REPORTSC. P. McKay, S. C. Martin, C. A. Griffith, R. M. KellerIcarus 129, 498 (1997).J. R. Holton, An Introduction to Dynamical Meteorol-ogy (Academic Press, San Diego, CA, 1992).E. Lellouch et al., Icarus 79, 328 (1989).The LFC for supersaturated conditions in the mid toupper troposphere, as suggested by Courtin et al.(27) and Samuelson et a!. (28), significantly exceedsthose for saturated conditions. At 140% ambientsupersaturation, the LFC lies at 13 km altitude for aparcel of 60% humidity starting at the surface.M. AWal, J. L Lunine, Geophys. Res. Lett. 21, 2491(1994).The entrainment of local air breaks the conservationof 0e for the rising plume. The increasing mass of a

1 dm aplume parcel, m, is parametrized as =

In popular metaphor, landslide processes be-gin spontaneously and gain momentum asthey proceed, but what determines how reallandslides move? Can small differences ininitial conditions cause some landslides toaccelerate catastrophically and others tocreep intermittently downslope? The distinc-tion is important because rapid landslidespose lethal threats, whereas slow landslidesdamage property but seldom cause fatalities(1).

A longstanding hypothesis holds thatlandslide behavior may depend on initial soilporosity, because soils approach specific crit-ical-state porosities during shear deformation(2-4). Tests on small soil specimens indicatethat dense soils (initially less porous thancritical) dilate as they begin to shear, whereasloose soils (initially more porous than criti-cal) contract (5-7). Dilation can reduce porewater pressures and thereby retard continueddeformation by increasing normal stressesand frictional strength at grain contacts,whereas contraction can increase pore waterpressures and thereby reduce frictional

'U.S. Geological Survey, 5400 MacArthur Boulevard,Vancouver, WA 98661, USA. zU.S. Geological Survey,345 Middlefield Road, Menlo Park, CA 94025, USA.'Department of Geological and Atmospheric Sciences,Iowa State University, Ames, IA 50011, USA.

following laboratory simulations (29) in studies ofclouds in Earth's tropics (30). Observations of terres-trial clouds indicate a value of a = 0.2 and range ofplume radii of r = 0.6 to 4 km.

25. The presence of rain agrees with the paucity ofnucleation sites available in Titan's atmosphere(Table 1). In the terrestrial atmosphere, the initialsize of a raindrop is roughly determined by thenumber of drops and the mass needed to lower asupersaturated atmosphere back to saturation. Theexistence of only a few nucleation sites bringsabout few cloud particles and large particle sizes.The only known source of nucleation sites on Titanis its haze, the number density of which is severalorders of magnitude smaller than typical terrestrialvalues. Considering the low number density ofnucleation sites compared to the high methaneabundance, Toon et a!. (31) realized that particles

strength (8-10). Positive feedback betweenfrictional strength reduction and soil contrac-tion may cause some landslides to transforminto liquefied high-speed flows (11-13).

To isolate the effect of initial soil porosityon landslide style and rate, we conductedlarge-scale experiments under closely con-trolled conditions. In each of nine landslideexperiments, we placed a 65-cm-thick, 6-m3rectangular prism of loamy sand soil (Table1) on a planar concrete bed inclined 31° fromhorizontal and bounded laterally by verticalconcrete walls 2 m apart (Fig. 1). The down-slope end of each soil prism was restrained bya rigid wall, which ensured that deformationoccurred at least partly within the soil mass(rather than along the bed) and that landslid-ing included a rotational component.

Different methods of soil placement yield-ed different initial porosities. The highestporosities (>0.5) were attained by dumpingthe soil in 0.5-m3 loads and raking it intoposition, without otherwise touching its sur-face. Lower porosities resulted from placingthe soil in 10-cm layers parallel to the bedand compacting each layer with either foottraffic or 16-Hz mechanical vibrations thatdelivered impulsive loads of -2 kPa atdepths of 10 cm (14). After placement ofeach soil prism, we determined porosities byexcavating four to nine -1-kg samples at

must grow to millimeter sizes to reduce severalpercent supersaturation to saturation conditions.

26. R. M. Goody, Y. L Yung, Atmospheric Radiation, The-oretical Basis (Oxford Univ. Press, Oxford, 1989).

Z7. R. Courtin et al., Icarus 114, 144 (1995).R. E. Samuelson et al., Planet. Space Sci. 45, 959(1997).H. Stommel, J. Meteorol. 4, 91 (1947).J. Simpson, G. W. Brier, R. H. Simpson, J. Atmos.24, 508 (1967).0. B. Toon, C. P. McKay, R. Courtin, T. P. Ackerman,Icarus 75, 255 (1988).P. Gierasch, R. Goody, P. Stone, Geophys. Fluid Dyn. 1,1 (1970)C.A.G. and J.LH. are supported by the NASA Plane-tary Astronomy Program under grant NAGS-6790.

18 July 2000; accepted 21 September 2000

various depths and measuring their volumes,masses, and water contents (15). No system-atic variations of porosity with depth weredetected.

Our suite of landslide experiments includ-ed individual tests with initial porosities rang-ing from 0.39 ± 0.03 to 0.55 ± 0.01 (± 1 SDsampling error for an individual experiment).Ancillary tests of the same soil in a ring-sheardevice and triaxial cell produced dilativeshear failure when initial porosity was Ls0.41and contractive shear failure when initial po-rosity was ^0.46 (Fig. 2 and Table 1). Land-slides with initial porosities that bracketed therange from 0.41 to 0.46 were therefore ofgreatest interest.

Landslide motion was measured with twoground-surface extensometers and 17 or 18subsurface tiltmeters arranged at depth incre-ments of -7 cm in two vertical nests (16).Pore water pressures were measured with 12tensiometers and 12 dynamic piezometers ar-ranged in three vertical nests at depth incre-ments of -20 cm (17) (Fig. 1). Data fromeach sensor were logged digitally at 20 Hz forthe duration of each experiment.

To induce landsliding, soil prisms werewatered with surface sprinklers and throughsubsurface channels that introduced simulat-ed groundwater (Fig. 1). Rising water tableswere kept nearly parallel to the impermeablebed by adjusting discharge from a drain at thebase of the retaining wall. Although prelim-inary experiments indicated that differentstyles and rates of water application influ-enced the onset of slope failure, this influencebecame negligible as failure occurred andinstigated changes in soil porosity (18, 19).

Landslides with differing porosities dis-played sharply contrasting dynamics (compareFigs. 3 and 4). Each of four landslides withinitial porosities >0.5 failed abruptly and ac-celerated within 1 s to speeds >1 m/s. Thesurfaces of these landslides appeared fluid andsmooth, and data from dynamic piezometersconfirmed that pore water pressures rose rapid-ly during failure and reached levels nearly suf-ficient to balance total normal stresses and liq-uefy the soil (Fig. 3). Three landslides with

Acute Sensitivity of LandslideRates to Initial Soil PorosityR. M. Iverson,' M. E. Reid, 2 N. R. Iverson,3 R. G. LaHusen,"

M. Logan,' J. E. Mann, 3 D. L. Brien2

Some landslides move imperceptibly downslope, whereas others acceleratecatastrophically. Experimental landslides triggered by rising pore water pressuremoved at sharply contrasting rates due to small differences in initial porosity.Wet sandy soil with porosity of about 0.5 contracted during slope failure,partially liquefied, and accelerated within 1 second to speeds over 1 meter persecond. The same soil with porosity of about 0.4 dilated during failure andslipped episodically at rates averaging 0.002 meter per second. Repeated slipepisodes were induced by gradually rising pore water pressure ; Id were ar-rested by pore dilation and attendant pore pressure decline.

www.sciencemag.org SCIENCE VOL 290 20 OCTOBER 2000 513

Tittmeter Dynamicpiezometer

Tensiometer

Sprinklers

r

Retainingwall

WaterSensor -channels nests

r Extensometeranchor

1 mi. Drain

1 mFig. 1. Schematic longitudinal cross section oflandslide experiments conducted at the U.S.Geological Survey debris flow flume, Oregon.The magnified ellipse depicts the positioning ofsensors in vertical nests.

.'recursory period46.33 minutes

2

Failure period3 seconds

Downslope displacement of ground surface

E0

0.

0

80 ■

Downslope rotation of tiltmeters at eight depthNumbers denote sensor depths (cm)measured vertically

ED_40

•_ 0

Occ

-40

1000 2000 2780 2781 2782Time since water appl cation commenced (s)

40o 2783

initial porosities indistinguishable from the crit-ical porosity (0.44 ± 0.03, 0.44 ± 0.03, and0.42 ± 0.03) displayed inconsistent behavior,including slow slumping of a single soil block,episodic slumping of multiple blocks, and mod-erately rapid (--0.1 m/s) slumping that ceasedafter <0.5 m displacement. Dynamic piezom-eter data from these experiments revealed acomplex mix of dilative and contractive soilbehavior during failure. The landslide with thelowest and least variable initial porosity(0.41 ± 0.01) displayed the clearest dilative soilbehavior as it underwent slow episodic motion(Fig. 4). Our attempt to induce a landslide withstill lower porosity (0.39 ± 0.03) ended un-eventfully because we could not impart porewater pressures sufficient to trigger slope fail-ure (20).

Figure 3 illustrates how landsliding ofloose soil (initial porosity 0.52 ± 0.02) canlead to rapid acceleration in the course ofonly 1 s. After about 2400 s of precursorysprinkling (with no groundwater inflow),positive pore water pressures developedfirst near the concrete bed and thereafter atshallower depths as a water table accretedvertically at rates -0.05 cm/s. This wettingcaused soil compaction, evidenced by aslight downslope rotation of tiltmeters at alldepths, downslope surface displacement ofnearly 10 cm, and vertical surface settle-ment of about 2 cm. As a consequence,average porosity declined to about 0.49, butthe soil remained looser than critical. Thesoil developed no surface cracks or othervisible signs of instability during this pre-cursory period.

Failure of the loose soil began at about

REPORTS2781 s (Fig. 3), when tiltmeters at all depthsbegan to rotate slightly upslope and porewater pressure heads below the water table(at depths of 50 and 67 cm) began to rise atrates >10 cm/s as a result of soil contraction.Visible slope rupture commenced about 0.5 slater (2781.5 s in Fig. 3), accompanied byrapidly accelerating surface displacement, di-vergence of tilts at different depths, and con-tinuing pressure-head rise. In about 1 s, pres-sure heads at 30-cm depth increased from

Fig. 2. Behavior of the loamy sand (Table 1) in aloose state (initial porosity 0.46) and dense state(initial porosity 0.41) when subjected to defor-mation in a ring-shear device (23). The deviceimposed shear displacements at 2 cm/s underconstant normal loads of 10 kPa and permittedfree drainage of water from the top and bottomof 7-cm-thick saturated soil specimens. Underthese conditions, measured shear stress is a sur-rogate for effective soil strength. Loose soil (A)contracted monotonically during shearing, re-sulting in decreased porosity, transiently elevat-ed pore pressure, and no peak in effectivestrength. Dense soil (B) initially dilated duringshearing, resulting in increased porosity, tran-siently reduced pore pressure, and a prominentpeak in effective strength. The porosity of thedense soil subsequently declined in response tobreakage of soil aggregates (24). Triaxial unload-ing tests using a protocol described in (7) alsoproduced contractive behavior for porosity 0.46and dilative behavior for porosity 0.41.

zero (atmospheric) values to hydrostatic val-ues (-30 cm) as soil contraction forced thewater table upward. Even larger increases inpressure heads at depths of 50 and 67 cmindicated that an upward head gradient devel-oped, which promoted soil liquefaction. By2783 s (Fig. 3), pore water pressures hadpeaked and declined as the soil containing thesensors approached the retaining wall,thinned, and began to spill over.

The divergence of subsurface tilts from

0.48

0.46

6

4

a.

A Loose soil

%re Shear stress

2 0.44 2o.

Porosity0.42 0 07

a.0.40 -2 0a0.46 6 -o

coDense soil0.44 4

a .• Shear stress ... .0.42

0̀a.-. Porosn.,.• Pore pressure

2 oa

0.40 0

5 10 1520.380

Shear displacement (cm)

Fig. 3. Data recorded ina landslide experimentwith loose soil (initialporosity 0.52 ± 0.02).To reveal details of be-havior during the 3-sfailure period, the timeaxis is expanded 927times. All sensors wereinitially positioned 2.3 mupslope from the re-taining wall (ellipse inFig. 1). Different sen-sors measured porewater pressure heads inthe precursory and fail-ure periods (17).

514 20 OCTOBER 2000 VOL 290 SCIENCE www.sciencemag.org

Precursory period244 minutes

Failure period15 minutes>4-

2 Downslope displacement of ground surfaceLetters denote slip episodes

A B

EcEC)

o.

a

Fyy

Pore-water pressure head at four depthsNumbers denote sensor depths (cm)measured vertically

80

6750

3010

tensiometer data dynamic piezometer data

80

-40

5000 10000 14640 14940 15240

15540

Time since water application commenced (s)

-400

2781.5 to 2782.5 s (Fig. 3) provides clues tothe kinematics of slope failure. During theprevious 1 s, tiltmeters at all depths rotatedslightly upslope (negative tilts), but at around2781.5 s, tiltmeters at depths ^45 cm beganto rotate rapidly downslope. This change inrotation coincided with a rapid increase indownslope landslide translation and likelyresulted from drag due to sliding along theconcrete bed. At the same time, accelerated

Fig. 4. Data recorded ina landslide experimentwith dense soil (initialporosity 0.41 ± 0.01).To reveal details of be-havior during the 15-min failure period, thetime axis is expanded16 times. All sensorswere initially posi-tioned 2.3 m upslopefrom the retaining watt(ellipse in Fig. 1). Differ-ent sensors measuredpore water pressureheads in the precursoryand failure periods (17).

Property (method or definition)

Mean texture (weight %)(wet sieving and sedigraph)

Initial moist bulk density (g/cml[excavation method (15)]

Initial water content(water mass/solid mass)

Initial porosity(1 — dry bulk density/2.7)

Hydraulic conductivity (cm/s)(permeameter tests*)

Hydraulic diffusivity (cm2/s)(drained compression tests*)

Friction angle at failure (degrees)(triaxial unloading tests*)

REPORTS

upslope rotation above a depth of 45 cmprovided evidence of superincumbent rota-tional landsliding. Despite this complex fail-ure geometry, pore pressure responses at alldepths indicated a relatively consistent pat-tern of soil contraction, which enhanced land-slide acceleration.

Data from the experiment with densesoil (Fig. 4) imply a failure geometry sim-ilar to that of the loose soil but reveal

markedly different landslide dynamics.Precursory pore water pressures necessaryto trigger failure of the dense soil devel-oped relatively slowly [owing to relativelylow hydraulic conductivity (Table 1)], andwere roughly twice as large as the porepressures necessary to trigger failure of theloose soil [owing to high peak strength ofthe dense soil (Table 1 and Fig. 2)]. Onaverage, motion of the landslide with densesoil proceeded about 300 times more slow-ly than motion of the landslide with loosesoil (compare Figs. 3 and 4).

Failure of the dense soil occurred episod-ically. At about 14600 s (Fig. 4), visiblesurface cracks and subsurface tilts of severaldegrees began to develop. At about 14,880 s(episode A in Fig. 4), visible landslide motionand soil dilation gradually commenced, witha consequent —10-cm decline in pore waterpressure heads. This initial episode of slowmotion lasted >100 s but produced <0.2 mof downslope surface displacement. Subsur-face tilts during this episode exhibited thesame divergence as in the loose soil experi-ment; deep tiltmeters rotated downslope asthe basal soil began to translate, whereastiltmeters near the surface rotated upslope assuperincumbent soil failed rotationally.

Subsequent slip episodes (B through Gin Fig. 4) were similar to episode A butwere somewhat briefer. Each episode wastriggered when pore water pressures recov-ered sufficiently from the previous epi-sode's dilation to help instigate renewedlandslide motion. Most episodes involvedconcurrent pulses of surface displacement,subsurface tilt, and pore pressure decline.Downslope displacements during all epi-sodes were <0.3 in and occurred at rates<0.1 m/s. Slip episode D (Fig. 4) caused aparticularly prominent decline in pore pres-sure that reduced pressure heads by over50% near the landslide base. Later slipepisodes caused smaller pore pressure de-clines and in some instances a local porepressure increase. Nonetheless, soil dilationcontinued to retard landslide motion, andsoil porosity did not reach a homogeneouscritical state even after displacements ex-ceeded 1 m.

Although several factors in addition toinitial porosity might influence landsliderates, in many instances these factors areeither obvious [such as inertia (21)] or insig-nificant [such as intrinsic rate dependence ofsoil strength (22)]. In contrast, the depen-dence of landslide rates on soil porosity is notreadily observable but can be pivotal. Thedependence arises from the coupling of po-rosity change and pore pressure change, andit exists even if soil strength is otherwiseconstant.

The magnitude of pore pressure changeinduced by porosity change during landslid-

Table 1. Mean physical properties of loamy sand used in landslide experiments. N denotes the numberof samples on which measurements were made in each experiment or soil test, and ± values indicate 1SD from the mean.

Loose soil experiment Dense soil experiment

89% sand, 6% silt, 5% clay(N = 8)

1.44 ± 0.06(N = 6)

0.12 ± 0.005(N = 6)

0.52 ± 0.02- (N = 6)

0.025 ± 0.007(N = 3)11 ± 4

(N = 2) (N = 2)

89% sand, 6% silt, 5% clay(N = 8)

1.82 ± 0.03(N = 6)

0.14 ± 0.007(N = 6)

0.41 ± 0.01

(N = 6)0.0022 ± 0.00005

(N = 3)28 ± 6

29 ± 2 41 ±11(N = 2) (N = 2)

*These tests were conducted on reconstituted soil compacted to the desired porosity.

www.sciencemag.org SCIENCE VOL 290 20 OCTOBER 2000 515

ing depends not only on initial porosity butalso on the relative time scales for soil defor-mation and pore pressure diffusion (18). Iffluid pressure can diffuse into or away fromcontracting or dilating soil as quickly as thesoil deforms, pressure equilibration keepspace with deformation and the effects of po-rosity change diminish. However, the timescale for pore pressure diffusion is 1721D,where h is the typical thickness of the de-forming soil mass and D is its typical hydrau-lic diffusivity. Even sandy soils with highdiffusivity commonly have D < 100 cm2/s(Table 1). Thus, the time scale for diffusivepore pressure equilibration in deforming soilmasses with h 1 m typically surpasses 10 s.hi comparison, the time scale for landslideacceleration in response to basal pore-pres-sure change is \/h/g (21), which yields val-ues <I s for h 1 m. We conclude that porepressure diffusion can seldom keep pace withsoil deformation and that relatively smallvariations in porosity can influence landslidebehavior profoundly.

References and NotesA. K. Turner, R. L Schuster, Eds., Landslides Investiga-tion and Mitigation (National Academy Press, Wash-ington, DC, 1996).Soil porosity (pore volume/total soil volume) rangesnaturally from about 0.3 to 0.7 as a result of geolog-ical and biological modification of parent sediment orbedrock. An alternative measure of pore space is voidratio (pore volume/soil solids volume). Critical-stateporosity depends not only on the physical propertiesof soil but also on the ambient state of stress andstress history.A. Casagrande, J. Boston Soc. Civ. Eng. 1936, 13( January 1936).A. N. Schofield, C. P. Wroth, Critical State Soil Me-chanics (McGraw-Hill, New York. 1968).G. Castro, thesis, Harvard University (1969).R. W. Fleming, S. D. Ellen, M. A. Algus, Eng. Geol. 27,201 (1989).S. A. Anderson, M. F. Reimer, J. Geotech. Eng. 121,216 (1995).O. Reynolds, Nature 33, 429 (1886).On the scale of individual grains and pores, thecoupling of fluid pressure and displacement of adja-cent solids is described by viscous lubrication theory.On a continuum scale involving millions of grains andpores, the same coupling can be described by poros-ity change and attendant development and diffusionof pore fluid pressure.Soil strength typically obeys the Coulomb-Terzaghiequation r = (a — p)tany + c, where r is mobilizedshear strength, Cr is total normal stress, and p is porefluid pressure, all on the same failure plane; N is thesoil angle of internal friction; and c is soil cohesion.Increased pore fluid pressure therefore reduces soilstrength if all other factors are constant.R. M. Iverson, R. G. LaHusen, Science 246, 769 (1989).D. Eckersley, Geotechnique 40, 489 (1990).G. Wang, K. Sassa, in Environmental Forest Science(Kluwer, Dordrecht, Netherlands, 1998), pp. 591-598.All compaction loads were applied normal to theslope. The longest compaction periods produced thelowest porosities, and vibratory compaction pro-duced more uniform porosities than did foot traffic.G. R. Blake, in Methods of Soil Analysis, C. A. Black,D. D. Evans, J. L. White, L E. Ensminger, P. E. Clark,Eds. (American Society of Agronomy, Madison, WI,1965), pp. 374-390.Tiltmeters were Crossbow model OCTA01-CAN, rig-idly mounted in smooth cylindrical tubes 2.5 cm indiameter, fitted with rough exterior vanes 1 cm highto provide good frictional contact with soil. Exten-

REPORTSsometers were Celesco model PT101-250AS, at-tached to anchors embedded in the soil surface. Anyuse of trade, product, or firm names in this publica-tion is for descriptive purposes only and does notimply endorsement by the U.S. government.Dynamic piezometers were fabricated to have a con-figuration that promoted rapid dynamic response andminimal signal attenuation (11). The sensing ele-ments in these piezometers were Honeywell Micro-switch differential pressure transducers (model26PCCFA3D, with nominal range 0 to 15 psi)Identical pressure transducers mounted to thepressure ports of Soil Moisture Equipment Jet-Filltensiometers (model 2725, equipped with porousceramic tips with nominal 1-bar air entry pres-sures) were used to measure pore water pressuresless than atmospheric.R. M. Iverson, M. E. Reid, R. G. LaHusen, Annu. Rev.Earth Planet. Sci. 25, 85 (1997).

19. M. E. Reid, R. G. LaHusen, R. M. Iverson, in Proceedingsof the First International Conference on Debris-flowHazards Mitigation, C. L Chen, Ed. (American Societyof Civil Engineers, New York, 1997), pp. 1-11.

Ecological speciation occurs when organismsexposed to divergent selective regimesevolve reproductive isolation as a by-productof adaptation (1-3). Mechanisms contribut-ing to ecological speciation include matechoice based on traits under divergent selec-tion (4, 5), hybrid or backcross inferiority (2),and reinforcement of assortative mating whenhybrids are inferior (6, 7). Ecological specia-tion appears to be relevant in allopatry andsympatry and has been supported by theoret-ical models, laboratory experiments, andstudies of natural systems (1-9). Here we

'Organismic and Evolutionary Biology Program, Uni-versity of Massachusetts, Amherst, MA 01003-5810,USA. 'Marine Molecular Biotechnology Laboratory,University of Washington, 3707 Brooklyn AvenueNortheast, Seattle, WA 98105-6715, USA. 3 School ofAquatic and Fishery Sciences, University of Washing-ton, Box 355020, Seattle, WA 98195, USA. 'Washing-ton Department of Fish and Wildlife, 600 Capitol WayNorth, Olympia, WA 98501, USA.

*To whom correspondence should be addressed. E-

mail: ahendry@bio.umass.edu

As illustrated in Fig. 2, dense soils generally display aprominent peak in strength (due to dilation), whichimpedes landslide initiation. After peaking, thestrength of dense soils gradually decays. With suffi-ciently large deformations, dense soils and loose soilshypothetically approach a state of constant porosity(the critical state) and constant (residual) strength.Breakage of soil aggregates complicated this behaviorin our ring-shear experiments (Fig. 2).R. M. Iverson, Water Resour. Res. 36, 1897 (2000).S. Leroueil, M. E. S. Marques, in Measuring and Mod-eling Time Dependent Soil Behavior, T. C. Sheahan,V. N. Katiakan, Eds. (American Society of Civil Engi-neers, New York, 1996), pp. 1-60.N. R. Iverson, R. W. Baker, T. S. Hooyer, Quat. Sci. Rev.16, 1057 (1997). •J. E. Mann, N. R. Iverson, R. M. Iverson, Eos (FallSuppl.) 80, F443 (1999).Supported in part by grant EAR9803991 from NSF.We thank K. Swinford for assistance with experi-ments and S. Ellen, J. Roering, and B. Muhunthan forcritiquing the manuscript.7 July 2000; accepted 14 September 2000

focus on an unknown aspect of ecologicalspeciation: How quickly can reproductiveisolation evolve?

Rapid evolution of adaptive traits often oc-curs in populations exposed to divergent eco-logical environments (10, 11): Although thisimplies that reproductive isolation may alsoevolve rapidly, the best examples of ecologicalspeciation are seen in groups that began diverg-ing thousands of years ago (12, 13). Unfortu-nately, inferring evolutionary rates on the basisof long-standing groups is questionable, be-cause averaging disparate rates across time willobscure biologically important short-term evo-lution (11). Thus, reproductive isolation mightevolve in only a few generations, or it mayrequire a long and gradual accumulation ofisolating mechanisms. Some insects that colo-nized new host plants 100 to 200 years agohave evolved ecologically mediated reproduc-tive isolation (14, 15). We ask whether repro-ductive isolation can evolve even faster by test-ing for evidence of intrinsic barriers to gentflow between two populations of sockeye salm

Rapid Evolution of ReproductiveIsolation in the Wild: Evidence

from Introduced SalmonAndrew P. Hendry,'" John K. Wenburg, 2 Paul Bentzen,"

Eric C. Volk,4 Thomas P. Quinn3

Colonization of new environments should promote rapid speciation as a by-product of adaptation to divergent selective regimes. Although this process ofecological speciation is known to have occurred over millennia or centuries,nothing is known about how quickly reproductive isolation "ctually evolveswhen new environments are first colonized. Using DNA microsatellites, pop-ulation-specific natural tags, and phenotypic variation, we tested for repro-ductive isolation between two adjacent salmon populations of a commonancestry that colonized divergent reproductive environments (a river and a lakebeach). We found evidence for the evolution of reproductive isolation afterfewer than 13 generations.

20 OCTOBER 2000 VOL 290 SCIENCE www.sciencemag.org