Life-Cycle Components of Selection in Erigeron annuus: I. Phenotypic Selection
Author(s): Donald A. Stratton
Reviewed work(s):
Source: Evolution, Vol. 46, No. 1 (Feb., 1992), pp. 92-106
Published by: Society for the Study of Evolution
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Evolution,46(1), 1992, pp. 92-106
LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERONANNUUS:
I. PHENOTYPIC SELECTION
DONALDA. STRATrON'
of New York,
DepartmentofEcology and Evolution,State University
StonyBrook, NY 11794 USA
dateand seedling
selectionon emergence
ofphenotypic
and direction
-The magnitude
Abstract.
amongsitesand
ofselection
theheterogeneity
annuuswasmeasuredto determine
sizeinErigeron
date.Threedisturbance
sizeandemergence
varianceexplainedbyseedling
offitness
theproportion
of
wereimposedto testthehypothesis
vegetation)
perennial
treatments
(open,annualvegetation,
Selectionwas mostheterogeneous
environments.
stronger
selectionon seedlingsin competitive
on a local
ofselection
inthemagnitude
spatialheterogeneity
earlyin thelifecycle,withsignificant
and selectionwas
selectiongradients
onlyfecundity
affected
treatments
scale. The disturbance
date
on emergence
differentials
in thesignofselection
variation
in openplots.Significant
strongest
varied
episodes;at laterstagesselection
selection
andfallviability
wasobservedforestablishment
during
highmortality
Seedlingsin theearliestcohortexperienced
in magnitude
butnotdirection.
anddisruptive
laterinthelifecycle.Bothstabilizing
sizeandfecundity
butincreased
establishment,
but in generalselectionwas
date wereobservedduringestablishment,
selectionon emergence
through
dateoperatedindirectly
At StonyBrookmostselectionon emergence
purelydirectional.
Therewerepersistent
seedlingsize,whereasat theWeldPreservedirectselectionwas stronger.
and
components,
date and rosettediameteron adultfitness
effects
of bothseedlingemergence
Overall,
variancein fecundity.
Octoberrosettediameterexplained18% of thetotalphenotypic
selection.Wintersurvithanfecundity
weremuchmoreimportant
components
viabilityfitness
episodeofselection.
vorshipwas thesinglemostimportant
seedlingsize.
selection,
date,phenotypic
germination
-Erigeronannuus,fitness,
Keywords.
ReceivedJuly8, 1990. AcceptedJune28, 1991.
In most plantpopulations,manymore oftenbecomemoreskewedwithtime,recontribution
seedlingsgerminatethan become estab- sultingin a disproportionate
lished(Grubb,1977; Cook, 1979) and the ofzygotesfromthefewlargestindividuals
seedlingstageis oftentheperiodof maxi- (Solbrig,1981;CookandLyons,1983;Weiis cor- ner,1985). Small variationsin thetiming
Ifseedlingmortality
mummortality.
rate
or in theinitialgrowth
relatedwithsome phenotypictrait,large ofgermination
intolargefitness
shiftsin thepopulationmeanvalue ofthat ofseedlingsmaytranslate
(Stanton,1985; Waller,1985).
trait(i.e., selection)may occurduringthe differences
phaseofthelifecycle. Thus,seedlingsize maylocka plantintoa
criticalestablishment
so thefinal
hierarchy,
Harper(1977) labels thisperiodthe "en- positionin thefitness
selection fitnessdistribution
may be largelydetersieve."Conceivably,
vironmental
enoughto over- mined by events occurringin the first
on seedlingsmaybe strong
selectionpressuresin later monthsofthelifecycle.
rideconflicting
stagesofthelifecycle.Ifseedlingcharacters Severalstudieshave shownthe imporare correlatedwithotherphenotypesex- tanceof emergencetimeforseedlingsurand lifetime
as wellas fecundity
pressedlaterin thelifecycle,selectionmay vivorship,
of adult fitness(Howell, 1981; Lee and Hamrick,
indirectlyshiftthe distribution
notevenexpressedin seedlings. 1983). The firstseedlingsto emergegeta
characters
their
andmaymaintain
Variationin seedlingsize maybe one of headstartongrowth
increased
exhibiting
offitness relativesizeadvantage,
determinants
themostimportant
at laterstagesof
and fecundity
duringtheearlystagesofthelifecycle.Size survivorship
with thelifecycle(Ross and Harper,1972;Wal(skewedsize distributions
hierarchies
a fewlargeindividuals)developearlyand ler, 1985). In some speciesearlygerminacreatinga tradeoff
tiondecreasesviability,
(Arsurvivorship
initial
and
size
between
1
Evoand
Presentaddress: Departmentof Ecology
1981),
Prince,
and
Marks
1973;
et
al.,
thur
lutionaryBiology,PrincetonUniversity,Princeton,NJ
whileotherstudieshave showndirectional
08544 USA.
92
PHENOTYPIC
SELECTION IN ERIGERON
93
1) how
overallcom- weedErigeronannuus.Specifically,
forearlygermination
selection
enponentsof fitness(Howell, 1981; Kalisz, doesselectionvarywiththecompetitive
Previousdata suggestthatse1986; Miller,1987).
vironment?
The above studieshave shownseedling lectionfor earlyseedlingsize should be
in morecompetitive
environments
withfit- stronger
size to be a traithighlycorrelated
expectedto be under (Stratton,
ness, and therefore
andErhardt,
1990).
1989;Schmitt
consistentpositive directionalselection. 2) Are therecriticalstagesin thelifecycle
However,the magnitudeof selectionmay for selectionon these traits?(i.e., which
se- components
In particular,
most
andfecundity
varyamongenvironments.
ofviability
lectionon earlyseedlingsize maybe more strongly
covarywithseedlingsizeandemerintensein densepopulationswithhighin- gencedate?)Finally,3) How muchof the
can be excompetition(Shaw, finalvariancein relativefitness
tra- or interspecific
1986; Schmittet al., 1987).Largeseed size plained by eventsat the seedlingstage?
size) Measurements
earlyseedling
ofgeneticvariationarepre(whichinpartdetermines
is favoredin moreclosed habitats(Salis- sented in a companionpaper (Stratton,
oftheseed 1992).
bury,1942) and themagnitude
inexperiments
with
increased
sizeadvantage
MATERIALS AND METHODS
high levels of competition(Black, 1958;
1989).
Stanton,1984; Stratton,
Erigeronannuus(Asteraceae)is a comof monearlysuccessionalweednativeto eastand eventhedirection
The magnitude
as a winselectionmaynotbe constantacrossselec- ernNorthAmerica.It is classified
tionepisodes(Cleggetal., 1978;Arnoldand ter annual; seeds typicallygerminatein
and
expressedat the August,seedlings
overwinter
as rosettes
Wade, 1984b).Characters
com- plantsbolt and flowerthe following
somefitness
sumseedlingstagemayaffect
first
year
ponentsbut not others,or theremay be mer.Someplantsdo notreproduce
betweenfitnesscomponents.For butinsteadremainas rosettes
untilthesectradeoffs
may increase ond or thirdsummer.Erigeronannuusis
example,earlygermination
but at triploidand apomictic,reproducing
fecundity)
solely
seedlingsize (ultimately
ifthereare bytinywind-dispersed
thecost of increasedmortality
seeds.Seed producclimaticconditionsduringes- tionis high:a singleplantmayproduceas
unfavorable
selectioninto manyas 100,000genetically
identicalseeds
tablishment.
By partitioning
episodesone can exposethe (Stratton,
1988).
thecomponent
was set up at two sites
criticalstagesofthelifecycleforselection The experiment
tradeoffs,near StonyBrook,New York, USA. The
and identify
potentiallifehistory
evenwhenthereis littleorno netselection. Weld Preserve(WP) sitewas an old field,
fiveyears,and had a
It is usefulto separatenaturalselection fallowapproximately
co- densecoverofprimarily
Solidagospp.The
into two components:the phenotypic
which StonyBrookcampussite (SB) was an old
variancebetweena traitand fitness,
meanswith- gardenplot dominatedby grasses,Artegovernschangesinphenotypic
in a generation,
and thegeneticvariancefor mesiavulgaris,
and Daucus carota.The SB
thetrait,whichallowsa responseto selec- site was moremesicthanWP, beingpartionacrossgenerations
(Landeand Arnold, tiallyshadedbyan oak forestto thesouth,
thatde- and thesubstrate
was less sandy.
1983). The ecologicalinteractions
rate
ofthree
blockconsisted
Eachexperimental
terminea plant'ssurvivorship,
growth
and ultimatefecundity
dependon pheno- 1 m2plots,each randomlyassignedone of
withthe envi- threedisturbancetreatments.
The treattypesand theirinteraction
ronment.Focusingsolelyon the mean re- mentsincludedbare tilledsoil ("open"),
whileimportantfor tilledsoil witha sparsecoverof summer
sponse of genotypes,
evolution,obscuresmuchoftheecological annuals("annual"),and naturalvegetation
thereproductive withsmall 10 x 10 cm disturbances
("pedynamicsthatdetermine
success of an individual.Here I address rennial").Plots in the open and annual
be- treatments
the relationship
were tilledone week priorto
questionsconcerning
I transdate planting.For theannualtreatment
tweenseedling
(germination
phenotypes
inthecommon planted16 seedlingsof Ambrosiaartemiandseedlingsize)andfitness
94
DONALD A. STRATTON
overeachcellandlightly
seedswasscattered
coveredwithsoil to helphold theseedsin
mom
place. I used seeds from10 Erigeronge~
EE
ENC1Eu
notypes,all collectedfroma largepopulation at the Weld Preserveand therewere
two replicatesof two maternalfamiliesof
each. See Stratton(1992) forthe genetic
details.Seedswereplantedbetween31 July
and 2 August1987,neartheendofthenorPerennial malperiodofseeddispersal
/
on LongIsland.
weekly
foremerwere
censused
The
plots
1
~~~~Annual
w
|
/
El Open
o o o o0
from15 Augustto
genceand survivorship
o-e
o-o
o-o
during
6 September.
Seedlingsthatemerged
o o o
0
* o
o-a
o-o@l
each censusweekweredefinedas an emerDe
0
oe
gencecohort.Therewerefourcohorts.A
ineachcellwere
subsetofup to 10seedlings
0
0o
o
o0
cohortusingcolmarkedineachemergence
Whenmorethan 10
or coded toothpicks.
I ranseedlingsin a givencohortemerged,
domlyremovedthe excess seedlings.Because mostoftheexcessseedlingsemerged
in cohort1,thinning
increasedthevariance
date by an averageof 13%.
design.Three in emergence
FIG. 1. Summary
oftheexperimental
(Open,Annual, Afterseedlingswereestablishedand after
treatments
disturbance
experimental
Perennial)werereplicatedin six blocksat two sites, mostearlymortality
had occurred,I perStonyBrookand WeldPreserve.Seedsweresownin
ofthesurviving
marked
a
subset
manently
40 10 x 10 cmcellswithineach 1 m2plotand several
and mappedtheirlocationwithin
weremeasured
percell.Thesolidcirclesshow seedlings
seedlings
thepositionofAmbrosiaplantsin theAnnualplots. thecell. An averageof three(maximum=
ineachcellwerechosensuch
four)seedlings
thattheywereapproximately
equallyspaced
af- and evenlydividedamongcohorts.Again,
siifoliaintothetilledplotsimmediately
tertheErigeronseedswereplanted(Fig. 1). because initial densitieswere not conI removedexcessseedlings.
The Ambrosiawereabout25 cm tallwhen trolled,
Seedling
This treatment
resultedin a removalin Octoberincreasedthe phenotransplanted.
50 cm canopywithsparseshade by two typicvarianceof emergencedate by only
Duringthe first 3%.
weeksaftertransplanting.
were I measuredthemaximumrosettediamweek,theopenand annualtreatments
plots,I clipped eterofeach seedlingand monitoredsurviverysimilar.In theperennial
to 10cmtofacilitate
thevegetation
planting vorshipduringcensusesin October1987,
removaltook March,May, July1988, and April,July
butno tillingor whole-plant
covergrewbackreadi- 1989. In theJulycensusesI estimatedthe
place.The perennial
to condi- seedproduction
correspond
ly. These treatments
ofeachreproductive
plant.
tions experiencedby Erigeronannuus in Because of thelargenumberof seeds prorangingfrom duced per plant (25,900 + 1,660 in this
successionalenvironments
newlyfallowfieldsto threeto fiveyearold experiment),
I countedthenumberofflower
All can be con- heads(capitula)oneachplantandestimated
successionalcommunities.
sidered"natural"habitatsof E. annuus. thenumber
ofseeds/capitulum
fromthedisk
Thereweresixblocks(4 at WP and 2 at SB) diameter.The numberof florets
percapitfora totalof 18 plots(Fig. 1).
ulumwas determined
bytheequation:floEach plotwas dividedinto40 10 x 10 rets= 38.9 + 7.27(DiskArea)(in mm2;r2
ina checkerboard
cmcells,arranged
design. = 0.81; Stratton,
1988). The diskdiameter
A plugof soil was removedfromeach cell increaseswiththeage ofthecapitulum,so
and theresulting
hole was refilled
withau- all measurements
weremadeon headswith
toclavedsoil.A small"pinch"(600-900) of dehiscentanthersin onlytheoutertwo or
SB
WP
END
PHENOTYPIC
SELECTION IN ERIGERON
95
fraction,
s is surEach floret
threewhorlsofdiskflorets.
pro- whereg is thegermination
ofreproducducesone single-seeded
acheneso, assum- vivorship,r is theprobability
in each year.
ingequal seedset,fecundity
is proportional tion,and f is the fecundity
Fitnesswas made relativeby dividingby
to thetotalnumberofflorets.
in each 1 m2plot.
SelectionEpisodesand FitnessCompo- themeanfitness
nents.-I dividedthelifecycleintoa series Measuring PhenotypicSelection.-While
focusedon the
primarily
of selectionepisodesthatmultiplicativelythis experiment
and fitness
Vi- covariancebetweenphenotypes
combineto determine
lifetimefitness.
thethreeexandcorrelational),
abilityselectionwas dividedintofivecom- (descriptive
disturbance
treatments
testedthe
ponents.Theestablishment
episodewasde- perimental
thatthemagnitude
ofdirectional
finedas emergence
through
theSeptember hypothesis
census;fall,winter,
spring,
and secondyear selectionon seedlingsize shouldbe greater
viabilityselectionepisodescovertheperi- in theplotswithhighlevelsofcompetition
ods September-October,
October-March,(perennial)comparedto the open and anMarch-July
1988,andJuly1988-July
1989, nual plots.In addition,the replicateplots
respectively.
The divisionsare somewhat withinsitesallowedad hoc testsforspatial
ofselectionbearbitrary
buttheycorrespond
to themajor variationin themagnitude
phasesofthelifecycleofE. annuus(estab- tweentheStonyBrookand Weld Preserve
lishment,
winterdormancy,
springgrowth) sites.
and foreach selection
and represent
periodsof distinctenviron- For each character
mentalsourcesofmortality.
Desiccationwas episodeI calculatedthedirectional
and staa particularly
fromlinearand
important
sourceofmortality bilizingselectiongradients
of relativefitnesson
and fallselection quadraticregressions
duringtheestablishment
was appar- the phenotype(Lande and Arnold,1983).
episodeswhereasfrost-heaving
sicharacters
areconsidered
overthe Whenmultiple
entlythemajorsourceofmortality
thevectorof partialregreswinter.Survivorsof a particularselection multaneously,
estimatesthedirectselecvalueofone, sion coefficients
episodewereassigneda fitness
otherwise
was zero forthatepisode. tion on each characterwhile controlling
fitness
The fecundity
component
wasestimated
by indirecteffectsof selectionon correlated
I standardized
all characters
to
theseedproduction
ofplantsthatsurvived characters.
to reproduce.Each replicate1 m2plotwas unit variancebeforeestimatingselection.
definedas a populationforthepurposeof Standardizedselectiongradientshave the
computingrelativefitnessand estimating advantage of being directlycomparable
Because
amongpopulationsandcharacters.
phenotypic
selection.
werenotmeasured
Most estimatesof lifetimefitnesswere theseedlingphenotypes
zerobecauseofthelargenumberofplants simultaneously,
themultipleregression
apI dis- proachcouldnotbe extendedto analysesof
thatfailedto surviveto reproduction.
of fitcountedthefecundity
ofsecondyearplants lifetimefitness.Multipleregression
andMarch
date,October,
by the populationgrowthrate,usingthe nessonemergence
wherewiis the rosettediametercould only includedata
equation:wi= z X-Xsi(X)mx)
ofindividuali,Xis thepop- from individualsthat survivedthrough
absolutefitness
earlierselectionepiulation growthrate, si(X)is survivorship(0 March,thusignoring
or 1) and mi(x)
is thefecundity
ofindividual sodes. Insteadthe componentsof lifetime
i at timex (Serviceand Lenski,1982). I selectionwereestimated
separately
andlifecalculatedX separatelyforeach of the 18 timeselectionwas computedas thesumof
overallepisodes(Arnold
gradients
replicateplotsby populationprojectionof selection
were
theLesliematrix(A) untiltheestimatesof andWade,1984a). Selectiongradients
X (ratioof populationsizes in successive made additiveby substituting
the initial
within0.001 (Table phenotypiccovariancematrixP0 forthe
generations)
converged
1). The transition
matrixwas oftheform conditionalcovariancematrixPi observed
at thebeginning
ofeachepisode(Wadeand
The initialphenotypiccoKalisz,
1990).
g sl rl)
gs2r2.f21
A [gS2
variancematrixwas constructed
byrepeatrl )
?
ISi. (1
edlysolvingforPi in theequation
96
DONALD A. STRATTON
1. Demographic
parameters
usedin calculating
thepopulation
growth
ratein the18 plots.The values
oflambdaare sensitive
to theestimated
germination
fraction
and thusto myestimateoftheinitialnumberof
seedsplanted(600-900/cell).
I usedthehighestimate,
buttheactualvalueofXmaybe as muchas 50% higher.
Highervaluesof X willfurther
decreasethevalue of delayedreproduction.
N = totalnumberof seedsthat
= N/(900*40);s = survivorship;
emergedperplot(germination
fraction
r = probability
ofreproduction,
given
thatit survivesthrough
year1 or 2;f = fecundity;
X= populationgrowth
rate.
TABLE
Year I
Site
Block
N
Open
SB 1
SB 2
WP 3
WP 4
WP 5
WP 6
Mean
Annual
SB 1
SB 2
WP 3
WP 4
WP 5
WP 6
Mean
Perennial
SB 1
SB 2
WP 3
WP 4
WP 5
WP 6
Mean
SI
ri
1,355
1,246
483
387
669
146
714
0.041
0.036
0.025
0.027
0.009
0.009
0.025
0.257
0.476
0.806
0.829
0.542
0.875
0.631
1,187
632
298
713
881
146
643
0.048
0.022
0.033
0.031
0.004
0.009
0.025
169
320
643
930
600
464
521
0.016
0.012
0.017
0.047
0.014
0.011
0.020
Year 2
fi
S2
r2
f2
A
4,152
6,216
66,739
43,839
11,656
47,053
29,943
0.75
0.86
0.57
0.62
0.20
1.00
0.67
0.27
0.55
1.00
0.80
1.00
0.50
0.69
6,854
7,220
20,195
20,036
8,705
13,949
12,826
2.32
4.21
18.06
10.64
1.17
1.54
6.32
0.229
0.524
0.971
0.955
0.800
0.750
0.705
4,572
6,517
54,990
42,318
6,870
82,437
32,951
0.58
0.87
1.00
1.00
0.67
0.75
0.81
0.18
0.75
1.00
0.67
0.50
0.33
0.57
24,241
10,244
10,952
9,962
6,321
24,798
14,420
2.77
1.95
14.63
24.81
0.61
2.31
7.85
0.143
0.000
0.636
0.292
0.000
0.186
0.210
6,583
0
11,534
13,266
0
5,687
6,178
0.83
0.67
0.46
0.83
0.21
0.76
0.63
0.60
0.50
0.67
0.74
0.00
0.52
0.51
5,490
6,942
58,541
15,757
0
12,916
16,608
0.46
0.52
2.92
6.07
0.00
0.84
1.80
ferentials
and selectiongradients
werecomputed
from
1,000
bootstrap
replications
of
wherehyis a 3 x 3 matrixof stabilizing
the
simple
or
multiple
regression
(Efron,
selectiongradients
and / is a 3 x 1 vector
ofdirectional
selection
gradients
(Landeand 1982). The data wererandomlyresampled
1,000timesfromtheorigArnold,1983; Lynchand Arnold,1988). withreplacement
inal
distribution,
restandardized,
and used
Notethatpriorto thethirdcensus,hy
and A
to
compute
new
regression
coefficients.
I
are only partiallyobserved.The unmeasuredelementswereassumedto be zero, used theupperand lower2.5% ofthedisofbootstrapped
estimatesto find
undertheassumption
thatselectioncannot tribution
act directlyon charactersbeforetheyare the95% confidence
intervalsfortheselecexpressed(LynchandArnold,1988).Given tion gradients.Individualselectiongradian estimateofPO,conditional
selectiongra- entswereconsidered
tobe significant
iftheir
dients, d', were made additive by /i =
95% confidenceintervalsdid not include
PO-P4i/ (Wade and Kalisz, 1990).
zero.Becausethereis a highprobability
that
StatisticalAnalyses.
-The distribution
of at leastone of 18 selectiongradients
willbe
lifetime
fitness
was highlynonnormal,
hav- nominallysignificant
duringeach episode,
inga largeexcessof individualswithzero experimentwise
errorwas controlledusing
fitness.Similarly,survivorship
could only the sequentialBonferroni
method(Rice,
assumevaluesof0 or 1, violatingthenor- 1989).
malityassumptionof regression
analysis.
Overallsignificance
of theselectiongraThus, significance
levels forselectiondif- dientsacrossthe entirerangeof environP
=
Pi
+
Pgiy,P,
(P-
)(P,O)
PHENOTYPIC
97
SELECTION IN ERIGERON
100
signed
mentswas testedvia nonparametric
rankstests,usingvaluesfromthe 18 plots
80
Even whenindividualesas observations.
fromzero,
timatescannotbe distinguished
60
ofthesignofselectiongradients 0
consistency
providesevidencethatselectionis nonzero,
40
ofselectionacross
andtestsforrepeatability
20
environments.
of selectionwas testedby
Heterogeneity
0
samplesforeachpair
300 bootstrap
drawing
0
for
of plotsand computingthe difference
each bootstrapreplicate.The twoselection
if the
different
weresignificantly
gradients
intervalof thedifferbootstrapconfidence
100
ence did not includezero (Dixon et al.,
ofthe 153 pairwise 80
1987). The proportion
differ-Q0.
comparisonsthatweresignificantly
entwas usedas an overallindexofthehet660
of selectionduringa givenepi- 0
erogeneity
sode. To correctformultiplecomparisons, 40
ofselectionwas onlyconsidheterogeneity
whenat least 12/153pairs (0
eredsignificant
confidenceintervals
had nonoverlapping
(frombinomial probability;P = 0.95; N
=
0
SB
20
40
80
60
WP
1
cohort
2
cohort
cohort
3
-
-*-
20
100
40
60
cohort
4
80
100
about
153). Notethatthistesttellsnothing
Weeks
only FIG. 2. Survivorship
which
pairsofplotsshowheterogeneity,
curvesforfourcohortsofE.
overall. annuus.Mortality
thatselectionwas heterogeneous
few
thefirst
wasconcentrated
during
at
Episodesofhighmortality
Comparisonsof themagnitudeof selec- weeksafteremergence.
plantsthatdied foland sites 46 and 95 weeksshowflowering
treatments
tionamongdisturbance
Kruskal-Wal- lowingreproduction.
weretestedbynonparametric
lis testsor Friedman'smethodforrandomized blocks(SAS, 1988) usingthe 18 selecto
Thesedo not 50% ofthetotaldeathsfromemergence
as observations.
tiongradients
A secondperiodofhighmorbutrather
require reproduction.
justtestforheterogeneity,
especially
in therankof selec- talityoccurredduringthewinter,
differences
consistent
tion gradientsamongsitesor disturbance forplantsin latercohorts.The mean survivorshipfromOctoberto Marchwas0.57.
treatments.
Plantsin theperennialplotsweresignifRESULTS
and
hadlowersurvivorship,
icantlysmaller,
inallplots. lowerfecundity
thanplantsin theopenand
Over12,000seedlings
emerged
The averageseed proOfthese,6,251weremarkedbycohortand annualtreatments.
2,160ofthe ductionin perennialplotswas 14.2 x 103
censusedforearlysurvivorship;
numberedand comparedto 27.1 x 103 and 32.3 x 103
wereindividually
survivors
rate, seedsin theopen and annualplots(F[2,440]
growth
measuredforrosettediameter,
Most = 3.43, P < 0.05). Only 19% of surviving
and lifetime
fecundity.
survivorship,
seedlingsemergedin the firstgerminationplantsreproducedas annualsin perennial
cohort,whichincluded53% ofall seedlings. plots,comparedto63% and70%intheopen
Cohorts2, 3, and 4 contained12, 20 and and annualtreatments
(F[2,121= 9.58, P =
0.003). As a result,estimatesofpopulation
respectively.
15% ofall seedlings,
dur- growthrateshoweddecliningpopulations
was concentrated
Seedlingmortality
ing the firstthreeweeks followingemer- (X < 1) in 4/6perennialplots(Table 1).
Selectionon EmergenceDate.-Direcgence(Fig.2). On average,42% ofall seedlings died withinthreeweeks. Mortality tionalselectionon emergencedate during
phase was significantly
accountedfornearly the establishment
duringestablishment
DONALD A. STRATTON
98
15
.
Establishment
1.0
0.5
OiAi
0.2-
0.0
0.0
Winter
's *
p
a
0d.
I
0.
0.
(1)
.0.5~
0
.10.5L
0.
0.
.0.
-
1.5
- -
--
i
-
j ftjrh
Fecundity
0.5
C)
_r
-1S8-
1.0
0
t
I
0.6-
*
'
.0.
Fall
0.8
o. o
I
Fiatns
01.0'
.1.8
FIG. 3. Standardized
on emergence
selection
differentials
forselection
datein 18 plotswith95% confidence
intervals.
The unitsarephenotypic
In thisand all otherfigures,
standarddeviations.
opensymbolsshowplots
at StonyBrookandclosedsymbols
showplotsat theWeldPreserve.
SquaresareOpenplots,circlesareAnnual
arePerennialplots.
plotsand triangles
treatments
or bein 6 of tionamongdisturbance
lateremergence)
positive(favoring
the 18 plotsand significantly
negativein 4 tweenthetwosites.
plots(Fig. 3). In addition,thereweresig- Threeplotsat theWeldPreserve
hadsignificant
quadratic
components
tothefitnessnificant
in the
selection
forlateremergence
thepresence
ofbothsta- fallselectionepisode(September
surface,
indicating
to Octobilizingselection
(fourplots)anddisruptiveber)whereasearlyemergence
was favored
selection(threeplots)duringtheestablish- in fourplots(all at StonyBrook)(Fig. 3).
mentepisode(Fig.4). Fourmoreplotshad Disruptive
datewas
selection
on emergence
significantly
positivequadratic
componentspresent
inthreeplots(Fig.4). Onemorehad
minimum
wasoutside a significantly
butthesurvivorship
positivequadraticcoefficient
therangeof phenotypes.
Thus theseplots butshowednetdirectional
selection
forearhadmonotonic
directional
selection
forlate ly emergence.
of disTherewas no effect
thefitness
emergence,
although
surface
was turbancetreatment
on the magnitudeor
significantly
curved.Both directional
and shapeoffallviability
buttherewas
selection
stabilizing/disruptive
selection
differentials
of direcsignificant
spatialheterogeneity
during
establishment
weresignificantly
het- tionalanddisruptive
selection
amongblocks
erogeneous
overalland showedspatialde- and sites(Table 2).
pendenceamongblocks(Table2). However Once seedlingssurvivedestablishment
therewas no significant
variationin selec- and fallmortality,
directional
selectionto-
PHENOTYPIC
SELECTION IN ERIGERON
99
wardearlyemergence
dominatedall other
1.5 Establishment
selectionepisodes(Fig.3) and therewas no
significant
stabilizingor disruptiveselection.Winterviabilityselectionfavoredearlyemergence
in 10/18plots,withan average
directionalselectiondifferential
of -0.26
0.5standarddeviations.Wintersurvivorship
wasovertwiceas highforthefirst
twoemergencecohortscomparedto thetwolatercohorts (0.70 versus 0.33; G = 199.6; P <
-10
.00.0001). Selectionwas strongerat Stony
C)
Brookthanthe Weld Preserve,but again
c 00. 0.5therewas no effect
of the competitive
environmenton wintersurvivorship
(Table
2). The netwinterselectiondifferential
was
partially
theresultofcorrelations
withseedlingsize. On average,51% oftheselection
differential
was due to indirectselection FIG. 4. Stabilizingand disruptive
selectiondifferthrough
seedlingsize.Indirectselectionwas entialson emergencedate duringtheestablishmentand
moreimportant
at SB, whereit accounted fallviabilityselectionepisodes. Negativeselectiondiffor87% of thetotalwinterviabilityselec- ferentialsindicatestabilizingselectionand positiveselectiondifferentials
indicatedisruptiveselection.Open
tiondifferential,
comparedto 19% at WP symbols
= SB, closed = WP; squares = Open plots,
(Friedman'stest,P < 0.003). Directwinter circles = Annual plots, triangles= Perennialplots.
viabilityselectionon emergencedate was
observedin seven plots (Table 3A). The
threedisturbance
treatments
had no signif- selectiongradientson emergencedate (conicanteffect
on eitherdirector indirect
win- trollingfor selectionon size) were not sigterviabilityselectionon emergence
date.
nificantbut generallypositive,especiallyin
The timingof reproduction
was highly the open plots (Table 3A). Direct fecundity
dependenton the date of emergence.Of selectiongradientson emergencedate were
plantsthatreproduced,
78% ofplantsin co- significantly
heterogeneousamong thethree
hort1 flowered
theirfirstyear(annuallife disturbancetreatments,but the patternof
cycle)comparedto only61% of plantsin heterogeneitydid not support my a priori
latercohorts(G = 11.2, P < 0.001). No predictionsof strongerselection in perenplantsthatgerminated
after20 Augustflow- nial plots (Table 3A). Most of the fecundity
eredat StonyBrookduringyear1. Similar- selection on emergencedate operated inly,theprobability
ofreproducing
at all was directlythroughseedlingsize.
muchhigher
forplantsintheearliestcohort The cumulative effectof phenotypicsecomparedto all others(0.235 versus0.073, lection on emergencedate was to decrease
G = 513.8, P < 0.000 1). Among plantsthat mean emergence time in most plots, but
reproduced,
earlyemergence
resultedin in- blocks 5 and 6 at WP had net positive secreasedfecundity
atbothsites(Fig.3). Plants lectionon emergence
date (Fig. 5, upper).
in cohort1 had 26% higherfecundity
at The cumulativeselectiondifferential
was
StonyBrook(7,420 versus5,860)and 41% -0.69 standarddeviationsat SB. Thiscorhigherfecundityat the Weld Preserve responds to a decrease in mean emergence
(38,000versus27,000)comparedto plants dateof5.2 days.Lifetime
selection
onemer= 10.2,P < 0.001). gencedate had a significant
in latercohorts(F11,4511
positivequaIndividualfecundity
selectiondifferentials
draticcomponentin 7/18plots(Table 2),
werenot significant,
partlya resultof the primarily
becausecohorts3 and 4 bothhad
smallnumberofplantsthatsurvivedto re- uniformlylow fitness(Fig. 6) ratherthan
produce,butselectionwasconsistently
neg- disruptive
selectionon emergence
date.At
ativeacrossall plots(P < 0.01,signedranks SBnmostofthelifetime
selectionon emertest).Whilethenetfecundity
selectiondif- gencedate was theresultof indirectselecferentials
favoredearlyemergence,
direct tion throughseedlingsize (Table
In
3..
100
DONALD A. STRATTON
2. Heterogeneity
ofselectiondifferentials.
Heterogeneity
ofselectionamong sites,blocks,and disturbance
treatmentswas testedvia Kruskall-Wallistestsusing the 18 selectiongradientsas observations.Heterogeneity
among plots is given by the proportionof the 153 pairwise comparisonsthatare significantly
different.
Overall
heterogeneity
among plots may be considered significantif it is greaterthan 8% (see text).
TABLE
Heterogeneityamong
treatments
2
Directional selectiondifferentials
Emergencedate
Viability
Establishment
0.25
Fall
0.50
Winter
0.14
Spring
1.13
Year-2
2.71
Fecundity
2.47
Lifetimefitness
0.18
October rosettediameter
Viability
Winter
Spring
Year-2
Fecundity
Lifetimefitness
0.78
0.78
2.16
1.22
5.23
Sites
Blocks
Plots
X2
X2
%
0.71
7.89**
8.43**
0.43
0.13
1.38
11.00***
0.56
1.06
0.13
1.45
0.65
selectiondifferentials
Stabilizing/disruptive
Emergencedate
Establishment
1.27
2.25
Fall
0.98
0.08
Lifetimefitness
0.57
0.82
October rosettediameter
Winter
2.14
5.93*
11.99**
13.66*
13.19*
1.56
1.33
10.35*
14.56*
51*
48*
17*
11*
35*
17*
54*
+
No. significant
-
4+
3+
1+
0
0
9.59
4.30
3.45
9.10
3.27
16*
6
15*
11*
29*
8+
0
0
1+
9+
9.94
11.12*
4.29
70*
20*
29*
7+
4+
5+
6.28
12*
3410-
10-
4-
6-
*** P < 0.001; ** P < 0.01; * P < 0.05.
highwintersurvivormostoftheselectionat WP acted mm had uniformly
contrast,
In all plots, ship (Fig. 8). Althoughthe fitnesssurface
on emergence
dateitself.
directly
curved,therewas netposlifetimeviabilityselectionon emergence was significantly
witha sizethreshse- itivedirectional
selection
thanfecundity
date was muchstronger
at
lection(Table 3C).
old. The thresholdeffectwas stronger
in significant
heteroSelectiononSeedlingSize.-Viabilityse- StonyBrook,resulting
lectionon Octoberrosettediameterpri- geneityof quadraticselectiondifferentials
variationinwinter amongsites(Table 2). Overall,winterviamarilyoperatedthrough
The mean directionalselec- bilityselectionaccountedfor80% of the
survivorship.
(Table 3).
was +0.41 standarddevi- lifetime
tiondifferential
selectiondifferential
were
had onlya weak
diameter
ations; winterselectiondifferentials
Octoberrosette
in 8 ofthe18plotsafteradjusting effect
in partbecauseof
significant
on springviability,
error,but all 18 were thelow opportunity
forselection.The variforexperimentwise
at P 5 0.056 (Fig.7). anceinrelative
was0.15,
nominallysignificant
survivorship
spring
selec- comparedto 1.43forwinter
There
Most of thevariationin phenotypic
viability.
of Octoberdiameteron
tionoccurred
amongplotsandamongblocks was also no effect
(P = 0.07), and therewereno significantsurvivorship
duringthesecondyear(Table
(Ta- 2).
treatment
effects
ofsiteor disturbance
were
ble2). Quadraticselectiondifferentials
On average,Octoberrosettediameterexin 9/18plots (all negative)but plained18% ofthetotalvariationin fecunsignificant
therewas no evidenceof an intermediatedity.Individualfecundity
selectiondifferin anyplots,but
maximum.Seedlingslargerthan20 entialswerenotsignificant
fitness
PHENOTYPIC SELECTION IN ERIGERON
101
TABLE 3. Meanphenotypic
selection
gradients
(A) andphenotypic
correlations
(B) foremergence
date,October,
andMarchrosette
diameter.
Becauseofheterogeneity
within
theWeldPreserve,
meanswerecomputed
separately
forblocks3,4 and 5,6. Asterisks
denotesignificant
heterogeneity
amongdisturbance
treatments
or locations
usingFriedman'stestforrandomized
blocks.Valuesin boldfaceindicateconsistency
ofthesignof selection
gradients
within
thelocationortreatment.
Underhoofno selection,
theprobability
thatall sixselection
gradients
havethesamesignis P = 0.03. No stabilizing
or disruptive
selection
gradients
weresignificantly
different
from
zero.The numberofplotswithindividually
significant
selectiongradients
(afteradjusting
forexperimentwise
error)is shownat theright.Compositeselectiongradients
(C) werecalculatedas thesumofselectionoverall
component
episodes.
Site
Variable
SB
WP(3,4)
Disturbancetreatment
WP(5,6)
A) Viabilityselectionepisodes
Establishment
Emergence
0.0849 -0.0996
0.2926 ***
Fall
Emergence
-0.0744 -0.0138
0.3130 *
Winter
Emergence
-0.2933 -0.4320
0.0763 *
October
2.8992
0.1361
0.7463 *
Adult(Spring+ Year 2)
Emergence
0.0321
0.0561
0.0543
October
-0.2615
0.1176 -0.2942 **
March
0.5443
0.1363
Fecundity
selection
Emergence
0.0350
0.0707
October
-0.1391
0.3150
March
0.2181
0.1632
B) Phenotypic
correlations
EmergenceOctober
-0.534
-0.473
EmergenceMarch
-0.443
-0.329
October-March
0.917
0.334
C) Compositeselection
gradients
Emergence
date
Totalindirect
-1.5044 -0.3575
Totaldirect
-0.2157 -0.4185
Totalviability -1.6980 -0.6424
Totalfecundity -0.0220 -0.1335
Octoberrosettediameter
Total indirect
Totaldirect
Totalviability
Totalfecundity
*
0.6720
2.4985
3.1253
0.0453
0.0513
0.5688
0.2744
0.3457
0.5761
**
0.2232
-0.0187
0.2707
+
-0.277
+
-0.162
0.611
*
-0.2213
0.9595 *
0.5190 *
0.2192 +
0.4949
**
0.5438
0.7735 *
0.1549
No0.
sig.
Open
Annual
0.1254
0.0306
0.1220
0.0365
0.1092
0.0792
+
7
-0.2152 -0.3409
0.6696
0.7312
0.0928
2.3809
+
7
10
Perennial
7
0.0371
0.0592
0.0462
-0.0056 -0.0864 -0.3461
0.3144
0.3549
0.2710
0.0711
0.3124 -0.2451
0.2150
0.4258
0
0
0.5874
3
0.0158 **
0.1079 *
0.0112 *
-0.432
-0.494
-0.358
-0.351
0.554
-0.310
0.598
-0.273
0.711
0
0
4
**
-0.6527 -0.4679 -1.0737
0.2547 -0.0709 -0.0815
-0.4379 -0.6299 -1.0743
0.0399
0.0911 -0.0808
0.3145
0.9763
0.8633
0.4275
0.4865
0.3996
0.8609
0.0253
0.4173
2.5734
2.7881
0.1117
**
+
P < 0.001; ** P < 0.01; + P < 0.05 but not significantwith Bonferroniadjustment.
overallfecundity
selectionwas significantly
thetotalfecundity
selection
on seedlingsize.
greaterthanzero (P = 0.02, signedranks Thethreedisturbance
treatments
differed
in
test).In mostplots,fecundity
selectionwas the directfecundity
selectionon October
muchweakerthanthetotalviabilityselec- rosettediameter(Table 3A). Howeverthe
tion(Table 3A; Fig. 5, lower).The effect
of phenotypic
selectionforlargeseedlings
was
seedlingsize on fecundity
was muchstron- highest
inopenplots,a pattern
oppositepregeramongplantsthatreproducedthefirst dictions.
year(fi= 0.26; significant
in 8/18)thanthe
Therelationship
between
seedling
sizeand
effect
on thefecundity
ofsecondyearplants lifetimefitnessshowedpurelydirectional
(ns). Thus delayedreproduction
decreased selection,
withno evidenceofcurvature
of
102
DONALD A. STRATTON
10000
1.0
2000
8000
r-0.5
--SB
CL~~~~~~~~~~~~~~~~~C
* 6000
0.0
Esab FalWntrAul:
E00
euniyFins
-0.5
LL
4000
U
2000
Winter
Adult
Fall
Estab.
FecundityFitness
0_ _0
0
1
2
3
4
5
0
Cohort
Episode
FIG. 6. Mean fitness
(?SE) of plantsin the four
emergence
cohortsat StonyBrookand theWeldPreserve.Notethedifference
in scaleon theordinate.
E oo
2.01.5-
.0
(0
E o 0.5 -:
0
.0
-0 .
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
- -
was weakestin highcompetition
perennial
In
plots,a patternoppositeto predictions.
contrast,Van der Toorn and Pons (1988)
did reporta trendforstronger
selectionon
dateclosedhabitatsintwospegermination
cies of Plantago.
Fall and winterviabilityselectionfavoringearlyemergence
cohortswas stronger
at
Episode
StonyBrook than at the Weld Preserve,
relatedto thecomselectiondiffer- whichmaybe partially
FIG. 5. Cumulativestandardized
date(upper) petitiveregime.One of the mostobvious
entials(?SE) forselectionon emergence
and Octoberrosettediameter(lower)at StonyBrook environmental
betweenthetwo
differences
bothspring siteswasthedifference
Adultviability
includes
andWeldPreserve.
inpercent
vegetation
forplantsthatdelayedrepro- cover. The
and year-2survivorship
at
Brook
wereall
plots
Stony
duction. Open symbols= SB, closed = WP; squares =
rapidly
colonized
by
other
species
(primar=
=
Open plots,circles Annualplots,triangles PerenWinter
Aduft Fecundity Fitness
nialplots.Data fromblocks5 and 6 at WP areshown ilygrasses,Artemesiavulgaris,and Stellaria
withdottedlines.
media) and by Octoberhad >80% cover.
Seedlingsin latercohortsfacedincreased
withthesespecieswhichmay
competition
thefitnesssurface(Fig. 9). Octoberrosette partiallyaccountforthestronger
selection
diameterexplained22% ofthephenotypic on emergence
dateat SB. At theWeldPrevariancein lifetime
fitness.
servetheopenandannualplotsmaintained
low percent vegetation cover (< 40%)
DIscuSSION
AtStonyBrook,
theexperiment.
throughout
HeterogeneousSelection.-The differ- theopenplotsresembled
theperennial
plots
encesintheenvironment
thatcausethehet- aftertwo monthsand werenot equivalent
of selectionamongthereplicate to theopentreatment
at theWeldPreserve.
erogeneity
plotsoperateovera local scale,butremain Becausethethreedisturbance
treatments
did
I foundfewconsistent
unknown.
that
itis notsurprising
differencesnotremaindistinct,
in the magnitudeof phenotypicselection estimatesofselectiondid notshowconsiswithrespectto thethreedisturbance
treat- tentdifferences
amongtreatments.
ments,so my a priorihypothesisof inWithintheWeldPreservesite,therewere
creasedselectionon seedlingsize in com- largeand consistent
in thepatdifferences
wasnotsupported
petitiveenvironments
by ternof selectionbetweenblocks 3 and 4
thesedata. Therewas significant
heteroge- versusblocks5 and 6. Thisspatialvariation
neityamongdisturbance
treatments
forfe- in selectionwas generally
muchlargerthan
selection
envicundity
forbothseedling the bioticeffects
of the competitive
gradients
sizeandemergence
date.Howeverselection ronment.The relevantenvironmental
dif-
PHENOTYPIC
Winter
2.0
~
1.0"I5
1.5-~~
._
103
SELECTION IN ERIGERON
1.0 -
f~
Fecundity
0.5
1.0~~~~~~~~~~~001
0.5
N
X
X5
4.0-
.
3.0-
i
Fitness
-1.0 _
diameter.
forOctoberrosette
and 95% confidence
intervals
Open
selection
differentials
FIG. 7. Standardized
= Perennial
plots.
symbols= SB, closed= WP; squares= Open plots,circles= Annualplots,triangles
ferences
are not known,howeverblocks5 selectionwas observedamongthe 18 plots
and 6 appearedto have sandiersoil that in nearlyall selectionepisodes.Selection
earlyin thelifecydriedout fasterduringthefalland winter, was mostheterogeneous
suggesting
thepossibleimportanceof abi- cle, withmost of the variationoccurring
oticfactors.
The severedesiccationmayac- amongblockswithinsites.Severalrecent
countforthelargeviability
selectionagainst studieshave examinedthe spatialheteroofphenotypic
selectionwithinplant
in thoseblocks.However, geneity
earlyemergence
suchinferences
aboutthecausesofselective populationsand thesehave shownsimilar
and direction
of
heterogeneity
are merelyspeculative.To variationin themagnitude
makecausalstatements
aboutparticular
se- selectionamongplots(Kalisz, 1986; Stewconlectiveforcesthe consequencesof abiotic artand Schoen,1987). The emerging
selectionon life
variationin soiltypemustbe testedexper- sensusis thatphenotypic
characters
is strongand variableon
as was done forthecompetitive history
imentally,
a local scale, even withinseeminglyhoregime(cf.Wade and Kalisz, 1990).
Significant
variationin themagnitude
of mogeneouspopulations(but see MitchellOldsandBergelson,
1990).Unliketheother
0
1.0
I=
0
0
0
0
30000
0 SB
I
@~0.8I
j
20000
C~~~~~~~~~~~
SB0
0.6
IL
0.4
10000
0.2
0.0
II
WP
*~~~~~~~
?
0
*
20
40
60
J
80
October Rosette Diameter(mm)
FIG. 8. Mean wintersurvivorship(?SE) as a function of October rosettediameterat Stony Brook and
the Weld Preserve.Each mean is based on minimum
of 12 plants.
0
20
40
60
October Rosette Diameter
fitness
FIG. 9. Mean lifetime
(?SE) as a function
of Octoberrosettediameterat StonyBrookand the
WeldPreserve.
104
DONALD A. STRATTON
studies,the fitness-related
size characters Direct versusIndirect Selection.-Much
examinedin thisexperiment
did not show ofthetotalselectionon emergence
dateand
variationin thedirection
ofselection:large Octoberrosettediameterwas theresultof
seedlings
hadhigher
viability
andfecundity,directselection,in additionto indirectseas is expectedoffitness-related
lectionthrough
traits.
effects
on sizeat laterstages
The phenotypic
selectionon emergence ofthelifecycle.The largedirecteffects
on
daterevealedan apparenttradeoff
between seedlingcharacters
resultfromtheimporthesize advantageofearlygerminating
in- tance of viabilityfitnesscomponentsfor
dividualsand survivorship
duringestab- lifetimeselectionin Erigeron.In contrast,
lishment.
Similarpatterns
wereobservedby Mitchell-Olds
andBergelson
(1990) showed
Arthuret al. (1973) and Marksand Prince littledirectselectionon seedlingsin Im(198 1) forcomparisons
offallvs. spring
ger- patiens,wherefitnesswas entirelydeterminating
plants.Viabilityversusfecundityminedbyfecundity.
In thisexperiment,
the
tradeoffs
withrespectto germination
date mostimportant
singlecontribution
wasthat
have generally
notbeen observedin single throughwintersurvivorship.
Winterviaseasoncomparisons
(Kalisz,1986;A. Biere, bilitydifferences
explainedoverhalfofthe
pers.comm.),althoughsuch tradeoffs
are selectionon Octoberrosettediameterand
oftenassumedforlifehistory
characters
at weretwiceas largeas selectionduringthe
equilibrium.
Directionalselectionon seed- adultlifestages.
lingsize may be expectedto decreasethe
Therecan not be "direct"fecundity
semeanemergence
dateuntilthisselectionis lectionon emergence
dateinthesamesense
balancedby conflicting
viabilityselection thattherecan be directfecundity
selection
againstindividuals
thatgerminate
tooearly. on adultphenotypes.
The directfecundity
In blocks1 through
4, negativedirectional selection
gradients
simplyindicatethatthere
selectionat laterstagesofthelifecyclewas are residualeffects
of emergence
date that
much stronger
thanthe positiveviability can not be explainedby the increasedfeselectionduringestablishment,
whereasin cundityof largerplants.In particular,if
blocks5 and 6 at WP thenetselectionon seedlingsize is heldconstant,
lateemerging
remainedpositive.
emergence
seedlingsmusthave grownfaster.PresumShape oftheFitnessSurface.
-Quadratic ablythepositivefecundity
selectiongradicoefficients
in earlyviabil- ents show a continuing
weresignificant
expressionof the
ity selectionepisodes,revealingboth sta- fastergrowthratesofthoseseedlings.
bilizingand disruptiveselectionon emer- Each of theselectiongradients
are meagencedate.Thisresulted
from
largevariation suredwitherrorand the errorsare comintherelativesurvivorship
ofcohorts2 and poundedwhen severalselectiongradients
3 duringestablishment.
The highmortalityare combined. Quantititiveconclusions
of cohort3 was largelyresponsibleforthe about the precisemagnitudes
of the comobserveddisruptiveselection.Laterselec- positeselectiongradients
maynotbe wartion episodesshowedonlymonotonicdi- ranted.Nevertheless,
therepeatability
withrectionalselectiontowardearlyemergence. in each block(Table 3) providesevidence
Quadraticselectiondifferentials
were sig- thatthe compositeselectiongradientsare
nificant
forlifetime
selectionon emergence reasonablyaccurateindicationsofthemadate,and forwinterviabilityselectionon jor patterns
of selection.
Octoberdiameter,but all had monotonic Othershave used pathanalysisto assess
fitnessfunctions.
FollowingMitchell-Olds therelativefitness
effects
ofvariouscausal
and Shaw (1987) theseare interpreted
as paths(Crespiand Bookstein,1989; Mitchdirectional
selection.The lifetime
selection ell-OldsandBergelson,
1990).Standardized
on seedlingsize was verynearlylinear.In selectiongradients
canbe converted
topath
the multipleregressions,
no quadraticse- coefficients
by multiplying
them by the
lectiongradientswere significant
forany standarddeviationof relativefitness.The
ofselection.Thusoverall,there techniquesof path analysiscould thenbe
component
was littleevidenceforstabilizing
ordisrup- usedto partition
thedetermination
ofeach
tiveselectionon seedlingsizeor emergence majorcomponent
offitness.
However,path
date.
analysisis less usefulwhenfitness
is mea-
PHENOTYPIC
SELECTION IN ERIGERON
105
suredin multipleepisodes.Pathsfromma- Ecology and Evolution, State Universityof
fitness
can New York at Stony Brook.
tolifetime
components
jor fitness
not be includedbecause path analysisasLITERATuRECITED
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S.
J.,
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M. J.WADE. 1984a. On themea(a logarithmic
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Evolution 38:709-719.
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1984b. On the measurementof naturaland
Also, in pathanalysiseach fitnesscomposexual selection: Applications. Evolution 38:720734.
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Corresponding
Editor:J.Schmitt
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