Considerations Regarding the Evolution of Hawaiian Animals

WILLIAM A. GOSLINEl

INTEREST IN THE BIOTAS of oceanic islands isof long standing. There are several reasons forthe continuing interest. One is the possibilitythat insular evolution may in some respectsrepresent a small-scale model of what has oc­curred on continents. Another is that , despiteall the work on the subject, the "hows" andthe "whys" of insular evolution remain inade­quately answered. Finally, there is the realiza­tion that , with the rapid decimation of nativeinsular habitats , it will soon be impossible tostudy many aspects of the subject (Hubbell,1967) .

The general field of evolution in oceanic ani­mals has been reviewed many times, most re­cently by Miller (1966) and Carlquist (1 965).Zimmerman's summary (1948) for the Ha­waiian terrestrial forms is classic. Here, I shalldeal with only certain facets of the subject, andI shall cite only those references from the tre­mendous literature most pertinent to the matterat hand . This selective method of presentationhas serious faults , but any attempt to be compre­hensive would seem only to obscure the threadsof thought that it is the purpose of the paperto present.

Recently, in writing of land plants, Carl­quist (1966 :433) has spoken of an "insularsyndrome of interrelated evolutionary phe­nomena." Insofar as Hawaiian animals are con­cerned, what is more striking to me is the di­versity of evolutionary results. Such variationsoccur not only between groups but within somegroups as well. For example the evolution ofthe Hawaiian drepaniid finches has been verydifferent from that of the sea birds.

This diversity of evolutionary results couldbe exemplified from various animals groups ,most notably insects. However, I shall not dealwith Hawaiian insects at any length, primarilybecause of unfamiliarity with them but also

1 Department of Zoology, Uni versity of Hawaii,Hon olulu, Hawaii 96822 . Manuscript received June13, 1967.

because at the present time they are the subjectof an intensive continuing investigation (Zim­merman , et al., 1948-; Spieth, 1966: 246).Rather, I shall emphasize the evolutionary prob­lems of three Hawaiian animal groups : the in­shore fishes, the achatinellid land snails, andthe drepaniid finches. As an introduction to theproblems involved the evolutionary status ofthese three groups in Hawaii is summarizedbriefly.

The Hawaiian inshore fishes (Gosline, 1958;Gosline and Brock, 1960) form part of amarine biota that is essentially similar to, butsomewhat impoverished, as compared with thatof the Central Pacific islands to the south andwest. There are few conspicuous gaps in theHawaiian marine biota, and, of those that dooccur, at least one-the coral genus Acropora­was present in Hawaiian waters in the past(Menard, Allison and Durham, 1962) . There:seems to be a more or less constant infiltrationof non -resident species into the Hawaiianmarine biota today, some of which have be­come established (Doty, 1961), while somehave not (Brock, 1948). Intentional introduc­tions of purely marine forms into Hawaiianwaters have been mostly unsuccessful. (By con­trast, a number of int roductions into areas ofreduced salinity, e.g., Kaneohe Bay on Oahu,have done quite well.) One of the few that hassucceeded, the "Marquesan sardine," has spreadthroughout the waters of the high HawaiianIslands in a matter of a few years (Murphy,1960). Endemism above the species levelamong Hawaiian fishes is dubious. However,about one third of the inshore species are rep­resented by endemic forms. These can usuallybe distinguished from Central Pacific counter­parts in 100% of the individuals (for someexceptions, see Gosline, 1955). Aside from afew expected correlations between morphologi­cal traits and the relatively cool Hawaiian watertemperatures (see, for example, Strasburg ,1955), the morphological characters by whichthe Hawaiian endemics differ from their Cen-

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tral Pacific counterparts appear to be of a ran­dom nature. Within any family of fishes rep­resented in Hawaii, the endemic forms areoften the most abundant.

In striking contrast with the Hawai ianmarine biota, the native terrestrial biota ishighly disharmonic or unbal anced. Greatgroups of animals, e.g., the amphibians, werecompletely unrepresented, whereas others, e.g.,the land snails and drepaniids, proliferatedgreatly. N ot only new Hawaiian species, butalso new genera and famili es evolved. Amongthe achatinellid land snails, the genus Achati­nella is restr icted to the island of Oahu, butsome 100 allopatric forms have been described.N o relationship between the peculiarities ofthese forms and the environment they inhabithas .ever been demonstrated. The drepaniidfinches seem to have evolved in quite a differentway. They inhabited all of the major islandsof the Hawaiian chain and some of the smallerislands as well. The most notable differentati onwithin the group is in beak shape, which isassociated with feeding habits (Baldwin,1953). Several different drepaniids were oftensympatric.

One of the main differences between theterrestrial and marine environments in Hawaiiis in the amount of change caused by man.The terrestrial environment has been largelytransformed, in part directly by man via agri­culture, etc., but perhaps more by the indirecteffect of animals and plants which man hasintroduced, intentionally or unintentionally.Many of these introductions have now replacedor are replacing the native biota and are directlyor indirectly responsible for the restrict ion orextinction of native forms.

With this brief background, the question ofevolutionary processes will be discussed.

Basic to the evolution ( or lack of it) thatwill occur on any island is the matter of whichorganisms are there and which are not. Toexist, an organism must first arrive, and it thenmust find an environment in which it can sur­vive and reproduce. Both of these aspects de­pend in part on the isolation of the island­not isolation in terms of geograph ical-physicalbarriers alone, but in terms of these in relationto the ability of the organism to cross them.

The day when isolation could be considered

PACIFIC SCIENCE, Vol. XXII, April 1968

a causal factor has long since passed. However,that it is a powerful controlling factor is gen­erally recognized. This control acts in two re­lated ways. First , it determines which organismswill get to an island and which will not. Theselectivity of this filtering factor will increasewith increasing isolation and hence will deter­mine in part the extent to which the islandbiota resembles its parental biota. The greaterthe difference between these two biotas, thegreater will be the change in biological selec­tion pressures on any organism arriving on theisland . This point will be discussed later.

Second, any species that establishes itself onan island should , at least for a while, be pre­served from contamination by gene flow fromthe parental populati on. If the recent introduc­tion to Hawaii of numerous species (e .g ., theMarquesan sardine , the African snail, thegarden spider, etc.) is any criterion, the initialimmigrants can build up a population of mil­lions of individuals in a few years. Beyond th ispoint contamination from gene flow from afew subsequent immigrants will probably havelittle effect (Gosline, 1958) . There are, how­ever, certain important except ions to th is state­ment. If, in the process of building up a popu­lation from initial immigrants, the populationbecomes debilitated in some way or loses itsability to cope with diseases or parasites whichlater immigrants may bring with them, thensubsequent immigration may matter a greatdeal.

Of factors actually causing insular evolutiononly two will be considered. One is naturalselection, and the other the series of featuresassociated with small population size.

It is generally agreed that differentation pro­ceeds more rapidly in animal populations onsmall islands than on large ones. The questionis : to what extent is this caused by differencesin the selective forces on small islands, and towhat extent to factors associated directly withsmall population sizes. A rather large body ofdata suggests that many of the peculiarities ofsmall-island forms are not directly selected bythe enviroment. Two examples will suffice.Dowdeswell and Ford ( 1953 and elsewhere)have shown that on the larger islands of theScilly group the spotting on the wing of thebutterfly M alliola [urtina remains about as it

Evolution of Haw aiian Animals-c-Gosi.n-rs

is on the adjacent Cornish mainland. On thesmall islands of the group, however, the num­ber of spots on the wing of the females notonly varies from island to island, but increaseson some and decreases on others. Second, Mer­tens (1934:116) has pointed out that the sameisland may contain a dwarf form of one reptileand a giant form of another. It would be dif­ficult to postulate environmental factors thatwould select animals in these ways.

If small-popul ation forces are to be postu­lated for such differences, three possibilitiesmust be considered . The first is the random lossof genes which may occur in small populat ionsby genetic drift (Wright, 1931, etc.). Such afactor would presumably be operative in allsmall populations. A special case of geneticdrift is the phen omenon often called foundereffect. Thi s merely expresses the fact that theoriginal immigrants to an island are frequentlyfew in number , and, whether or not they con­stitute a representative sample, they can bringwith them only a small proportion of the allelespresent in the parental population (see, forexample, Zimmerman, 1948 :122, 123). Thethird possible small-population factor is whatMayr ( 1954) calls internal selection. In largepopulations where each gene often has manyalleles those which work best as heterozygoteswill tend to be selected; on the other hand , insmall populations there will be a larger prop or­tion of homozygotes, and alleles which workbest in the homozygotic condition will tend tobe selected. Thus some shift in intern al selec­tion pressure between large and small popula­tions would be expected.

These small-population factors, acting perse, should affect insular immigrants duringthose initial stages when the population is stillsmall ( Fig. 1). But there appears to be noknown instance in which a change at this stagehas been recorded (d. Mayr, 1954). Further­more, it is a generally accepted dictum that,other things being equal, the older the islandthe greater will be the differentati on in itsbiota; this implies continuous, not just initial,change.

Then how is the differentation that occurs onislands, and more rapidly on small islands, tobe explained? King (1955) conducted selec­tion experiments for DDT resistance on two

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cultures of Drosophila melanogaster. After adozen or more generati ons some degree ofDDT resistance began to be built up in bothlines. But, as judged from crossbreeding ex­per iments, the resistance had been built updifferently in the two lines. King (195 5:314)states: "The manner in which a line could re­spond to selection was to some extent deter­mined by the genetic nature of the sample fromwhich it started , and having started along onecertain road, it kept on. The inevitable sam­pling error which occurs when a line is takenfrom a larger population is very likely the an­lage of the genetic individuality of the line .This is, of course, an example of the principleof genetic drift. . .." The second example isthat reported on by Dobzhansky and Pavlov­sky (1957) . In this experiment ten culturesfrom a specially developed laboratory stock ofDrosophila psendo-obscura were started with20 flies each and compared after 17 monthswith ten other cultures that did not begin witha reduced number of individuals . Th ose stockswhich had started with 20 flies showed morevariation than the controls. Again, Dobzhanskyand Pavlovsky conclude ( 1957:316) : "Al­though the trait studied (g ene arrangement inthe third chromosome) is subject to powerfulselection pressure , the outcome of the selectionin the experimental populations is conditionedby random genetic drift."

One aspect of these experiments by Kingand by Dobzhansky and Pavlovsky may well beof importance for insular evolution. In bothinstances not only were the original samplessmall, but the selection that was exerted uponthem was far different from the selection of thenatural environment from which the flies came.It is as though the samples in the experimentswere subjected to an intense selection pressureat right angles to the pressures to which theancestral "wild" forms had presumably adaptedthemselves. Possibly some of the alleles in­tensely selected under the laboratory conditionshad been of only peripheral significance to thewild stocks and hence variably represented inthem. Such alleles would be more subject tosampling error among small founder popula­tions drawn from the parental stock than thosepreviously under intense positive selection pres­sure.

270 PACIFIC SCIENCE, Vol. XXII, April 1962

FIG. 1. The oretical popul ation size (above) andallele variety ( below) plotted against time in a ter­restrial animal that successfully immigrates into theHawaiian Islands for the first time. A, D ate ofarr ival; B, time when pop ulation becomes sufficientlylarge that small-popul ation genetic factors will per secause no further loss of alleles; C, initial peak ofabund ance; and D, subsequent equilibrium. For dis­cussion, see text.

Hawaiian endemic fishes cannot, apparently , beattributed to small -popul ation losses alone isthat in many instances the Hawaiian endemicsare not characterized by a simple increase invariability ( as in Dobzhansky and Pavlovsky'sflies) but rather by new and fairly constantcharacters entirely outside the range of vari­ability of the ancestral populations (as inKing's results) . Presumably such charactersmust have arisen through a reintegration and/or evolution from the ancestral genetic systemvia direct or ind irect selection. The same rea­soning would seem to apply to at least some ofEisentraut's melanic lizard popul ations.

Judging by personal observation and com­mon knowledge concerning recent successful ter­restri al int roductions of animals to the Hawaiianislands (see also Mead, 1961: 180-1 82 ; Tomich,et al., in press) , there is often (presumablyfollowing a longer or shor ter period of smallnumbers) a tremendous initial build-up and"overshoot" in popul ation numb er (Fig. 1) .During my 18 years in Hawaii this has hap-

Insular selection pressures (except, perhaps,for species int roduced by and dependent onman) are similarly at an angle to those exertedon the mainland parental form. Insofar as theisland biota is different from that whence theimmigrants came, it is inevitable that the bio­logical selection pressures on islands will differ.Any immigrant to an island will leave behindat least some of the predators, competitors,diseases, and parasites that the parental main­land stock had to cope with. On the other handthe ini tial immigrants may well have to adaptto new forms of food, cover, etc. (This will beless true only in degree if a species arrives bya series of island hops .)

There is also evidence that selection pressureson small islands are likely to be more radicallydifferent than they are on large islands. Thus,on Manana Island, a small outlier of Oahuwithout domestic cats, the cat flea (Ctenoce­ph alides felis f elis) has developed an ecto­parasitic existence on rabbits (OryctolaguscllllicltlltS) (Tomich , et al., in press). Again,in the Balearic Islands off Spain , Eisentraut(1 949 ) showed that on the smaller outliers thefood of lizards (Lacerta) differed considerablyfrom that on the main islands . As the normalinsect food became more restricted, these lizardsadded the normally avoided ants to their diets,and on very small island s ate even flower petalsand young plant shoots.

Eisentraut believed that this change in diethad a direct metabolic effect resulting in themelanism frequently found in the small-islandpopulations. To me (d. Dowdeswell and Ford ,1953) it seems more likely that the morpho­logical changes so frequently found in smallpoulations are in part the indirect effect ofaltered selection pressures working with timeon the, in part randomly, depleted gene pool ofsmall popul ations. A gradual reintegration ofsuch a gene pool in response to altered selec­tion pressures would likely involve a change inphenotypic characters that are not themselvesselected . Such an interpretation (d. Mayr,1954; D obzhansky and Pavlovsky, 1957) seemsto me to provide the best available explanationfor the often rather heterogeneous differenti­ating characteristics of insular endemics, e.g.,H awaiian inshore fishes.

The main reason why the peculiarities of

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Evolution of H awaiian Animals-s-Gosr.ms 271

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FIG. 2. On a background (dotted contours 's rep­resentin g Sewall Wright's adaptive peak concept areshown (hatched areass: above, a theoret ical popul a­tion just before reaching the initi al maximum (C inFig. 1), and below, the same popul ation after anequilibrium size had been reached (D in Fig. 1) .For discussion, see text.

drepaniid finches, with their various beak types,in mind .

Another possibility is that, following theinitial pop ulation explosion, some factor otherthan food supply develops to keep subsequen tnumbers low. This could be disease or para­sitism, some change in other environmentalfeatures, or some other factor which wouldlower the reprod uctive rate. That the reproduc­tive rate may be diminished has been stressedby Lack (1954). Lack deals especially withchanges in egg number in birds. But there is

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pened with a garden spider (Argiope appema )and the giant Af rican snail (Acha tina flt/ica) ,among the more conspicuous unintentionallyintroduced forms. If it can be assumed that thiscycle happened in the past with our "native"biota, then certain postulates concerning selec­tion pressures would seem to follow.

First , during the period of initia l buildup ofan introduced form , selection pressure must bevery low. (Apparently the other members of thebiota are not initia lly able to cope with or defendthemselves from the new introduction.) H ow­ever, at some point in the buildup, the popula­tion becomes excessive, after which it fallsdrastically to a new fluctuating equilibrium wellbelow the previous maximum. The nature of thefactor that sooner or later kills back the initialovershoot is unkn own in any particular instance.There is no reason to believe it is the same in allcases, or that it may not be a combination offactors. W hat is important to the present argu­ment is that after a period of relaxed selectionduring the population buildup a very severeselection pressure of some sort appears . Some ofthe various possibilities are as follows.

First, the animal may eat out the availablefood supply and then die of starvation. Thisapparently happened to the rabbits introducedto Lisianski Island (Bryan, 1942 :192, 193),and almost but, perhaps significantly, not qui tewith the rabbits on Laysan (Warner, 1963 :6,7; cf . also Tomich, et al., in press).

On a larger island with a more varied biotaa second possible situation might occur afterthe immigrant population had overeaten itsoriginal food supply . Assume that an immi­grant adapts itself to an insular food supply asclose as possible to that of its parental stock.Assume that , having adapted itself to this in­sular food source, the immigrant builds up atremendous pop ulation under greatly relaxedselection pressure. At some point it will over­shoot its new food supp ly and a severe compe­tition for food will take place. This selectionmay preserve the best adapted individuals ofthe original immigrant type, if enough of thefood supp ly is left. It may also preserve thoseindividuals that have differen tiated fart hest inthe direction of adapting to a new food source(Fig. 2) . This theoretical possibility has beenset up with the evolut ion of the Hawaiian

272

another method, in plants at least, by which thereplacement rate may be held in check. Ratten ­bury (1962: 354) has said of New Zealandforms:

"Furthermore, the germination of seeds ofmany native species is a matter of extreme dif ­ficulty, as is evidenced by the sporadic ap­pearance of seedlings which often seem torequi re special conditi ons for their develop­ment. Competent nurserymen have experiencedgreat difficulty in germinating native seeds,often resorting to powerful treatments forbreaking the dormancy. In many cases the via­ble period is very short."

Under conditions of severe interspecific com­petition, a reduction in the reproduction rate,however accomplished, would seem to be feasi­ble only to the extent that it enables the speciesto raise a greater number of offspring (Lack,1954) . T o drop below that rate would invitereplacement by competit ors (including possi­ble subsequent immigrants of the same spe­cies) . If , however, there is very slightinterspecific compet ition, the reproductive ratemight theoretically fall to and somewhat belowthe maximum possible replacement rate with ­out immediate harm. In my opinion, this iswhat seems to have occurred in many formsamong the native terrestrial Hawaiian biota .

If this is true, a species that experien ced essen­tially no natural selection from inf ra- and inter ­specific competition during the initial increasemight again avoid natural selection after anequilibrium had been reached. In the process,however, the species would of course lose itsaggressiveness, especially as compared withsubsequently introduced rapidly breedingforms .

Possible examples of a loss in fertility , ap­parently before any overpopulation has takenplace, are provided by the Hawaiian hawk (B II­teo solitarillS) and the Hawaiian crow (C oroustropims) , both of which are, and so far asknown always have been, confined to relativelysmall areas of the single island of H awaii. Inview of the general adaptability of relatedmainland forms this geographic restriction ismost difficult to explain. Possibly here we havea "depauperization of biotype" (Hulten, 1937)arising from small populati on effects. Such a"depauperization" of course could and prob-

PACIFIC SCIENCE, Vol. XXII, Apr il 1968

ably often does result in a lowering of repro­ductive capacity.

Returning finally to the achatinellid snailsof Oahu , it seems obvious from the isolationof many of the colonies of Achatinella (evenbefore their extensive extermination by theintroduced carnivorous snails Englandina andGonaxis; see Krauss, 1964) that they were notspreading. Presumably they can only have beendeveloped from some more "aggressive" an­cestral form (and/ or a less dissected topogra­phy than exists today) . But given the morerecent isolation in separate colonies there seemsto be no reason why, in the absence of furthergene flow, each colony should not evolve in itsown way as do so many other small popula­tions, particularly land snails. A basically sim­ilar provisional hypothesis has been advancedby Carson (1966 :405 ) to explain the forma­tion of Hawaiian species of Drosophilidae.

ACKNOWLEDMENTS

My acquaintance with this subject has beengained over the years through the kind helpof colleagues too numer ous to mention. Forspecific suggestions and comments on the manu­script I wish sincerely to thank Dr. A. H. Ban­ner, Dr. E. A. Kay, and Dr. C. H. Lamoureuxof the Un iversity of Hawaii, and Dr. Y. Kondoof the Bishop Museum, all of whom have beenresidents of Hawaii and interested in theHawaiian biota for much or most of their lives.

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