Historical and Ecological Factors in the Biogeography of ...

AMER. ZOOL., 22:453^171 (1982)

Historical and Ecological Factors in the Biogeography ofAposematic Neotropical Butterflies1

KEITH S. BROWN, JR.

Departamento de Zoologia, Institute de Biologia, Universidade Estadual de Campinas,C.P. 1170, Campinas, Sao Paulo IB.100 Brazil

SYNOPSIS. Quantitative analysis of low-level (mostly subspecific) geographic differentia-tion of aposematic color patterns in three groups of forest butterflies (Heliconiini, Ith-omiinae, and Troidini), in 1,550 half-degree quadrants throughout the Neotropics, in-dicates four principal species-endemicity regions, 44 principal subspecies-endemicitycenters, and 300 quadrants of subspecies hybridization. The endemism centers are dif-ferent in outline but nearly identical in position for each of the three ecologically differentgroups. Basic data on present and past physical environments (climate, soils, geomor-phology, and vegetation structures), when integrated into maps, show 1) regions of fa-vorable ecological conditions for forest organisms in the present, 2) regions of environ-mental conformities and rapid transitions in the present, and 3) regions of high probabilityfor humid forest persistence during the major landscape changes at the end of the lastglacial period, 13,000-20,000 yr ago. Species diversity (a local phenomenon) is readilyrelated to favorable modern ecological conditions and environmental microheterogeneity,promoted by unpredictable mild disturbance. Biogeographical patterns of evolution atthe level of regional subspecies correlate very well with presumed paleoecological condi-tions. The biogeographical data also support an hypothesis of differentiation resultingfrom areas of rapid environmental change; most of the hybridization quadrants fall withintransition zones. There is lower correlation between centers of endemism and zones of en vi-ronmental conformity, but some individual cases of differentiation in the butterflies maybe adequately explained by modern ecological factors. Species presently undergoing dif-ferentiation in isolated habitat islands (Amazonian savannas and high-altitude Andeanforests and paramos) may offer the best opportunities for observation of evolutionaryprocesses in the present.

INTRODUCTION

Neotropical biogeography, traditionallyhampered by a lack of data on distribu-tions and environments, has recently be-come an open and exciting field. The con-ditions which made this possible becameavailable in the 1970s, through remotesensing surveys (RADAMBRASIL, 1973-1981) and the opening up of new roadsand airstrips in the Amazon Basin, per-mitting extensive ground sampling ofplants, animals and soils. A few detaileddistribution maps are now appearing, de-scribing differentiation patterns in a lim-ited number of better-sampled organisms.These are compared with detailed maps ofclimate, soils, and vegetation. New infor-mation available on the geological andvegetational history of the Neotropical re-

1 From the Symposium on Alternative Hypotheses inBiogeography presented at the Annual Meeting of theAmerican Society of Zoologists and the Society of Sys-tematic Zoology, 27-30 December 1980, at Seattle,Washington.

gion is used in relating the biogeographicalpatterns to historical as well as ecologicalfactors. What do these new data-sets revealabout evolution in the Neotropics?

New biogeographical data continuallyshow, of course, that even the best-knownpatterns may be simplified, distorted, de-ficient, or wrong. Keeping this caveat inmind, I will analyze here a large data-seton regional differentiation in Neotropicalforest insects, and interpret some of itsmore salient features in relation to histor-ical and ecological forces which may havehelped to shape the observed patterns.

BIOGEOGRAPHY OF NEOTROPICALFOREST BUTTERFLIES

Timing of butterfly differentiationduring the Cenozoic

Coope's elegant analysis from fossils, ofPleistocene migrations of north-temperatebeetle faunas (1978, 1979), shows thatthere is no evidence for extensive specia-tion or extinctions in spite of the great cli-matic fluctuations in the same period (Liv-

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454 KEITH S. BROWN, JR.

ingstone and Van der Hammen, 1978).Such a generalization is also reasonable toapply to Neotropical and other butterflies.Although few butterfly fossils are known,they suggest the general presence of mod-ern species in the late Pliocene and earlyPleistocene, and modern genera (at leastten, by no means the most primitive) inmiddle Tertiary fossils (Brown, 1980a);two Eocene genera {Praepapilio, Riodinella;Durden and Rose, 1978) might easily beincluded in the modern Baronia and Rio-dina, respectively. Many genus-groups,tribes, and subfamilies span South Amer-ica and Australia or even all the derivativesof ancient Gondwana, supporting a pre-Tertiary origin for these taxa.

Thus, there is reason to assume thatmost butterfly species are older than themiddle of the Pleistocene, and it is unlikelythat late Pleistocene paleoecological phe-nomena will correlate with geographicalpatterns at the species level in Neotropicalbutterflies. More likely, such phenomenamight have caused differentiation of rec-ognized regional subspecies. Species-levelpatterns may correlate better with majorTertiary-early Pleistocene orogenies andocean introgressions. Since objective bio-logical information is often lacking for dis-tinguishing butterfly species from subspe-cies, however, it is possible that somegroups will show evolutionary patterns atone taxonomic level similar to those seenat higher or lower levels in other groups.This could also be ascribed to inadequatebiosystematic information, or fundamentalvariations in rates of differentiation in thediverse groups.

Systematics, ecological preferences, andaptness for biogeographical analysis

A survey of all recently revised groupsof Neotropical butterflies (Brown, 1980a:Table 1) shows that most include speciesof widely differing ecological preferences.This ecological diversity, when combinedwith the difficulty in designation of con-phyletic units (forced by the paucity offield data and the variable reliability of ex-ternal and internal characters), can lead tochaos in the analysis of biogeographicalpatterns. All 33 butterfly groups consid-

ered in the survey show suggestive clustersof possible subspecies (or allospecies)whose biogeography should be consideredas a unit; in less than a quarter of the pub-lished revisions have such widespreadunits been specifically recognized andgrouped phyletically.

The same survey reveals adequate geo-graphical sampling for only a third of thegroups. Most Neotropical butterflies arewidely scattered in sparse populations, andonly a few combine habitat or geographicalrestriction (best for fine-grained biogeo-graphical analysis) with local abundance,permitting rapid and ample sampling.Only one of the 33 groups (Heliconiini)has been thoroughly studied genetically, toyield some idea of the degree, velocity, andadaptive importance of the differentiationof taxonomic characters. Several of thegroups show extensive polymorphism,high dispersal capacity, or other characterswhich blur their geographical patterns ofevolution. A numerical rating of the 33groups on the basis of all these charactersand the knowledge available reveals thatonly six are really apt for biogeographicalanalysis. The patterns seen in the twohighest rated groups are analyzed quanti-tatively here, along with some additionalinformation from the third.

The biogeographical patterns

Three large groups of butterflies, whosebiogeographical patterns have been thor-oughly examined during intensive fieldwork over the past ten years (Nymphali-dae: Ithomiinae and Nymphalinae-Heli-coniini, and Papilionidae: Troidini) havelarvae that feed upon poisonous plants.The adults are thus aposematic (warninglycolored) and accompanied by many mim-ics, and are usually common. The charac-ters used in their systematics (gross wingcolor patterns) are subjected to strongadaptive selection for mimicry stabilizedover large regions. Their taxonomic ar-rangement thus tends to reflect a coherentand easily visualized biogeographical pat-tern closely related to natural selection.Their large size and abundance lead tothere being extensive museum materialand literature citations. Ithomiine and hel-

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BlOGEOGRAPHY OF NEOTROPICAL BUTTERFLIES 455

iconiine populations can be easily sampledby attraction with pyrrolizidine-alkaloid-decomposition baits and red cloths, re-spectively. The troidines usually come tomany of the same flowers (Verbenaceae,Compositae, Rubiaceae) as the other twogroups. The biology of the three groupsis well studied, especially for the Heliconi-ini (Brown, 1981); almost all species aretied to humid tropical forest and specificclimate/soil associations, from Mexico toArgentina.

These favorable characteristics havepermitted species lists to be made for thecommunities of the three butterfly groupsin over 4,000 localities, covering over1,550 half-degree quadrants, a third of theNeotropical forests. While some of theselists are still very incomplete, most reflectadequately the endemicity characters atspecies and subspecies levels of the popu-lations in the quadrants. No large areasstill remain to be sampled, but the cover-age is light in parts of the Brazilian Ama-zon, eastern Colombia, northern Peru,northeastern Bolivia, and eastern Hondu-ras and Nicaragua.

A full set of the lists for Heliconiini andIthomiinae (through 1979), and of distri-bution maps for these two groups, is inBrown (1979). Part of the distributionmaps may be found also in Brown (l980a,b,1982, maps for the Troidini will be pre-sented in Brown (in preparation).

Over 80% (129) of the 160 species ana-lyzed in the three groups, and 15 of the 34genera, are endemic to one of four slightlyoverlapping species-endemicity regions (Fig.1). These regions correspond to the broad-est divisions of the Neotropical realm, usu-ally recognizable in any group of organ-isms examined.

Only 34% (55) of the species are restrict-ed to areas which are less than half as largeas the principal species regions in whichthey are included, but cover more thanone subspecies-endemicity center (see iso-line concentrations in Fig. 1). These small-er areas, representing less than half of theendemic faunas, correspond closely withsome finer species-endemicity regions ob-served in a wide variety of other forest or-ganisms such as birds (Haffer, 1974) and

trees (Prance, 1982); such species alsoshow similar ranges to some widespreadbutterfly subspecies.

A mere 13% (21) of the species showhighly restricted ranges, equivalent toareas usually characterized by subspeciesendemism in these butterflies. This couldbe due to faulty biosystematic placementof these species or subspecies; indeed, thespecies analyzed are often considered in abiogeographical and phyletic sense, withbiological information often derived onlyfrom zones of field contact or hybridiza-tion. More likely, the restricted ranges forspecies represent extreme relictualism ordifferent rates of achievement of repro-ductive isolation in differentiating lines. Inany case, their small number emphasizesthe relatively large discontinuity betweenthe few species-endemicity regions and themany recognizable centers of subspeciesendemism.

Forty-four regions show a marked en-demicity of subspecies, each with from tento as many as 40 local endemics (usuallywell over half the locally occurring subspe-cies) recognizably differentiated fromthose in immediately adjacent regions.These regions of subspecific endemismwere most clearly defined by projectionmaps prepared for the total of 864 well-defined subspecies of forest Heliconiiniand Ithomiinae (the total range limits ofthe 170 geographically restricted forestTroidini analyzed are still not fully enoughdefined to permit a similar quantitativeanalysis). The smallest region of subspeciesendemism covers 10,000 km2, while thelargest represents over 800,000 km2 of rel-atively homogeneous phenotypes (mimeticwing color-patterns) in a large fraction ofthe local aposematic butterflies; thus theyare clearly regional, not local, phenomena.Subareas of endemism, with less than halfof the local species differentiated, werealso identified; at least two of these, at thenorthern and southern extremes of theNeotropics, were more strongly represent-ed in the Troidini (Guerrero, in westernMexico, and Santa Catarina, in southernBrazil).

The endemic centers for subspecieswere quantitatively mapped by isolines (by

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TRANSANDEANHel 15It h 12

isol 27

Troi 8

GEN 0

ANHelIthisolTroi

GEN

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TABLE 1. Peripheral diversity in subspecies endemic centers (Heliconiini and Ithomiinae).

(and maximumendemic value)

Belem (15)Choco(18)Huallaga (18)Imataca (9)Imeri(13)Inambari (19)Loreto (13)Madeira (7)Manaus/Guiana (21)Oyapock (22)Rondonia (25)Suciia (18)Tapajos (26)Ucayali (26)Yungas (25)Averages for 15 centers

Quadrantsexamined

15459

10957

12555

117

118

Nuclear region

Speciespresent (x)

9.58.09.44.65.3

11.87.24.77.3

11.211.411.213.415.913.59.6

Average percenthybridization

5.8%11.7%11.4%5.7%4.8%

18.6%5.9%7.7%

10.5%3.8%3.8%1.4%7.4%

14.1%6.4%8.1%

Quadrantsexamined

65484463

14555

11956.3

Peripheral region

Speciespresent (x)

10.711.614.56.09.5

13.510.76.0

12.012.216.214.015.518.116.812.5

Average percenthybridization

39.3%42.2%48.0%58.3%18.0%45.6%39.1%50.0%50.0%21.8%35.7%63.0%43.2%61.3%17.7%

42.2%

* Peripheral, very small, and poorly known centers are not considered. After Brown (1978, 1979, 1982).

quadrant, using the species lists and as-signing each subspecies to the center in-dicated by its position in the projectionmaps). This gave a picture correspondingto the projection maps, with much overlapof adjacent centers. These endemism mapsalso showed multiple peaks of speciesnumbers within most of the centers, notorganized into smooth concentric isolines(Brown, 1980a, 1982). In order to obtain acoherent picture of endemism levels for eachcenter, a double subtraction was appliedfor hybridization of any species on thequadrant list. When half the local popu-lations showed mixture between subspe-cies characters from two or more centers,the value for corrected endemism in thequadrant fell below 0. In this way, valueswere obtained which showed peaks ofendemism in the nuclei of the centers, andtroughs (negative values) between them(Fig. 2). Most of the highest uncorrectedvalues could then be seen to fall on a bro-

ken ring near the 0 corrected isoline foreach center. There was thus a correlationbetween larger values for uncorrectedendemism (approximating local species di-versity) and larger corrections due to hy-bridization (Table 1).

Figure 2 quantitatively reflects the dis-tribution of regionally differentiated mi-metic color-pattern genes (four to elevendifferent basic color patterns or "rings" ineach center of endemism) in a large num-ber of Neotropical aposematic butterflies.The resulting biogeographical picture iscomposed of units much larger than thelocal population or the small habitat island,but much smaller than the general do-mains seen in most species-group patterns.Its detail was made possible in part by theclear and strong regional differentiation inwing color of the aposematic butterflies(also detectable in other butterfly groups,but in more muted form), but more yet bythe broad sampling network in the Ama-

Fic. 1. Principal species-endemicity regions for Neotropical forest butterflies. Numbers of endemic speciesof Heliconiini (Hel), Ithomiinae (Ith) and Troidini (Troi) are indicated, along with endemic genera (GEN—all in the Ithomiinae except for one genus of Heliconiini in the Andean region). Ranges of the total speciesin the first two groups (total as isol) are used to plot isolines, of increasingly darker shading: 0-4 species,single hatching; 5—9, double; 10-19, triple; 20-29, quadruple; and 30 or over (only in Amazonian region),black. Dotted lines suggest maximum ranges for any tropical forest butterfly species associated with eachregion. Modified from Brown (1980a).

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zonian interfluvials and other previouslylittle-known areas, and the ease of captureof these common species. It will be contin-uously refined as more data come in frompreviously unsampled regions, but aftersuccessive refinements since 1973 it seemsto be fairly near to an accurate represen-tation of the phenomenon it strives topresent quantitatively: the complex re-gional differentiation of organisms in themodern Neotropical forests.

As mentioned above, the three groupsanalyzed, even though they may havemuch in common (large size, aposematiccoloration, forest habitat), have dissimilarecological and evolutionary syndromes.Heliconiini and Troidini have similaritiesin adult resources (favored flowers) andlarval foodplants (poisonous tropical vines)and have relatively constant chromosomecomplements, but their life-history strate-gies are very different: heliconians arelonglived as adults (2—9 mo) and completejuvenile development in about a month,but troidines are short-lived as adults (1-6wk) and take 2—4 mo to develop as juve-niles. The two groups also differ funda-mentally in light preferences (intensity andstructure), diapause ability, roosting andsocial behavior, climate and soil associa-tions, sexual dimorphism, and degree ofinfrageneric and infraspecific differentia-

tion. Ithomiines have a widely variablechromosome complement, feed on decay-ing animal and plant matter as adults, andshow patterns of larval behavior and re-source utilization strongly divergent fromthose of the other two groups; they areusually found in cooler or seasonal cli-mates and on very rich soils. Dispersal abil-ity also varies between and within the threegroups, as do responses to physical bar-riers, chemical stimuli, periods of unfavor-able weather, and foodplant phenology.

In spite of these differences, the groupsshow the same general centers for subspe-cies endemism—at least in position, thoughnot always in shape (Fig. 3). This impliesa strong, possibly historical, factor oper-ating in the determination of these centers,overriding the differences in populationecology and environmental preferencesand responses.

CORRELATION OF HISTORICAL ANDECOLOGICAL FACTORS WITH

BlOGEOGRAPHY

The relatively simple patterns of speciesendemism in forest butterflies (Fig. 1) arelinked to major physiognomic charactersof the Neotropical realm (the Andes andthe Guianan and Brazilian Shields) and donot need further comment here. Likewise,the smaller areas occupied by 34% of the

FIG. 2. Principal subspecies-endemicity centers for forest Heliconiini and Ithomiinae, expressed as areaswithin isolines of one-third (single hatching) and two-thirds (double hatching) of the maximum correctedendemism values for half-degree quadrant species lists (with a double subtraction for hybridization). Quad-rants with no positive corrected endemism value are shown in black (hybridization zones). Names of thecenters, in alphabetical order, with maximum corrected (and uncorrected) endemism values in any quadrant,are: Ab, Abitagua 12(22); An, Andes (8) (corrected values not possible due to narrowness of this medium-elevation strip); Ap, Apure 10(10); Ar, Araguaia 7(7); Ba, Bahia 8(10); Be, Belem 14(15); C, Chanchamayo11(19); Ca, Cauca 9(13); Cb, Chimborazo 24(24); Cc, Choco 12(18); Ch, Chiriqui 18(24); Ct, Catatumbo 7(9);Da, Darien 10(11); Gp, Guapore 6(10); Gu, Guatemala 19(19), note subcenters; H, Huallaga 13(18); In,Inambari 10(19); Ir, Imeri 11(13); It, Imataca 6(9); Lo, Loreto 9(13); Md, Madeira 5(7); MG, Manaus/Guiana8(21); Mg, Magdalena 4(11); Mj, Marajo 4(4); Mn, Maranon 9(10); Ne, Nechi 13(23); Np, Napo 7(21); Oy,Oyapock 18(22); Pe, Pernambuco 6(6); Pn, Pantepui 8(8); Pt, Putumayo 10(20); RG, Rancho Grande 19(20);RJ, Rio de Janeiro 12(13); Ro, Rondonia 16(25); Rr, Roraima 6(8); SM, Santa Marta 6(6); ST, Sucre/Trinidad10(10); Su, Suciia 14(18); Tf, Tefe 5(12); Tp, Tapajos 22(26); U. Ucayali 14(26); V, Villavkencio 17(25); Ve,Ventuari 3(9); Y, Yungas 19(25).

FIG. 3. Subspecies-endemicity centers for analyzed forest Heliconiini (left) and Ithomiinae (center), as one-third of maximum corrected isoline, with "Andes" center not shown; and Troidini (right), approximateoutlines onlv. The 44 centers seen in the Heliconiini and the Ithomiinae are reduced to 43 in the Troidini(less Suciia, Andes, Pantepui and Tefe, but adding Guerrero, 1 ULuman and Santa Catarina, each with se\eralwell-differentiated subspecies), which ho\\e\er show six centers with onlv one endemic subspecies known(hatched).

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\ \ Mj,ITHOMIINAE TROIDINI

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A N N U A L R A I N F A L L

1 1 < 500 mm500-1000 mm1000 -1500 mm1500 -2000 mm2000-2500 mm2500-3000 mm3000-4000 mm4000-5000 mm>5000mm

SOURCES: Bryjon « Hare (1974); Schwerdt-feger (1976); Paes de Camargo et a l .(1977); Hoffmann (1975); Brown~Tl9T9)

FIG. 4.

DRY S E A S O Nand

TOPOG RAPHY

>-4 months dry season

5 months dry season

6-12 months dry season

( a l l data from 1967-1970; ootrecorded foe Andes above

1500 m elevation)

\"11«" \ «;;.!„ F U t

land

Low mountainsSOURCES: Miller & Feddes (1971);

Brows (1979).

FIG. 5.

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species are clearly related to importantbarriers of long standing, such as the Ni-caraguan depression, the Isthmus of Da-rien, the Maracaibo Depression, the Llanosand Cerrado domains, the sandstone por-tion of the Guiana Shield, the dry valley ofthe upper Maranon river, the lowest flood-able portions of the Amazon Basin, andthe Salvador and Campos depressions onthe Brazilian coast (note isolines in Fig. 1).The more complex patterns which beg ex-planation are 1) those of the centers ofsubspecific endemism and hybridizationzones (Fig. 2) and 2) the local species di-versity not correlated with endemism (Ta-ble 1). The diversity/endemism dissocia-tion has been confirmed by a number ofstandardized transects across endemic cen-ters, and is not simply an artifact of sam-pling intensity or site accessibility.

To help in the understanding of thesepatterns, it is necessary to examine for theAmerican tropics a variety of basic ecolog-ical and historical factors. These data arepresented, in somewhat reduced forms, inFigures 4-9: modern rainfall (Fig. 4), im-portance of the dry season in 1967-1970(Fig. 5), basic topography (Fig. 5), contem-porary vegetation (Fig. 6), present-day soilcharacters (Fig. 7), paleoclimate during thecool, dry period (for tropical regions; Liv-ingstone and Van der Hammen, 1978)which terminated the last glacial age,13,000-20,000 years ago (Fig. 8), andvegetation domains suggested on the basisof geomorphological studies of the verymodified landscapes of the same past pe-riod (Fig. 9). Further data may be foundin Brown (1979, 1980a, 1982) and Brownand Ab'Saber (1979).

Three separate syntheses of these dataare presented in Figures 10—12: 1) areaswhich especially favor contemporary hu-mid forest species (Fig. 10); 2) areas of rap-id environmental change in the present(Fig. 11); and 3) areas with high likelihoodthat their humid tropical forests persistedthrough the cold dry period at the end ofthe last glaciation (Fig. 12). These will beseparately compared with the biogeo-graphical patterns in an attempt to answercertain key questions about differentiationin the Neotropical butterfly fauna.

What are the relative contributions ofevolution and ecology to local speciesdiversity in the Neotropical forests?

Many recent authors (e.g., Vanzolini,1973) have sought to relate the legendaryhigh species diversity in Neotropical for-ests with recent evolutionary processes(dramatic climatic and vegetational fluc-tuations during the late Pleistocene, lead-ing to increased competition, divergence,and extensive speciation in "refuges" dur-ing regressive phases). Others have doubt-ed the importance of Pleistocene catastro-phes in species formation, and describedlocal diversity as an ecological phenome-non, fundamentally related to favorableenvironmental factors coupled with mi-croheterogeneity—the latter promoted byfine mosaics of topography and soils, aswell as continual reorganization of thevegetational communities and food websthrough unpredictable mild disturbance(Brown, 1978; Connell, 1978; Fox, 1979;Gilbert, 1980). Figure 13 shows a correla-tive test of these two hypotheses aboutspecies diversity. Quadrants of maximumspecies numbers in Heliconiini and Ith-omiinae (230 quadrants with 15 or morespecies recorded among those analyzed)are superimposed with A) regions of highlikelihood for forest stability during a ma-jor regressive climate phase of the late Pleis-tocene—a potential powerful evolutionarymultiplier (Fig. 12), and B) regions of es-pecially favorable environment for humidforest organisms today—a potential eco-logical determinant of diversity (Fig. 10).The acceptable correlation of B (76%) butpoor overlap of A (55%; see also Table 1)suggest that the diversity at this level ismuch more of an ecological than an evo-lutionary phenomenon in the Neotropicalforests. Local communities may becomesaturated at species levels far below thosepotentially present from nearby evolution-ary pools, and these levels may be some-what increased in favorable present-dayenvironments. That they can also be in-creased by microheterogeneity and regu-lar mild disturbance is shown in Fig. 13C:the same maximum quadrants are super-imposed upon a map of "ecological tension

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V E G E T A T I O N(ORIGINAL)

Rich dense and open, lowlandor premontane rain forest

Rich gallery forest or allu-vial forest,eutrophic soils

Rich semi deciduous forestwith evergreen inclusions

Poor dense forest on levelterrain and dystrophic soils

Poor transitional and openforest types (palm, liana,bamboo, deciduous thicket)

Floodable forests: varzea,i gapo, mangroves

white sand formations:campinarana, campina

Mixed poor forests (palm,dense, open, alluvial,floodable, white sand)

Non-tropical-forest vegetation(including floodable fieldand Amazonian Bavannas)

SOURCES: FAO/UNESCO ( 1 9 7 6 ) ; Hueck 4S i e b e r t ( 1 9 7 2 ) ; RADAMBRASIL ( 1 9 7 3 -1 9 8 1 ) ; Brown ( 1 9 7 9 , 1 9 8 1 c ) .

FIG. 6. Brown, 1981c in legend = Brown, 1982.

Eutrophic "terra roxa" andred-yellow podzolic soil

Dystrophic "terra roxa" andred-yellow podzolic soil ,mesotrophic latosol, eutrophiccambisol and lithosol mosaics

Eutrophic alluvial soil andhionic gley

Dystrophic cambisol andlithosol mosaics, andosol

Alic or sandy podzolic soilor latosol, includingyellow latosol

Plinthic podzolic soil andlatosol, vertisol, dystrophicalluvial soil and humic gley

Hydromorphic podzol, ferralicquartz sand

Plinthic soils plusconcretionary soils (n saics)

• Quartz ssnd, concretionary soil**planosol , cold and ar id so i l s

SOURCES: FAO/UNESCO (1971, 1976);RADAMBRASIL (1973-1981); Brown(1979. 1981c).

'- uFIG. 7. Brown, 1981c in legend = Brown, 1982.

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®

July-August, 18,000 years ago

£ l L_ow or Higfr pressure cell

Depression of temperaturein relation to present, 9C

Predominantwinds

Predominant cool winds

Warm, dry winds (foehn)

Ar f high probabilityfor orographic rainfall

Same, but only in nohemisphere wet seas(July), not in dry

Shoreline (mean sea90 m below that of

SOURCES: Damuth & Fairbridge(1970), CLIMAP (1976), Gates(1976), Brown (1979)

FIG. 8.

•

P A L E 0 D 0 M A I N S

Shoreline (m.s. l . -90

Tropical humid forestPossible larger areasOther t ropical humid

forests (gal lery ,s ubiDontan6, deciduous

Parano, subparancAndean forests (>1500fqt

Pacific deser t , >tonte

Araucarias * open field

Gran Chaco

Caatinga and othersemi-arid flora

fc^^j Cerrados\ -j Llanos, savannasI j Mixed non-forest typ

SOURCES: Haffer (1974).Ab'Saner (1977)

r- A

r ^ \mr

FIG. 9.

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FAVORABLE HABITATFOR HUMID TROPICALFOREST ORGANISMSIN THE PRESENT

Areas which satisfy allof the following

conditions: 1

More than 2000 mi annualprecipitation (Figure 4)

0-4 months annual dryseason (Figure S)

Any type of continuoushumid tropical forest(dense or open, flat orrolling topography, richor poor soils) (Figure 6)

FIG. 10.

EN VIRONMENTALC O N F O R M I T I E SAND R A P I DT R A N S I T I O N S "IN THE PRESENT

Areas within or on the edgesof tropical forest donains,which satisfy at_ least twoof die following conditions:

a. Rapid transitions in annualprecipitation (more than 1000am in less than 100 km)

(Figure 4)

b. Rapid transition from denseor open rain forest to deci-duous or floodable forest,scrub, or non—forest vegeta-tion (Figure 6)

c. Rapid transitions in soilstructure (from red-yellowlatotol or podzolic soil toyellow latosol or shallower,plinthic or hydromorphic soil)

(Figure 7)

FIG. 11.

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PALEOECOLOGICALFOREST REFUGES

13,000-20,000 years ago

Axeas which show a totalof 3 (hatched) or 4-5

(black) in the summation:

a. Vegetation types favorableto long stability (first 3categories, heavy hatchingin Figure 6 - +1), and sa-vannas within forest domain*(white in Figure 6 - -1) ;

b. Soil types indicative oflong stability (first ca-tegory, black in Figure 6= + 2, next two, heavilyhatched in Figure 7 = +1),

the past (last three cate-gories, light hatching inFigure 7 = -1);

c. Areas of higher orographicrainfall in July-August,18,000 y.b.p. (Figure 8-+1);

d. Tropical forests in sameperiod, from georaorphology(Figure 9 - +1).

FIG. 12.

zones" (areas of fine mosaics of topogra-phy, climate, soils, and vegetation, repre-sented by Fig. 11 with addition of areas ofdry-season variability and cold-wave liabil-ity, signs of climatic unpredictability whichproduces local mosaics in vegetationalsuccession and animal populations). Onceagain, correlation is very good (over 90%),much better than that with historical fac-tors (Fig. 13/4). Naturally, some of thehigh-diversity quadrants are related tolong residence by naturalists or collectors,but the number of 15 species in the twogroups is often achieved in a few hours'sampling in favorable areas, and neverreached after years in less rich sites. Manyevolutionary factors have also logicallycontributed over long time to the total re-gional pools of species, from which localmeasured diversity may be drawn. Thefacts seem to affirm, however, a negativerelationship of diversity with recent paleo-ecology and both short- and long-term sta-bility, and positive relationship with veri-fiable contemporary ecological factors andtheir patterns of spatial and temporal vari-

ation—all of which are highly amenable toinvestigation and experimentation in thefield, at the present.

Can subspecies endemism and hybridizationpatterns be explained by currentenvironmental factors?

Endler (1977, 1982) and Benson (1982)have proposed that past isolation is unnec-essary to account for the more fine-grained biogeographic patterns seen inNeotropical organisms. Modern environ-mental transitions, coupled with the pos-sibility of differentiation along clines,should be sufficient to produce the ob-served discontinuities in genetic featuresof plants and animals. The biogeographi-cal data-set for forest butterfly subspecies(Fig. 2) is ideal for testing this hypothesis,since it represents genetic differentiationwithin species, revealed as large regions ofrelative character stability (the centers ofendemism) divided by zones of mixing andinstability (hybridization between subspe-cies). By Benson's hypothesis (1982), thelatter should correspond with regions of

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rapid environmental transition (Fig. 11);by Endler's argument (1982), the formershould coincide with regions of environ-mental homogeneity (conformity). Super-imposition of Figure 2 with Figure 11 (Fig.14) gives some support for this hypothesis.An acceptable majority (69%) of the 297butterfly subspecies hybridization quad-rants corresponds roughly with zones ofrapid environmental transition. A lowercorrelation, however, is revealed betweenendemism and zones of environmentalconformity. Of the 43 centers of ende-mism (not considering "Andes"), 24 showstrong internal transitions (that is, crossseveral borders between environmentalconformities); nine show peripheral inva-sion of transitions, and only ten are clearlycontained by zones of environmental ho-mogeneity. Thus, although hybridizationzones tend to concentrate in predictableareas, they fail to occur in many other pre-dicted areas; the biogeographical divisionsat a recognized genetic level should bemuch more numerous than those actuallyobserved, if similar environmental transi-tions promoted stepped clines as efficient-ly in one place as another. Indeed, manygenetic transitions and individual subspe-cies limits do occur within the endemiccenters, but the generalizable hybridiza-tion zones, representing over half of therespective faunas, correspond to only apart of the environmental transitions, notalways the strongest. Furthermore, threegroups of butterflies with very differentecological preferences show endemic cen-ters with outlines conditioned by contem-porary ecological conditions, but essential-ly in identical locations (Fig. 3). Theseregional patterns seem to suggest a moredrastic, possibly long-term and historical,factor in the formation of the differentiat-ed sets of butterfly subspecies, with thepresent-day zones of hybridization un-doubtedly affected by current environ-mental discontinuities—which, however,are still not uniform in their action, as seenin Figure 14.

Among the biogeographical patternspossibly influenced by recent environ-ments are those related to forest structure,particular!) light diffusion (Papageorgis,

1975; Benson, 1982); resource or habitatspecialization, especially in phytophagousinsects with unstable chromosome comple-ments like the Ithomiinae (Bush, 1975; K.Brown, T. Emmel, P. Eliazar, B. Drum-mond and E. Suomalainen, in prepara-tion); and the fixation of hybrids (Brownand Mielke, 1972; Brown, 1976, 1977;Vasconcellos-Neto and Brown, 1982). Pos-sible examples of such processes of differ-entiation are collected in Table 2; thesemay eventually be disqualified from inclu-sion in sets resulting from any historicalfactors in evolution.

Are regional patterns of evolution relatedto major historical factors in therestructuring of Neotropical landscapes?

If indeed the Neotropical forests suf-fered fragmentation during the cool, dryperiod at the end of the Wiirm-Wisconsinglaciation, the fragments ("paleoecologicalforest refuges") might have served as res-ervoirs for genetically diverging faunas,selected by differences in local environ-ments. Inference of the existence, or de-termination of the positions, of such "ref-uges" on the basis of biogeographicalinformation only is circular, however, andthus not appropriate in the testing of hy-potheses about evolution. On the otherhand, indication of their existence and po-sitions on the basis of independent evi-dences, from the geosciences, not involv-ing considerations of systematics or speciesdiversity, could provide a valid model forthe evaluation of evolutionary hypotheses(Fig. 12). Superimposition of Figure 2 andFigure 12 (Fig. 15) reveals a strong corre-lation between subspecies endemism and"paleoecological forest refuges." Essential-ly all endemic centers enclose one or moreareas of high likelihood for forest stabilityin the harsh climate in the final period ofthe last glaciation; the notable exception isMarajo, a clear case of island endemismwhich was under water in the warm moistperiod ("climatic optimum") 5,000 yearsago (Fairbridge, 1976). Those endemiccenters which enclose more than one "ref-uge" area usually also show some internalsubdivision; different endemic centerswithin a continuous "refuge" area are usu-

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TABLE 2. Geographically restricted butterflies whose evolution may be related to current ecological factors or hybridization.

Possible factor Species or subspecies (with details)

Light structure

Habitat or foodplantisolation

Hybridization

Heliconiini: yellow subspecies of Heliconius ethilla and H. hecale (semi-deciduous forests)

orange-yellow-black subspecies of H. erato and H. melpomene(open Amazonian forests)

Ithomiinae: yellow subspecies of Tithorea harmonia (semi-deciduous forests)

Heliconiini: Philaethria pygmalion (Amazonian savannas)Dione juno miraculosa (W. Peruvian "lomas")Dione glycera (cloud forest, wooded paramos)Heliconius hecuba and H. cydno (montane forests)H. wallacei araguaia (gallery forests)Melanic H. numata (dense Andean forests)H. hecale hecale (coastal thickets, Guyana)H. demeler terrasanta (low alluvial thickets in western Para)

Ithomiinae: Elzunia pavonu (arid habitats, NW Peru)Melanic Mechanitis, Hypothyris, Napeogenes, Hyposcada and

others (dense Andean forests)Many Hyalyris (medium-elevation habitats)Many Hypothyris and Napeogenes (strongly restricted foodplants

or habitats)

Troidini: Pandes bunichus diodorus (cerrado, C. Brazil)P. ascanius, P. zacynthus zacynthus and z. polymetus (coastal

restinga, E. Brazil)P. hahneli (poor thickets, central Amazonia)P. anchises subspecies in northern Colombia (divided between

wet and dry habitats)P. panthonus subspecies in central BrazilMany Battus and Pandes in W. Mexico, S. Brazil

Heliconiini: Heliconius heurippa (Fig. 3, leftmost); many named uniquespecimens

Ithomiinae: Certain Mechanitis subspecies; many named unique specimens,especially in Melinaea

ally separated today by Andean ridges{e.g., Huallaga/Chanchamayo/Ucayali).Very little of the predicted area for sub-species differentiation (Fig. 12) is not rep-resented by known endemism in forestbutterfly subspecies; of this, a part has notyet been sampled, another part (poorly iso-lated from adjacent "refuges") showsendemism in other groups of butterfliesand organisms of lower dispersal capacity,and only a small fraction coincides withbutterfly hybridization zones (Fig. 15).

Whether one will regard Figure 14 orFigure 15 as showing a more convincingcorrelation between biogeographical pat-terns and contemporary or historical eco-logical factors, respectively, may dependmore on predisposition than objectiveevaluation. Historical hypotheses aboutevolution in this Neotropical system are

essentially non-falsifiable, representingonly "strong inference" based on good cor-relation. Hypotheses relating this regionaldifferentiation to current ecological fac-tors may be equally non-falsifiable, sinceall too often the best an ecologist may hopeto observe in his own lifetime is evolutionon a very local scale, with limited spread-ing to adjacent populations. Even goodcorrelation does not substantiate causa-tion, though a close fit between indepen-dent data-sets (as Fig. 15) carries a pow-erful suggestive influence.

Whatever may be the best interpretationor correlation for the data used to prepareFigures 1-12, they have already been veryuseful: over 100,000 km2 of new nationalparks and equivalent reserves have beenestablished in tropical America in 1975-1980 (with an equal amount still as pro-

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CORRELATION OFDIVERSITY WITH

PALEOECOLOGY(FAVORABLE PAST

ENVIRONMENT)Quadrants with over 15specie* recorded, amongthe Heliconiini andIthotniinae analyeedby Brown (1979)

Paleoecological forestrefuges (Figure 12)

Coincidence of both(125 quadrants -

552 of total)

CORRELATION OFD I V E R S I T Y WITHFAVORABLE PRESENTENVIRONMENT '

Quadrants with over15 species

Area most favorable fortropical humid fororganisms in the pscat (from Figure

Notable groups ofhigh-diversity qua-drants in areas oflow rainfall

C O R R E L A T I O N OF

D I V E R S I T Y WITH

E N V I R O N M E N T A L

T R A N S I T I O N S AND

D I S T U R B A N C E

Quadrants with over15 species

Rapid environmentaltransitions (Fig. 11)

Area of high risk forunpredictable longdry season or coldspells )

Over 90Z of thequadrants axecoincident

•

FIG. 13.

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C O R R E L A T I O N OF

E N D E M I S M W I T H

E N V I R O N M E N T A L

C O N F O R M I T I E S AND

RAPID T R A N S I T I O N S

FIG. 14.

CORRELATION OF

ENDEMISM WITH

PALEOECOLOGICAL

FOREST REFUGES

Paleoecological fores trefuges (Figure 12)

(Figun

enters (one-thi rd

d area (included<i the 43 centers)

Hybridization quadrants(Figure 2)

Overlap of hybridizationwith refuges (75 quadrants

- 25Z of total)

FIG. 15.

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posals under study by national govern-ments), many as a result of and taking intoaccount these data-sets and their indica-tions about the geographical distributionsand histories of different genetic re-sources. Hopefully, these patterns and re-sources will thus continue to be availablefor more careful long-term study and ex-perimentation during the foreseeable fu-ture.

SPECIES OF OTHER HABITATS: THEINVITATION TO RESEARCH

All of the above discussion has appliedto species of the humid tropical forests atlow to medium elevations (to 2,000 m).These forests are today mostly continuousin the Neotropics, though their faunas arenot (Fig. 2), pointing to subtle ecologicalbarriers and probable historical modifica-tions of the forest landscapes. The conti-nuity of these forests today has provokeda discontinuity in other habitats which for-merly were probably more continuous:high-altitude paramos (Simpson-Vuilleu-mier, 1971) and Amazonian and other sa-vannas (Eden, 1974). The study of thesepresently isolated habitats should givesome of the best evidence for evolutionaryprocesses in Neotropical floras and faunas,since these processes and their modern re-sults can often be directly observed andmanipulated (Brown and Benson, 1977).

A number of butterfly genera or genus-groups are characteristic of isolatedhigh-altitude vegetation patches in theNeotropics; these include especially thepronophiline satyrs and the Tatochila-Phu-lia group of pierine whites (see Brown,1980a). Open habitats at lower elevationsin the Neotropics are characterized by anabundance of endemic euptychiine satyrs,as well as many local theclines, riodininesand skippers. It is hoped that ecologistsand entomologists will consider devotingperiods of intensive study to these butter-flies in their presently isolated and disjuncthabitats, in order to provide informationon some of the important evolutionaryprocesses and ecological factors importantin the generation of the biogeographicalpatterns now becoming known in the Neo-tropical forest Lepidoptera.

ACKNOWLEDGMENTS

Condensed in large part from Brown(1979, 1980a). Contribution number 33 ofthe Programa de Ecologia, Instituto deBiologia, Universidade Estadual de Cam-pinas, and paper 9 in the series "Geo-graphical patterns of evolution in Neo-tropical Lepidoptera." I am grateful to W.W. Benson, J. A. Endler, A. N. Ab'Saber,D. R. Gifford, J. Haffer, G. Lamas M., I.Lepsch, P. M. Sheppard, S. S. Tillett, andT. C. Whitmore for major contributions tothis research program and the ideas in themanuscript, and to Drs. Benson and En-dler and especially C. L. Remington forcriticism of the manuscript. I also acknowl-edge travel funds from the National Sci-ence Foundation and the World WildlifeFund—U.S. (Program "Minimum CriticalSize of Ecosystems") to help refine the dataand permit its presentation in Seattle inDecember 1980.

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