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UNIQUENESS OF HAWAI'I'S BIOTA

Hawai'i fins oJen been referred to as the best 'halural laboruroty" for evofutio~~aty srud~s in the world.

Kenneth Y . Kaneshiro

In his book entitled Isl1~1d Poprrlufions, ~Williamson (1981) wrote of the importance of a group of Hawaiian ilics in the family Drosophilidae for studying island hiology. In a detailed chapter dcscribing the results of research conducted o t ~ these flies, he began:

Of all UIC groups of organisms, plants or inimalr, thal can hc slldicd on islands, the Hawaiian nrwslphilidac nrcsuyremc. This is why. Thmc are a grcal

He concluded his chapter hy saying:

Any her study uicvolutiun must surely seem inadcquatc alter thc gady of the. F~awaii;mL)rosopBilidac. In no nthcr group is so much known, arc there so mimy snccics. and can the ccnlorrical relalimsliips hc clarilicd .w ~vcll. Thcy also show

amount ol~vwk lo bednnc on the group, bnl rhc work carried out so far clcarly cstahlishcj lheir supremacy.

Williamson was cert:~i~ily accurate in his itsseument of the significance of the Hawaiian drosophilids for unde-standing evolutionary hioiogy of island populations. Ilowevcr, malty other grot.ps of organisms in Hawi ' i , hoth

R lam\ and animals, could be st.~dicd i n :he same way :liar thc (iro,ophilids ave b c e i and would hc eq11:l.l~ \ 3 l u b l e in unravelling lhe mysteries of how

new :ilc forms evolve. The native Imd biota of the H a w i i m Islands in- chdes 1,2(61,40t(species of plants, approximutely 8,000 species of insects, 1,000 species of land snails. anti over 100 species of birds (including many that h&e hecome cxrinct\. Amozitlo c x a n d e s of a d m & radiati& abound in these lslinds, but t ~ & / o f the o ~ h k g r o ~ ; ~ t h w e b&n s tcd~ed in the same way that the drosopllil~ds hwe been, r l u ! it, irom so n:mv d ~ t k r e n t ahpccls of binlow in a nluitidiscio~imarv team ~roiect. With the tremendous advances of . mole&har bidow in recen: ye&. of species t h t may h w e heen dilf ial t to s t d v renc!ici~lly for sunle reaton or another (for example duc lo dif<icuhy i n r?o;i'ng and mainti?ining in the la5or.z!ory, o r tlifficulty of ohtain- Ing adequilte sam?le tfzes from field populations) could also fit \\'i!iamcon's

description of why Hawaiian drosophilids are 'supreme' in the study of cvo- lutionarq phenomena. Much genettc information, especially from DNA analysei, Ean now be ohtained even from single specimens of tiny insect cnecias. But. what is ir about Hawai'i that makes the soectacular biota that ..r-..-..- ~ . , ~~

have evolved here zo biologically unique? First and foremost is the fact that the Hawaiian Archipelago is the most

isolated land mass on earth. situated in the middle of thevast Pacific Basin ~. - ~ . ~ ~ ~

with the nearest continent3! land mass about 2.200 mi (4,000 km) away. and the nearest tsland groups. about 1,000 mi (1,600 km). Because the Island?. arose from undersea volcanoes, they werenever part, of any continent, and their wative plants and animals arose from chance mgrants that survived the long journey across thousands of kilometers of ocean Thus, due to the infre- quency of arrivals from the same ancestral population, gene flow between the ancestral and the incipient population was effectively non-existent. Thts situ- ation gave these founding individuals a pre-condition for deslabilhation of the coadapted genetic system of the ancestral population formed over con- siderable evolutionarv time. This was followed bv reoreantzatton Into a new and, at least initially. H very dynamic genetic en&onm&t within the new uooulation Indeed. the rare individuals that reached the Hzwaiian Islands were lme oioneers. A ereat diversity of oDen niches was available as a result of dkfercnces in l t i tude. nroi&ure.'soils, and tou?grap!tv. Co!onists were oflcn opportunistic and able to aday to microewtronments unlike those in which their-ancestral stock evolved.

The flowerine of soecies did not occur iust on a binele island. The fact that each tsland di thc2Hawailan A r c h ~ ~ e l a ' ~ o was formed in chronologinl sequence and in cmgle f~ le h35 also plnyed an important role in the speciation ~ a i t e r n we observe in the native fauna~and flora; Punched thrott h the f ocean floor of the Pacific Plate from what is believed to be a sing e hot soot .~~~ ~-~ -

where volcan~c akivny ic most intensc, each of the Islands was ftrmed as'the Pacific Plate inched northwesterly over millions of years. A, each new Island aooeared to the southeast. chance colonists from oider Islands could become eiiablished and devehp pnp1a:ions which were genetically sliyhtlv dlffercnt from that of their progenitors, or ancestral forms. I t is estimated that nvre than 9% of the plants and animals in Hawai'i are hinple-island endemics; that is, few species are found on more than one island. Thn indiclrtes that, except for the nore vagile (dtspersih!e) hird species and plants w~th tiny a~r-borne weds, there is little actlve dispersal among ishnds. Even so, considering the available land mars of the present high islands (includin Kaua'i, N'ihau, O%hu, Moloka'i. Lanai, Kaho'olau)e, M ~ . I I , ard Haudiflotlay, the cumber of endemic species in most groups is great compared to continental standards. For exam le. nearly one-quarter of the species known worldwide in the family ~ r o s o ~ h i i d a e are endemtc to the Islands. Large numbers of closely related species forming single lineages that are derived from single founding events (one or a few individuals) are not unusual in the Hawa!ianfauna and flora.

Within individual Islands, special geological and cl!matological features presentedpopulations with an opportunity to dtfferent~ate into separate ye- netic entitles. On the older Islands, erosion formed dee valleys which sepa- R rated less dispersible populations from each other and t ,us prevented gene ex- change and fusion of the geographically isolated populattons. On the younger Islands (such as the island of Hawai'i), the formatton of kipuka (islands of vegetation in a sea of lava) also formed extrtnsic barriers to gene exchange between populations. Climatologically, rainfall patterns are much influenced

by topography in orographic rainfall p;ltterns. Warm, moist air is blown toward the Islands by,the prevailing northeastern trade winds. The result is hiph rainfall on the wltrduard orthe the as tern), and dry, desert-like conditions on the leeward (southwestern) side% of mountains a r d ltiphlands on the Islands. Thus, the iq!and of Hilwai'i pos,esses a range of habitats fron: sea level to nearlv 1~.W ft (4,300 m) in elevation. with some of rhe most dra- matic rainfaligradients in the world. These conditions create asituation where extremely different habitats for lants and animals can be found with- i n relitively short distances of one anotier. In their college-level textbook for introductory biology,,Raven and Johnson (1986) made the following state- ment about rapid cvolutron: "In general, rapid evolution takes place i n arcas where the Itabltats are varied and located near one another. The pace of evo- lution is accelerated further by rapid climatic change."

These extrinsic features of thc Ilawaiian biota have plaved a vital role i n formation ofncw species resultirrg from the founder effcct (Mayr 1963). be- lieved to be the printary rncchmistn of speciation in the native fauna and flora. A t least for suecles of HawaiinnDrosonhilu. and most likelv for other

~ ~~-~

cative groups, s~~tgl; fcrtili7etl propapules ( f h l i d ~ n ~ ind'vidwds) are helievcd to have been thc proeeniri~rs of new !wp~~lations on successivclg yourlger Islands as thev rore nbobc the s:~rfncr (,'the oceail m d hecame availnh:e for culoni7ution. ' O J ~ s t~d ies of tne Hawaiian Dro~op l~ i la indicate that d u r q the earlv stages of colo:rizat~on when the populnt~on size is small, natural and sexual wlection pessures that were pre5ent in the ancestral w~ul ; r t lon are relaxed, and yen& variants oreviot18lv eliminated or maintained at vew low level\ ;;re n& permitted to kcomhine in the ne\v population. Also, d d - ing the period when the ily population Gze is small. there might be sh:f:s in the mat.ng systern. favor'l~g females which are less selective in mate choice: females that are highly di,criminating may never encounter males ahk to sati,h their cwrrs t ip requirements. I h c to pleiotmpic (genes whicir p rw ducc more than otie effcct) effects on o!her scclltelttc of lne genome (chmn~o- some set). such shifts i n the matine svstem canhnve dramaticeffects i n - , rebui ld14 the geneticsystcnl of the ncw popdalwn ill response toextcrnal env'ronment:~l conditiuns. Thu,. II i s ndjustnrents witnin rlie "scxud environ- mcnt'of these pop.~l;trions that p13y an extrcntel!, :nrportunt role in directing the coursc rIt evolution in the Hawailan I)ruw,hlo. Allhough olher native organisna arc slro delic;ttely :td:iptcd to the d:rcrsity of hahitat3 in Hawaiian ecosystems. 2nd cxplos~vc adaptive radiation (that ic. genel:c adjustmen~s to the envlroorncntl IS e j ~ec~a l l v evldent in the I lsnaiinn h ~ o ~ a . i t 1s w r ~ e s t c d that %-a! seleciion n;:~v ha<c pli~yed an cvcn more im!wrtint role I%? u a l selectinn in the most dyn;~lnic slagcs of specics formntmn. The crewlv formed oo~ulotions continue to resoond i o the forccs of nalural as well as s c ~ t ~ u l sE~c'ction ill dc\clopiny suffi&nr geneticdiffcrencec from [he mccs- Ira1 population, untd WCII time 3s irreve-sible rc~rroduct:\.c isolation i s achlevid.

H a w i ' i h:ts oltcn heen referrcd to as the hcst "nutl~ral lahoratoe" fnr evolu~ionarv s!udies in tne uorld. As \Yilliams~m ~mpl:ed, cvolution:~ry studies o f I f:~ua;.i's native h i w ~ h:we heell e.\trcnlelv ;~wort;lnt :- devc l~~ r i l t r modem theories of speciation and evolution. Such <lud~es arc second to ion;. What is even rnnrc import;inl is the fact that I l w a i i n n ecmystems present a unique opportunity to investigate basic biological phenomena for so many different whdisciplines o i biology. However lhasic such research may he, the knowledge gained will he applicable to more mission-oricntcd research

oroerams. For exam~le . the work on the matine behavior of ilawaiian r - - o ~ ~ ~ ~-~ - ~

Dro~opAilu is nlreadih&n d direct impact onihe aeveloptnenl of more 7' effective control programs .or pest tephritid fnlit fly species (for example. the ~ e d i t e r r a n e i n fiuit fly Cehfifis c"pimta, the Oriental fruit fly Daws domnlir, and the melon f ly Dncuscucurbirue) in Hilwai'i. w h ~ l e minimizing the need to apply cxcesslvc amounts of insecticides. Mass release of sterile male riles can he used to reduce ~ o ~ u l a t i o n s . Results of research on the molecular gencrics of Hawai'j's riarive biota will no d o u h have far-reaching effects in medical wience as well as in genetic engineering and h;otechnology ocaericult~~ru! comtnndities. With the trcrnendntis 3dv3nces n l g e n e t : ~ encheerinc within the onst few vears. the crucial value of the e~<)rrnous bi610piral &versity in ~ i lu i l l ' i is 'hecdmq mcreasingly obvious (see,, , , , c . ,rmn, i h s vchtne). The single mo,t imporxnt remurce or H3wa1 I 1s ttr natlve hiom. The cnmmitmcn: of State and Federal :iuencies to implement stri~rgcnt conservation and managemenr policies is vitil to its preseivation,

Important References

Carson, HL 1982. Spcdatiun ns a mnjor rcorganiulion uf poIy,lyycnic balaner*. Pp. 411133 IN C. Barigozi (ed.). Mcrlrminnr ~JSpeciulim. Alan R. l k , Kcw York.

Canon, H.1- [this volume) Gcnc pml conrcnation. Kancrhiro, KY.; aad C.R.B. Buakc. I'B7. Scxual rclcclion wd specialion: issues raird b"

Hswitiian drosophilhls. TmoN ht Ecolop nnd E~:ol~rrion 2(?)117-212. Ma?,, E. 1G53. A,~imolSpcicr m8dErolrrrion. Hanwd Univ. PrGs, Cambridge.

Maswchurclls. Ravcn, P., and G.B. J o h m n . 1086. Biolcj.?. Timer Miirur/hlorb~ Cnllry Puhl. Co.,

Sr. Louis Miswuri. Willimwm, M. 1'81. Isla~tdPop~~lafion~. Orford Univ. Press. Oxford, England