-

use anal mis'

Ne'rl Yo¡k.

of the sof t -

102:243-282.

JJlooklt¿r'en

Vol . 106, No. 949 The Americ¿ur Naturalist May-June 1972

EDAPHIC ARIDITY AS A FACTOR IN ANGIOSPERM EVOIiUTION

D¡.Nr¡r, f. Axnr,non

l)cpaltrtrcuts of Geology :rnd J-lotany, University of Califolnia, I)avis, California 95616

Stebbins (1952) has sholvn that there are several reasons why planteYolution would be relatively rapid irr arid. to semiaricl regions. First,in areas where moisture is limited, diversity in local terrain, soil, andother factors has a greater effect on the flora and vegetation than inlegions where moisture is adequate. Second, semiarid climates with theirregional diversity promote the division of medium- to large-sized popula-tions into smalier units which are isolated from each other but can exchangegenes by occasional migration, and establish populations that may giverise to new t&xa. And third, in dry regions, many different specializedvegetative structures (e.g., reduced leaf size, specialized leaf covering

[scales, trichornes], deciiluous habit, deep root system, swollen trunks,bulbs, etc.) can evolve r'.'hich may enable plants to withstand periods ofsevere drought.

Stebbins invokecl considerabie topographic relief (e.g., Coast Ranges ofCalifornia), as well as aridity, to provide a diverse environmental frarne-rvorli for rapid. evolution. Fulthermore, hc also notecl that in clry regionsdivergent evolution is not necessarily limited to adaptations to xerophytismbecause some mesophytes, as in the genera Scorzonura, 'I'rapogtogon, Cassio,In,ga, Aacsal,pinía, Miruosa, and llatth,inia, appear to have been clerivedfrom xelophyt ic anccstors. I - Ie conchrd0s: "Tf the c l imate. . . is becoming

¡:rogressively rnore moist, lve can cxpect that some of the xerophytes withfavorable structure rvill becorne adapteil to rnoist couditiorts. In this\\'ay, tlte flola of thc samc mesophytic legioll could conlc to contain mem-bcrs of the s¿rme farnily or tribe, ali aclapted to esscntially similar condi-tions, but adaptcil in different ways and hence very differcnt frorn eachother because of their different evolutionary histories."

While I do not deny the efflcacy of topographic relief and climatic changein promoting rapid ancl divergent evolutiorr, there ale nonetheless alterna-tives to Stebbins's analysis which have not been discussed previously. Theseare suggested by the evolutionary potcntial lvhich may be inferrerl forIandscapes of crystaliine rock that provicled a¡id and semiarid edaphicsites throughout l\{esozoic and Cenozoic tirncs, nclt only in seasonally dryereas but in wet tropical regions as l 'ell.

EDAPIIIC DESERTS AND SDMIDESERTS

Tirc possibility that barren to semibarrerr areas ancl their immediateenvirons may have had an important evolutionary lole in angiosperm

3 1 1

312 TII I ' AMI. I ] i ICÁN NAI]URA.I , IST

evolution came to my attention on a recent visit to llrazil. There, high-

grade gneisses ancl granitic roclis of Archean age form barren to semibarren

tracts that are erlt jrely sull 'gunded by rvet tropicai rain forest' In adclit ion,

simila¡ tra,cts for:m the srrbstl '¿rte for the caa,tinga (tholn forest) of north-

easteln l lrazil, and they occur alsg in i¡terrnediate alcas whcre savann¿

vegetation is lvell clevelopecl. No one has previously raisecl for rliscussion

the evolutionary potential of such a lanclscape. It is of subcontinental

extent (Brazilian shield) ; is typified by persistent barren to semibarrcrr

bornhardts (inselbelgs), lorv hil ls, clomesr ridges, ancl rock plains; and is

found in diverse climates. li'urthermore, geologic evidence shows that simi-

lar landscapes of wide extent were preserrt on all the continents throughout

Mesozoie aud cenozoic times. 'lo set the background for the view that these

arid. to semiarid edaphic sites probably had an impoltant role in evtllution,

reference is macle first to ecological relations in Brazil, and then other

areas where similar terranes occur are noted briefly'

In the coastal parts of Brazil, where precipitation is high (over 2,000

mm, or 100f inches), regolith may be from 50 ft to over 200 ft deep

(Branner 1896, p.262). The transition to crystall ine basement is abrupt,

usually taking place witliin ¿r ferv rllillirneters. ]lecauso of the i[rpcrvious

nature of the basement, at times of torrential r¿rin the regolith on steeper

slopes may become saturated and unstable. As a I'esult' Iandslides occur

regularly and expose iarge patches of naherl clystalline b¿r.sement. These

persist for long periocls because the roeks are excecdingly hard and resis-

la,nt, and the prod.ucts of rock weathering ilre washed- away as quickly as

they form on the steep (or' vertical) slopes. Sugarloaf at Rio de Janerio

is a farniliar example, and ntany otirers in tire region are sirnilal, extending

north i]lto the drier parts of Brazil as lve]l as to t]re south. A visit to sugar-

Ioaf wil l clernonstr'¿rttt t lrat it is bnsicaily a "clcscrt islanrl" isolated in a

tropical jungle. sugarloaf is barren mouolith (bornhardt) of granite-gneiss

that supports a few specics of brorneliads, orchids, cacti, vellozias, and sim-

ilar petrophytes that attach in smail cr.eviccs. This exceeding)y xeric flora is

situated. only a ferv tens of feet frorn a tlense tropical rain forest composed

of. Anacard,iunt,, Asteroca,t'Aun1,, Bactris, Ilatuísteria, Caesalpinia, Cassia'

Cebia, Cecropia, Clt'orisia, C'íssampelos, Clusia, Covnbretium, Diospyros'

Ficus, IIelícinia, Inga, Jacaranda, Mi'mosa, M'imusoyts, Nactand'ra, Ocoteu,

Psgclr,ortia, Rol'lti'nia, Srtltindus, Simaruba, B'iparuna, and many others

(Mcl-.,ean 1919, p. 164-165). Above the level of the lowland rain forest

another type of forest commorrly rlevelops: it is more open, is composed of

smaller trees, and. I ives in a much drier (edaphically)'area. The lower

rain forest may gracle insensibly into it, though they are often sharply

separated because of the rapid change in soil depth as the slopes steepen'

In other parts of tlie nearby area' as on the flat-topped mountain Pedra

d.a vavea, .bhere are a more xeric type of forest and. associated tussock

grassland. They are isc¡Iated from the lower forest by a wail of inaccessible

|ranite-gneiss (Mcllean 1-919, p.6), and live in a cooler, more equable

climate.

The si¡throughnormalinteriorthe raintion, anddecreasecdevelopcrwhere cabroacl pleattingasumac, cdeciduouforrned.is exceeda hammeIligh, stiJanerio iplants e:iis trappeoccasiona

This arseasonal 'cipally trsuch rapno moist 'factor in,wise Jlre'S¿¡lvadorblankctcisoil rvhicon thc bfrain forclh i ) is of r !caa.tinga.the exactthe waterthe rvate:marily frforest to

ApaltterI'aneswp l l l f f o

rrrystaJlirrpr..diplaintimes, mr

&"

DDAPIIIC AITIDITY AND ANC;IOSPIjIiM llVOLUTION

The significance of dry edaphic sites becomes apparent as one progresses

through the region and realizes that they are not rare but make up a

normul part of the landscape. 1,'urthermorc, as one rnovcs to the drier

interior these sitcs beeome more flcqttcnt. .11't lr: exa.m¡rlrl, in coasta,l B¿hia

the rain forest receives about 1,000 mm (60-70 inches) yearly precipita-

tion, and the soii is very deep. About 20-30 miles inland precipitation has

decreasecl to 600-700 mm (30-35 inchcs), atrrl in that arca tlrere is a well-

developccl caatinga. The soil is scarcely 7-10 cm (3-4 inchcs) deep in swales

rvhere caatinga is dcnsest, b¡t is thinlter to abscnt on the slopcn of the

broad plains or on the crests of lor,v hills and ridges. In such areas the

cattinga is semi-open and inhabits a rock platform rvith bromeliads, palms,

sunrac, cacti, aca(lia, zizyphus, a¡tl similar plants-bgtlt evcrgr<lcrL anrl

ileciduous-growing out of roch crevices whele a little soil has lodged or

formed. The exposed rock is entirely fresh, has poor platy joirit ing, antl

is exceedingly harcl-emitting a sharp bcll-like ring wlten ¡rounclerl rvith

a irammer. The transition to soil is abrupt, taking place in scarcel.v 2 mm.

Ifigh, steep-sided borhardts that are counterparts of Sugarloaf at Rio de

Janerio arc scattererl in the area. They are bare and support onlv a ferv

plants except at their cooler (solttr-facing) bases r,vhere sufficient runof{

is trappecl in the gr:us and shallow soil to support a dense caatilga. or,

occasion¿rll.y, shoTt-trce forest.This arid flora is not due entirely to clecreased" moistttre irrlancl or to its

seasonal concentration. As Smith (1945, p.298) observed, it "is due prin-

cipaily to tlie impervious nature of thc frock] rvhich sheds t'¿rter rvith

such rapidity as to cause floods in pcriods of heavy rainfall and retains

no nroisture for fplants during] peliotls of r ' lrought." Th¿rt this edaphic

factor increases the degree of aridity far bcyond that rvhich lvould other-

rvise prevaii is consistent with relations observed along tlie roatl frorn

S¿lvador to Jacobina. l lhe coastal regign which suppolts rain forest is

blanlieted by Cretaceous and Tertiary sedirneritary rocks that have a deep

soil u'lrich is under intellsc cultivatitill. 'l'hc setlimentary cover laps out

on tlrc basement about 25 rniles inlancl. It is thcre that lelict patches of

rain forcst quicirly disappear, intense cultivation ceases, and the rolling

hil ls of Archcan basement rock with only a fcw mill imeters of soil support

caatinga. The change is abrupt ancl takes place lvithin scarcely a mile;

the exact transitiou is obscured now by centuries of land use. In any event,

the rvater-retaining properties of the sedimentary series, as compared. with

the Nater-shedding properiies of the crystalline basement, account pri-

marily for the marked. and rapid. change fi'om a mesophytic evergreen

forest to the caatinga, which is adapted to severe aridity.

Apart from this landsctrpe, which is rvell dr:r 'ulopecl in Brazil, similar

te.-¡anes occur elservilere in South Arucrica ¿rrtl on otltcr eontirtcuts as

rvell (f ig. 1) . These ancient landscapcs rvhich h¿rve been barved into old

crystalline loclis are lepl'esented rlow by dcfol'llctl and uplifted broad

pecliplains of earlier erosiott cycles. lTnti l colnparatively lecent geologic

tirnes, much of t l l is terl '¿ttre was lelatively lorv a,ltr l wa.s interluptr:cl bV

313

re, high-inibarrena.dclit ion,rli north-

savannaiscussion,rtittentalnibarrett1; and is

hat simi-roughouthat tltesevolution,en other

¡er 2,000ft deeP

j abriupt'rperviousr steePeries occurrt. Thesend resis-:ickly asr Jarrerioxtendingio Sugar-,ted in a

ite-gneissand sim-

ic flora is

somposed,,, Cassia,l iospyros,

t, Ocotea,iy othersLin forestnposed oflhe lowerr sharplYJ Steepen.rin Pedrad tussockaccessiblee equable

e l ^u a = THE AII]IIiICAN NATUITALIST

l¡tc, 1.-21. I)istril¡utio¡r of shieltl i:llcas ilr tho SotLtltcrrt I"Iemisphclc (fronr

KinS 1967). Ilxposures of younger bascment rocks ¿rc lot includcii' Edaphic

deserts ancl semideserts have been prese¡t in eaeh area du¡iug angiosperm

history. n, Distr.ibution of shield ¿].c¿s irl the Northclll I{crlisp}torc (from

Iting 1967). Exposures of younger bascrnent rocks not shorvn. Eclaphic deserts

and semirlescl.ts llave been present i}l cach area clul'ing rngiospet'm history.

croslon resrlomes, ridand had rLocally, hto those exSahara, tlAustralia.

Althougmetamorplrecailecl tlMountain)vcr:y aridIlcucheriaYucca, w)sen1ibarrelr'yherc thcous harcl$those fronalso probthave beena r n ¡ i n r q r

increasingcvolved (l

rocks.From h

cstablishcfloor sprecontinentntuaterl bylate Meso(1 ) Ju raIr¿Iurasiar

Gonclwani n o i n r ¡ p

wJiich mcto threomost extr(5 ) i n t cni r r o f n r l ¡

i l i ¿rl l art

rrrgul i t tc

cyclcs ¿iI ' r.o rn n r ¡ i n o

of b¿rselrr

ÍrYai lablc

EDAPI{IC ARIDITY AND ANGIOSPERM I]VOI'UTION 3l l¡

: ( f ron

0daphie

iospcrtu( f rom

aleserts, 'y .

erosion residuals of harcl basement rocl(s that formed scatteled bornhalctts,

d.omes, ridges, and. stripped. low surfaces that wele barren to semibarren

and. h¿rtL only thin soil (sec i l]ustrations in l( ing 1967; wil l is 1936).

I=rocally, higher mountains cornprised the cores of ancient ranges similar

to those exposetl lrow as thc Tibctsi and l{oggar Mourltains of thc sriuthcrrt

Sahara, the Guiana llighlaniis, and the Macdonnell Range of central

Australia.Although tire regions meiltioned ¿rc of allcicnt age, are¿ls of youngcr

rnetamorphic and granitic rocks also have dry edaphic sites' Here may be

recallecl the numerous granite-gneiss domes scattered fronr Georgia (Stone

l\{ountain) to Virginia at altitudes near 1,500-1'800 ft' They support a

l.er.)¡ aricl flora inclucling spccies of Alliunt,, Arenaria, Aster, Btt,Lltostylis,

Ileu,clteria, Hypericnw,, Paníutnt, L'oa, Opuntia, Sed'ttrtt, Yiguerüt, ancl

Yrtcco, rvli ich have lveste1'n 1a,thcr than castcrn ¿rffir l i t ics' Thcy l ivc orl

semibarren to barren granite-gneiss Slopes, chiefly of southerly cxposule)

\rhere they ¿rre within a ferv te[s of rncters of the rich mcsopllvtic decidu-

ous hardlvood forest. The dor¡relands of the soutlieu Sierra Nevacla, attd

those from central San Diego County southward into Baja Califoll ia, havc

.lso pr.obably ¡aii ¿r crit ical role in cvolutiort irr that I 'egiott bc<raltsc tltc\r

have been exposed since the iate c¡etaceous. rlL tlt is area, the trerlcl to

greater aridity during the Tertiary has selectecl p)ants able to rvithstancj

I 'c"r. 'nri,rg clro'g¡t. Jtt seems hig¡1y probabJc t¡at t¡. lrew t¿rx¿ rv¡ir¡

evolvecl ciid so in local clry sites provicle<l by exposecl slopes of basetrent

roclis.From his revierv of the landscapes of the earth, ancl as is now more firrnly

established on thc basis of ncw additional evidence which suppol'ts ocean-

f l o o r s p r e a i l i l r g , I ( i r l g ( 1 9 5 0 , 1 9 6 ? ) l r a s s ] i o w n t l r a t t h e p l a i n l a n t l s o f t l r econtinents reveal a history of rjenuclational ancl clepositional cycles purlc-

tuatecl by brief tectonic episocles th¿it have established ilew equilibra' Frorn

late Mesozoic to r.ccent t imc this history includcs (King 1967, p. 5l6-41 6)

(1) Jurassic-Early Cret¿iccous rviclespread planation (Gondrvarran alld

Laurasian) ; (2) partial planation during the Late Cretaceous (post-

Gondt'ana and post-Laurasian); (3) a long early Cenozoic cycle result-

ing in ver)' smooth planation tha't represent's the ancestral surfaee frorn

rvhich most of the world's scenery has been subsequently can'ed; (4) one

to three late cenozoic partial cycles, of which the earliest (Miocene) is

tnost cxt0nsive antl to rvhich rtrost of lJrc lvolld'Ñ sccncrv bclrlngs; a'n11

(5) intense Quaternary orogeny rvith valley cutting and glaciation' Allorv-

i ng fo r l oca l va r i a t i on , t hema jo rpa t te rno fcyc l i c l anc l scapcsseemss im i l a ri lr ¿rl l aleas an<l suggcsts a global ctlt l trol l iy tcctonism $'hich opcrates to

r:egulate lanclscape ¡y goo"r^irg base levels. Tn teuns of evolutirl¡r, these

c),cles are significaut because cleforru¿tion allcl uplift provide the basis for

Iernoving the overlying seilimentarV forlriations, thus exposing new tracts

()f baserne[t ter,r.ailr t lrat rrray bc }t,gariled as ¡l ionetlr areas l ' ]r ich beconc

avl i la l l lc for p larr t occupal . io t t ant l cxplo i t r t t io t r '

316 THI] AMEI],ICAN NATURALIST

EVOIJUTIONABY R,OI,E OF EDAPIIIC AR'IDITY

The geologic and geomorphic eyidence reviewed by lfing (1967) makesit abundantiy clear that dry edaphic sites composed of hard. crystallinerocks have been available for plant occupation throughout angiospermhistory in tropical, ternperate, ancl polar regions (fig. 1). Ir'ulthermore,nelv sites have become available as lands were elevatecl ancl stripped ofsedimentary cover. These areas were exposed graclually and in a patchlikemanner, resulting in a se¡ies of disconbinuolls baserrrent sites prior to thefinal stripping of the entire surface (see illustration in Willis 1936). Onother occasions large areas of basement have been flooded by seaways orburied under extensive floodplains, thus eliminating sites and. many of thetaxa restricted to them.

The presenee of dry edaphic sites of crystalline basement would favorall the positive features that tend to accelerate evolution under aridity, aselaborated by Stebbins (1952). Moderate changes in local relief wouldaccentuate microclimatic differences wherever barren to semibarren sur-faces rve¡e present, antl rvould favor the juxtapositiorr of divergent popula-tions. fn furms of adaptation to clrought, the gradients of aridity in theseedaphic dry areas would be comparable with rain shadows discussed earlierfrom the standpoint of rapid evolution (Axelrod 1967).'We can visualizea series of isohyets around each dry edaphic area, grading from the bo¡ders(humid-subhumid) to more xeric (scmiarid-arid) conditions toward thecenter. fn each major climatic zone, therefore, edaphic sites ¡rere availableto taxa adapted to all grades and intensities of rlrought, as weII as to theaccompanying thermal differences (hot, warm, mild, cold) that oecurredwith changing altitude and latitude.

Differences irr terrain, soil depth, anil barren to semibarren sites wouldtend to disrupt spatial relations of popuiations and lead to the emergenceof discrete units. In this connection, it is noted again that areas of crystal-line basement providing arid sites would. regularly have a spotwise orpatchy distribution during stages of uplift and unroofing of overlyingsedimentary rocks. This rvould result in disrupted. populations and leadto isolation in areas of slightly or mod.erately different climatic conditions

owing to differcnces in local reiief ancl alt itude. Furthermore, differencesin populations with respect to their inherent adaptations to a moderated.egree of drought would. be expected in some cases to have high selectivevalue for further adaptation to more arid conditions-to live in localdeserts anil semideserts provided by these edaphic sites.

The topographic, climatic, and edaphic (arid) diversity in areas ofcrystalline temane, together with local discontinuities in outcrop, seemsadequate to explain the fragmentation of populations and the ernergence

of incipient new t¿rxa resulting froru isolation. Ncr,v populations that were

adapted to live in diverse ways untler conditions of someu'hat greater

aridity would norv be able to cleploy into nearby edaphicaliy arid sites

rvherever there 1l'cre local differences in microclimate, iletcrmined by their

positionbaruen riancestralmay be rwith distrmarshes,I{owever,the normrrelativelyarray ofrelativelyof the flcgreat div

The pr,such as t]tion of pcsemia,ridlogical er,late Tert:there, to ;India, Afthe ro le rclimates rTransverrelevatecl thas beenfnasmucbephemeraand sumrthe clc¡se rranean clopportun.

The fo:climatic rclimates.mesophyttinuallyevidence.to the climoister a:is that w(and warmclister aadapt tocessfullyoccu r wi t

67) makescrystallinerrgiosperm;:thermore,,ripped of

¡ra,tchlikeior to the1936 ) . On

eaways or

rny of the

ruld. favorrlidity, as

ief wouldrrren sur-rit popula-y in theseled earlierr visualize're borders¡ward the: availableas to the

. occurred

tes would3mergencerf crystal-,otwise oroverlyingand lead

conditionslifferencesmoilerate

r selective: in local

areas of'op, seemsernergencethat wereit greater

arid sitesI by their

EDAP}IIC AIT,IDITY AND ANGIOSPERM EVOLÜTION q 1 n

position rvith respect to bornhardts, domes, koppies, and semibarren to

barren ridges or slopes as compared with those in adjacent areas where

ancestral or ¡elatecl taxa lived. 'Ihese semixeric and xeric eclaphic sites

may be regariled as pioneer areas, comparable in an evolutionary sense

rvith disturbcd areas (roadsicles, old ficlds, talus slides, riverine floodplains,

matshes, etc.) where evolution of diverse taxa is proceeding rapidly today.

flolr,ever, nerv tax¿ in the latter areas tend to disappcar quickly through

the normal lllocess of plant succession. J3y contrast, arid edaphic sites are

relatively persistent and hence would become well stocked with a great

array of taxa that ileployecl into them from adjacent more mesic (though

relatively tlry) areas. That this has happened is apparent from the richness

of the floras of dry tropical regions, including the desert, and from the

great diversity of adaptive types that live there''Ih.e precccling evidence suggests it is not necessary to call on high relicf,

such as the Coast Ranges of California, to play a guiding role in the evolu-

tion of populations during most of angiosperm history. Actually, the largest

semiarid. regions have always been in the subtropics (savannas), ancl geo-

logical evidence shorvs that relief there was not gleat from Mesozoic into

Iate Tertiary time (I(ing 196?). Furthermore, evolution did. not stagnate

there, to judge from the rich savanna flora of central America, the Guianas,

India, Africa, Brazil, ancl Australia. There is much eviclcnce to show that

the role of topographic diversity in promoting e'i'olution in the semiarid.

climates of California is rel¿rtively reccnt: the Coast Ranges as well as the

Transverse Ranges, the Peninsular ll,anges, and the Sierra Nevada were

elevated chiefly during the Quaternary. Since then, evolution in this region

has been primarily among herbaceous groups and at a loltr taxonomic level.

Tnasmueh as the Mediterranean climate to which they are adapted is

ephemeral, most of these taxa will probably disappear as the oceans warm

and summer rains return (Axelrod 1971). Nonetheless, it is evid.ent that

the close colrelation between mountain building, the appearance of Mediter-

ranean climate, and rapid evolution of herbaceous taxa demonstrates the

opportunistic nature of evolution, as Stebbins has stressed.

The foregoing evidence also inclicates that it is not necessary to invoke

climatic change as the stimulus for readaptation of xerophytes to moister

climates. In the first place, since the taxa that have readapted to more

mesophytic conditions are of diverse ages, we would have to call on eon-

tinually fluetuating climate deep in the tropics-for which there is no

evidence. While Some minor readaptation may have occurred in response

to thc climatic fluctuations of the Qnatcrnary, eviclencc of readaption to

moister aleas at that time has not been documented. The point to be stressed

is that while reaclaptation could occur in response to a trencl to moister

(and rvarmer) climate, retrclaptation coulcl ¿lso ottct.tt ' witlrout it. Since

moister areas are always adjaceut to eclaphically dry sites' taxa cou}cl re'

adapt to them as variable populations appearecl which could compete suc-

cessfully in moister sites in these landscapes, irt whicli arid edaphic deserts

6ccttr rvit lr it l a Stone'S throrv of I 'airr fol 'r. ls1,. Ftlrtherlnot'e' aS new el'osiort

318 TIIE AMERICAN NATURALIST

cycles commenced and splead wideiy acr.oss continents, numelous areas of

ancient clystalhne terranes wele continually exposecl for occupa,tion by

plants. such areas occurrecl in wet tropical areas, savanna regions, clry

l,emperate )ntit,ttr1es, ¿ltrcl cooler antl moistcr regions. ' l 'he opportunititrs hcrc

have at least in palt been pl'ovicled by a "restless earth," exposing the

basement of the sialic crust.

Tire ¡lreceding rclations also lr¿n'c an important bcarirrg on soll lc problcrns

raised. by early angiosperm evolution. As suggested earlier (Axelrod 1970,

p. 292), they evident)y evolved in the Arctrean terrane in the uplands of

the rvarmer parts of Gond¡vanaland. Climates were seasonally dry, as dem-

onstrated by saline deposits ancl aeolian sandstones fr:om the surrounding

Iowlands, and by geologic evidence which shows that arid edaphic sites

occurred throughout the areas of Arehean crystalline terrane. Tn view of

the diversity of relief, climate, and edaphic conclitions, carly angiosperm

populations would naturally be broken up into diverse scattered gloups.

Different adaptive mocles, defined by the nature of the root system, water

requirements, leaf size, leaf thickness, a1cl numero¡s other adaptive fea-

tules, can result in selection for very clifferent conditions in localized areas'

As a result, many very divergent taxa probably could arise in a brief time'

some of these may haYe been unique adaptive typcs-and it is infelred that

some have persistecl in scarcely modifiecl form, as exemplif ied by members

of the Aizoaeeae, caetaceae, crassulaceae, Didieraceae, Fouquieriaceae,

Euphorbiaceae (tl ibe Euphorbicae), and many others'

As Gondrvanaland broke up during middle cretaceous and later times.

there was a general trend to moister, milcler climate as the ratio of sea to

land. surface increasecl and as the Cenomanian seas spreacl widely' Somc

taxa that hacl evolvecl in drier areas probably then adapted to moister cii-

mates and retained some of their ancicrrt features even though they no

Jonger had "need." for them. As Stebbins has noted (1952, p'43), in dry

areas leaoes of ISatthinia with sepalate leaflets appear to be more prirnitive

lhan Ba,uhinia .rvith bifid lalge leaves in the rain forest-and large-leaved

(derivedg) species of Bau,hinia are iu rocks of Cenomanian age' Mcljean

ifSfO, p. 167) also com'retrted. o' the atiornalous occu'"ence of C'h'orisia

(thorny trunk and branches) in t.he rain forest near Rio de Janerio' and' chorisia may also haYe ailapted to moister areas during the cenomanian,

though the fossil record does not yet provicle eviclence for it. Although a

trend to increased rnoisture woulcl be effcctive in promoting a shift from

dry to moister envirouments, readaptation also may have occurred. when-

ever suitable combinations of new aclaptive features (physiologic' morpho-

logic, or biotic) arose, for humid and dry sites may cxist sicle by side in

ancient crystalline telranes.

As moister clinratcs sprcacl rviclcly clu¡i¡g Ce¡otnania¡ ¿lrf l latcr t irnes'

nllmerous taxa that lr¿rd been adapted to drought no cloubt became extinct

as their ¿rreas tended to shrirLk ancl disappear' But some appear to have

¡lersistetl, arrtl they rl l¿I)¡ haYe r-lt lr ic so irr t l le drv e<laphic sites which were

¡,idespread througho¡t lorver to mitft l lc ]atit¡r1es, ÍLl 'eas wltel 'c these unique

ItD

forms still oc<Cretaccous antthan they arethcn suppoltct)Baja Californithe Kalahari.areas, anil in 1probably rvereclomes, ancl ri'climates begantaxa in these aexpanding dry

Ancient terrsided mountaidry edaphic sienabiing edapsavanna ancl rmates.

Similar dryhistory. Theirmoisture havethe origin of r

These ariclreadapted to ras in Cenomallocal refugia ipersisted. to th

Inasmuch. aithe precedingthe fossil planthat are alreaadaptive typeiedaphic sites r

'Ihe presentmy study of NNational Scierl ived in volcaexplain some

The manus<Jtavcn, a.rrrl G.

ous areas of

cupation bY

regions, dry.unities here

xposing tl ie

nc problemsrelrod 1970,

uplands o{

lry, as dem-

surroundingdapliic sites

ln vlew or

angiospermrred grouPs.'stem, water

rlaptive fea-

alized areas.

l brief t ime.

nferred that

by membersrquieriaeeae,

lnter t imes,lio of sea to

irlcly. Somemoister cli-

rgh they no

43), in drY

re primitiveIarge-leavedqe. l\{clieanof Clt'orisia

Tanerio, and

)enomanian,Although a

r shift from

ulreil when-gic, morpho-e by side in

later times,

came extinctrear to have

rvhich were

these uniqlre

EDAP}IIC AIIIDITY AND ANGIOSPERM ]]VOLUTION 319

forms still occur today. The fossil record shows that during the Late

Cretaceor:s ancl early Tcrtiary, the ¡trcsent rlry regions 'wele more moist

than they are today (Axelrocl 1950). Alcas of the present tropical cleselts

thcn supported forests, a,s inrjica,tcd liy tlrc fossil recortl in sotltllcrn Alizona,

Baja California, coast-ccntral l'eru, rvcsl,ct'n Tndia, the Saharan region, and

the l(alahari. I)uring tle Late Cretaceous ancl early Tertiary in these

arcas, and irr borclering lcgions as wrrl l, plants of rrrortl arid rcquirenrcnts

probably were confined chiefly to clrier eclaphic sites provided by hills,

d.omes, and ridges composed of haril, rcsistant crystalline rocks. As clry

climates began to expand following the carly Eoeene, the rclict xeromorphic

taxa in these arid sites, as well as ne\\, taxa, could thon spread out into the

expanding dry areas.

SUT[I[ARY

Ancient terranes of granitic and tnetamorphic rocks regularly form steep-

sidecl mountains (bornharclts), domes, ridges, and plains. They provicle

dr¡' s¿l-ph'c sites in elimatic regio¡s ¡artging froln wet tropical to rlesert,

enabling eclaphic deserts to occur in proximity to tropical rain forest,

savaltrta arrcl otl ier types of vegetation, includirrg tlrr.¡se in temperate cli-

mates.Similar dry edaphic sites have becn present throughout angiosperm

histoly. Their sc¿ittered clistribution arrd steep gradients of decreasing

lloisture have proviclcd environmcntal opportunitics that have cncouraged

the origin of taxa adapted to varying graclcs of drought.

Tlrcse at.icl sitcs uray ¿lso havc scrvctl ¿ls loci frorn wlrich sotno t¿txa

reaclapted to nearby moister sites. IJuring pcriods of climatic amelioration,

as in cenomanian ancl later times, dry eclaphic sites appear to have been

local refugia for unique taxa adapted to clrought whose ilcccnclants have

persisted to the present.fuasmuch as evolutionarv rate woulil tend to acceler¿te in local clry sitcs,

the preceding lelations nlay account fol thc rarity of "missing l inks" in

the fossil plant record, for the "sudden appearance" in the record of taxa

that are alread"y flily eyolved, a¡d for the occurrence today of unique

adaptive types that h¿l,ve no fossil record becausc they developed in rlry

edanhic sites remote from areas where most of the fossil recorcl accumulated.

ACKNOWLIIDGMITNTS

The present pa,per is an outgrolvth of problems of paleoecology raised by

my study of Miocene floras in Nevada, a resealch project supported by the

I.{ational Science }'orurdatiou (grants GB 7480' GI} 19844)' Those flor:as

lir,ed in volcanic terranes that had dry edaphic sites, which appear to

explain some of the unusual paleoecological relations there'

i.h. *ur,orcript has been revierved crit ically b.v l)otr¿ld l iyhos, Prrl,e¡

Ravc.n, antl G. Tredyat.cl Stebbins, to \vh<lln thanhs ¿tre extclt<]cd.

a - a

320 THE AM]IIIICAN NATUIi,ALIST

LITER,AIIURE CITED

Axelrocl, D. I. 1950. nvolution of clcscrt vcgctation in wcstcln NortlL Arnorica. Carncgic

Inst. Washington Pub. 590 :215-306.1967. Drought, tliastrop)risnr, anrl qua,ntunr evolution. .llvolution 21 :201-209.

1970. Mesozoic paleogeography and early angiospelm history. llot. llcv. 36:

277-3L9.1971. Eisto¡y of the Mediterranean ecosystem in califoilia. 1¿ IL Mooney [etl..],

Convergence in st¡ucturo of ccosystcms in Meditcrl'¿nciln climatos. Springer-

Yerlag, Ber)in (in press).Branner, J. C. 1896. Decomposition of r.ocks in Brazil. Geol. Soc. Amer. Bull. 7 :255-314.

King, L. C. 1950. The study of thc worltlts plainlancls: a new approach in geomor-

phology. Quart. J. Geol. Soc. T,onclon 106:101-127.1967. The rnorphology of the earth: a stualy antl synthesis of worltl scenel'y.

Oliver & Boycl, Lontlon. 699 P.Mclean, R. C. 1.919. Studies in the ecology of tropical rainforest: with speeial refe¡e¡cc

to ths forests of South Brazil. J. I'col. 7:5-54, I2l-172'

smith, L, B. 1945. The vegetation of Brazil, p, 297-902. 1¿ F. Yertloorn [ccl.], Plants

and plant science i:r Latin Ame¡ica' Chronica Botanica, Waltha^rn, Mass'

Stebbins, G. L. 1952. A¡idity as a stimulus to evolution. Amcr' Natur' 86:33-44'

willis, B. 1"936. East African plateaus ancl rift valleys. carnegie Inst. Washington Putr'

470. 358 p.

Vol. 106, No.

SEX R,AT

The topof attenticording toselectivemales anciparents (e.apparent,1 9 6 1 ;Osorio,burt 1969;Wildish I

Accordinaturaltwo sexes.progeny obe thenote thatcreate a d.ias differenperio¿l of

lreigh (the periodfemale bifactor toRestrictedmigrationhave all btlie sex

Thepr0xl

openrngs,compre1968; Osor

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)