = 44+; CI = 0.84)based on mtDNA restrictionsites. Numbers along branches indicate percentage of timesthat clade was distinguished in 500bootstrapped trees (only percentages greater than 50 shown).Distribution of haplotypesamong sample localities shown to right.

DISCUSSION of a few rare haplotypes in the Pearl Islands not observed in central Panama raises the pos- The major feature of the mtDNA analysesis sibility that Pearl Island saltatorswere derived the distinctivehess of Lesser Antillean, Pana- from a different region of the mainland. Wet- manian, and coastal Peruvian Streaked Salta- more et al. (1984) suggestedthat the endemic tors. No mtDNA haplotypesare shared among subspeciesfrom the Pearl Islands,S. a. speratus, these localities, and the mtDNA clades ob- was more closelyrelated to S. a. striatipectusof served are distinguishedfrom one another by eastern Panama and Colombia than to the cen- multiple mutational steps.The averagegenetic tral Panamanian S. a. isthmicus.Alternatively, distance between the Antillean albicollis and the the rare Pearl Island haplotypesmay have aris- subspecies representing Panama and Peru en in situ either after the land bridge was sub- (0.063) exceedsall but a few of the published merged,or earlier but without dispersingfrom estimates of mtDNA differences between con- the area. The probability of origination within generic speciesof passerinebirds, and all of the last 10,000 years seems relatively remote those between populationswithin species(Ta- given the averagerate of mutation in mtDNA. ble 3). The average genetic distance between Finally, we may have failed to samplethe rare the Peruvian immaculatus and the two Pana- haplotypeson the mainlandsimply because they manian subspecies(0.035) also exceedsother are present there at low frequency. Our data intraspecificcomparisons and is consistentwith neither support nor excludeany of these pos- levels of differentiation between congeneric sibilities. They do, however, emphasizethe ob- species. servationthat subspecificdistinctions bear little The lack of sizable differentiation in mtDNA relationship to degree of mtDNA divergence between populationsin the Pearl Islands and among populations. various locations in central Panama is not sur- The genetic similarity among the three An- prising in that the Pearl Islandsare land-bridge tillean populations sampled suggestsa recent islands with connections to the mainland as re- expansion of the taxon within the LesserAn- centlyas 9,500 to 11,000years ago (Bartlett and tilles. All the mtDNA polymorphismwithin the Barghoorn1973, Fairbanks 1989). The presence LesserAntilles was revealed by HindlII and rep- January1993] GeneticDivergence in Saltatoralbicollis 123

TABLE3. mtDNA divergencebetween populationsand congenericspecies of passerinebirds. •

Sample sizes Local- Haplo- Taxa n ities Enzymes types Reference Intraspecific comparisons Quiscalusquiscula 35 8 20 29 -- 0.0028 Zink et al. (1991b) Parus bicolor 8 2 19 4 -- 0.0040 Avise and Zink (1988) Dendroicanigrescens I1 2 14 6 0.0024 0.0077 Berminghamet al. (1992) Passerella iliaca 46 9 20 5 0.0008 0.0086 Zink (1991) Ammodramus maritimus 40 2 18 II -- 0.0098 Avise and Nelson (1989) Agelaiusphoeniceus 127 19 18 34 0.0020 -- Ball et al. (1988) Melospizamelodia 27 7 20 15 0.0027 -- Zink (1991) Intrageneric comparisons Quiscalus 14 2 19 3 -- 0.0160 Avise and Zink (1988) Melospiza 9 3 18 3 -- 0.0290 Zink (1991) Quiscalus I0 3 20 4 -- 0.0295 Zink et al. (1991b) Zonotrichia 22 5 19 I0 -- 0.0320 Zink et al. (1991a) Sturnella -- 2 -- 2 -- 0.0340 Freeman (1991) Icterus -- 2 -- 2 -- 0.0370 Freeman (1991) Dendroica 11 5 17 5 -- 0.0440 Kessler and Avise (1985) Dendroica 35 8 14 18 -- 0.0523 Berminghamet al. (1992) Pipilo 26 4 16 11 -- 0.0640 Zink and Dirtman (1991) Parus 10 2 18 3 -- 0.0700 Mack et al. (1986) Ammodramus 58 8 16 19 0.0060 0.0730 Zink and Avise (1990) This study Saltator albicollis 81 7 13 14 -- 0.0630

a•r • genotypicdiversity within populations;D,y = geneticdifferentiation between populations. resentsa seriesof four single-substitutiondif- alsobe sufficientfor colonizationof neighbor- ferences.Haplotype I occurson the three is- ing islands. The presenceof a common geno- lands sampled. The Martinique haplotypes type (37%) on Martinique that was absent from representa series(V-I-IV) with the gain of a our samplesfrom Dominica to the north and single restriction site between I and V and the St. Lucia to the south reinforces the idea that loss of a site between I and IV. The St. Lucia movementof femalesamong islandsis limited. haplotypessimilarly representa series(I-II-III) The probability of missing a haplotype that with the lossof a single restrictionsite at each common in our samplesfrom Dominica (n = 9) step. Haplotype III also can be obtained by a and St. Lucia (n = 10) by chance alone is about single site lossfrom IV of Martinique, but geo- 0.01. The data raise two possibilitiesto explain graphical considerationsmake the sequenceII- the geographicpattern. Either Martinique was III more likely. the sourcepopulation for the recentexpansion We exclude the possibility that the popula- and haplotype IV was lost in the founding tions of Streaked Saltators on the individual events,or birds carrying the commonhaplotype Antillean islandshave had as long a history of colonized Martinique from another island fol- isolationfrom eachother asthey have had from lowing a period of isolationand geneticdiffer- continental populations; in such a case, sub- entiation. stantial levels of mtDNA sequencedivergence The genetic diversity on the Pearl Islands (•r would have been observed among our Antil- = 0.0009 on PachecaIsland and 0.0028 on Rey lean samples.We also believe it unlikely that Island) is similar to that in central Panama gene flow among the islands has persisted (0.0017 in an Atlantic coastal population and throughout their history in the LesserAntilles 0.0018in a central population),indicating that given that the speciesoccurs on neither St. Vin- some of the Pearl Islands suffered neither a cent nor Grenada, which lie between St. Lucia founder effect during colonization nor loss of and the South American continent. Presum- genetic diversity since isolation from the main- ably, dispersal abilities sufficient to maintain land. These values are similar to those observed genetic uniformity among the islands would in North American passetines(Table 3). The 124 SEUTX•œT ^L. [Auk, Vol. 110 genetic diversities on Martinique (0.0016) and from lines that produced Panamanian and Pe- St. Lucia (0.0008) also provide no compelling ruvian populations about three million years evidence of founder effects or of bottlenecks on ago. Streaked Saltators almost certainly colo- the Antillean islands. The Antillean popula- nized the Antilles from the coast of Venezuela, tions are old enough to obscurefounder effects but we have not collected material from that and presumably large enough to prevent loss area. Thus, we cannot date the colonization of of genetic diversity through drift. The absence the islands themselves, but can conclude that of genetic diversity in the Dominican sample differentiation within the speciesdates from be- suggests,however, the possibility of recent col- fore the Pleistocene. If the Lesser Antilles were onization from Martinique by a small number colonized by S. albicollisbefore the Pleistocene, of individuals. the agesof other bird taxawithin the Antilles, The high degreeof geneticdifferentiation ob- many of which are endemic at specificand ge- servedin S.albicollis suggests that a broadsurvey neric rank, must be much older than usually of tropical speciesmay reveal more pronounced thought. Thus, much of the biogeographic geographical structure than observed in tem- structure of both Antillean and continental perate species(Hackett and Rosenberg1990). Central and South American populations may Characterizing such differences in detail may have been establishedconsiderably before the elucidate processesof speciesformation and the onsetof Pleistoceneclimate cycles,as argued in generation of diversity in tropical avifaunas. Capparella (1991) for Amazona. Alternatively, The genus Saltator,for example,is moderately the mtDNA clock for these passerinesmay be diverse, containing 12 named speciesthat in- ticking considerablyfaster than the primate clude 52 subspeciesbetween Mexico and trop- mtDNA clock (concerning rate differences in ical South America (Paynter and Storer 1970). mtDNA sequence evolution, see Martin et al. As we have seen,continental populationsof S. 1992).In any event, our resultssupport Hackett albicollis(Panama and Peru) have been isolated and Rosenberg(1990), who found levels of ge- for a sufficient period that they share no hap- netic differentiation among Neotropical con- lotypes. Even though gene flow between Pan- generic speciesthat greatly exceed those re- ama and Peru would be possible owing to the ported from North America. availability of suitable habitats, the two popu- Taxonomicconclusions.--Our genetic data re- lations effectively are evolutionarily indepen- inforce the conclusion of Ridgway (1901) that dent. A similar relationship may be found for Antillean populations on Guadeloupe, Dom- other saltatorspecies. inica, Martinique, and St. Lucia should be treat- The degree of genetic differentiation be- ed as a distinct species,Saltator albicollis Vieillot tween named subspeciesof S. albicollisranges 1817,with continental populationsreferable to from none or very little (isthmicusvs. speratus, Saltatorstriatipectus Lafresnaye 1847. The ge- guadelupensisvs. albicollis)to levelscharacteristic netic difference between Panamanian and Pe- of congenericspecies (guadelupensis/albicollis, ruvian populations(0.035) also exceedsdiffer- immaculatus,and isthmicus/speratus).Thus, tax- encesbetween conspecificpopulations reported onomic differentiation must be used with cau- in other studies, and lies well within the levels tion in historical biogeographicanalyses (Ber- of differentiation observedbetween congeneric minghamet al. 1992).For example,Ricklefs and species.Because we have no genetic informa- Cox (1972) inferred age of specieswithin the tion on populations occupying the region be- Antilles partly on the basisof taxonomicdif- tween Panama and Peru, we cannot comment ferentiation at the subspecies/specieslevel. further on their relationshipsto one another. While taxonomymay provide a general indi- cation of age, estimatesof genetic differentia- ACKNOWLEDGMENTS tion based on methods such as DNA restriction- G.S. is supported by a Canadian NSERC Postdoc- site analysisand sequencingwill be necessary toral Fellowshipand J.B.was supportedby a Smith- for detailed work. sonJanInstitution postdoctoral fellowship. R.E.R. was Applying the conventional mtDNA molec- supportedat the SmithsonianTropical Research In- ular clock, which in primates ticks at a rate of stitute by a Smithsonian Regents'Fellowship. Field roughly2% divergence per million years(Brown work wasfunded by the National GeographicSociety, et al. 1979), to saltatorssuggests that popula- and laboratorywork was supportedby the Smithson- tions in the Lesser Antilles became separated ian Institution's Scholarly Studies Program and Ab- January1993] GeneticDivergence inSaltator albicollis 125 bott Fund. For assistance in the field, we thank Katie process.Pages 1658-1664 in Acta XIX Congressus Sieving, David McLain, William A. Schew, Victor Internationalis Ornithologici (H. Ouellet, Ed.). Apanius, Doug Wechsler,and GeorgesTayalay. Per- Ottawa, Ontario, Canada, 1986. Univ. Ottawa mits to collect birds in Panama were provided by Press, Ottawa. INRENARE. Robert M. Zink (Museum of Natural Sci- CAPPARELLA,A.P. 1991. Neotropical avian diversity ences, Louisiana State University) provided tissue and riverine barriers. Pages 307-316 in Acta XX samples from Peru. We thank Monica Medina, Tim Congressus Internationalis Ornithologici. Collins, and Marivi Walker for assistance in the lab- Christchurch, New Zealand, 1990. New Zealand oratory, and Angelo P. Capparella and two anony- Ornithol. Congr. Trust Board, Wellington. mous reviewers for comments that improved the DOWLING, T. E., C. MORITZ, AND J. D. PALMER. 1990. quality of the manuscript. Nucleic acids II: Restriction site analysis. 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