Paleontological Society
Genic Heterozygosity and Rate of Speciation
Author(s): John C. Avise
Source: Paleobiology, Vol. 3, No. 4 (Autumn, 1977), pp. 422-432
Published by: Paleontological Society
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Paleobiology. 1977. vol. 3, pp. 422-432.
Genic heterozygosity
and rate of speciation
John C. Avise
Abstract.-The hypothesisis proposedthat mean level of heterozygosity
is functionally
related to rate of speciationin evolutionary
phylads. Under this hypothesis,phylads which
speciatemorerapidlydo so because of increasedlevel of within-species
geneticvariability
whichis then available to conversionto species differences
under appropriateecological or
environmental
conditions.An important
corollaryis that rate of speciationcould be limited
in phyladswith low geneticvariability,
irrespective
of environmental
considerations.
This hypothesishas been testedwith respectto electrophoretically
detectablevariationin
productsof structural
genesin two familiesof NorthAmericanfishescharacterized
by grossly
different
ratesof speciation.Totals of 69 species of the highlyspeciose Cyprinidae,and 19
species of the relativelydepauperateCentrarchidae,
were assayed for mean level of heterozygosityat 11-24 geneticloci. Since Cyprinidaeand Centrarchidaeexhibiton the average
nearlyidenticallevels of genic variation(H = 0.052 ? 0.004, and H = 0.049 + 0.009,
respectively),the hypothesisthat level of heterozygosity
affectsrate of speciationin these
fishesis not supported.
the amountof genicvariability
in both Cyprinidaeand Centrarchidae
Nonetheless,
is large,
comparableto mean levels in previouslystudied vertebrates.The great wealth of genome
variability,reflectedin the electrophoretic
variationpresent in virtuallyall outcrossing
organisms,apparentlycan accommodateconsiderableflexibility
in rate and patternof evolutionaryresponseto the variousenvironmental
regimeschallengingorganisms.
JohnC. Avise. Departmentof Zoology,University
of Georgia; Athens,Georgia 30602
Accepted: June 15, 1977
Introduction
Rates of appearance of new kinds of organisms (such as species, genera,or families)
are clearly heterogeneousthru evolutionary
time and across evolutionaryphylads (Simpson 1944). To manyevolutionistsin the early
part of this century,mutationrates were an
importantlimitingfactorin the originof new
taxa. Hence, "At firstthoughtit mightseem
obvious that periods of explosive radiation
should be attributedto an increased rate of
mutation" (Rensch 1959, p. 103). This hypothesis was gradually abandoned (though
never stronglytested and falsified) as evidence accumulated that stores of genetic
variabilitywere normallygreat in most populations, and in any generation of sexual
reproducers, the variability generated by
recombinationfar surpassed novel variation
introduced by mutations. As expressed by
Huxley (1942, p. 517): "limits(to evolution)
are relative to the environmentalsituation:
if this is radically altered,evolutionaryradiation may again set in, showing that the
previous standstill was not due to lack of
genetic variability";and by Rensch (1959, p.
104): "mutation at normal rates provides
such a wealthof variantsthatwe need assume
no furtherincrease of mutation,but only a
more intenseselection,for . . . rapid adaptive
radiation."
Recent views emphasize organism-environment relationshipsas the principal factors
controllingspecies proliferation(Grant 1963;
Hutchinson 1959). Thus adaptive radiations
often occur when a species enters an unoccupied habitat with diverse "open niches,"or
when a population acquires a new complex
of adaptive charactersenabling it to exploit
an available environmentmore efficiently
(Stebbins 1971). The possibilitythat general
geneticfactors,apart fromhistoricallyunique
acquisitions,mightexerta profoundinfluence
on rate of speciationhas been largelyignored.
Notable exceptions are the suggestions of
Mayr (1963) that evolutionary rates are
stronglyaffectedby degree of genetic buffering or homeostasis and Carson's (1959,
1968) early hypothesisthat genetic systems
withopen or freerecombinationunder certain
conditionspromotespecies formation.
HETEROZYGOSITY
423
AND SPECIATION
TABLE 1.
Taxonomic and evolutionary informationon minnows (Cyprinidae) and sunfish (Centrarchidae).
Information from Branson and Moore (1962), Gosline (1971, 1974), Miller (1959, 1965), and Romer
(1966).
Attribute
1)
2)
3)
4)
5)
6)
7)
8)
# living species worldwide
# living genera worldwide
# living N. Amer. species
# living N. Amer. genera
# known fossil species in N. Amer.
# known fossil genera in N. Amer.
earliest known N. Amer. fossils
probable ancestors of N. Amer. forms
There is at least a minimal relationship
between level of genetic variabilitywithin a
species and its potentialfor speciation: rates
of divergence among populations lacking
genetic variabilityare no greater than rates
of appearance of new mutations. Is the relationshipbetween genetic variabilityand rate
of speciation strongerthan this? Lewontin
(1974, p. 186) argues "This last point, that
considerable evolutionarychange (including
speciationand divergenceof new fullspecies)
occurs withoutbeing limited by the rate of
appearance of novel genes is the chief consequence, for the process of speciation,of the
immensearrayof genetic variationthat exists
in populations of sexually reproducing organisms." Evidence gathered in the last ten
years,primarilythroughelectrophoretictechniques, suggests that not all evolutionary
phylads have similarlevels of genic variation
(Selander 1976; Powell 1976). The null hypothesis erected and tested in this paper is
that there is no correlationbetween rate of
speciation in an evolutionaryphylad and the
mean level of genic variability within its
member species.
Two families of fishes characterized by
greatly differentrates of speciation were
chosen for study: the highlyspeciose North
American minnows (Cyprinidae) and the
comparatively depauperate sunfish (Centrarchidae). Ancestors of both families entered the North American continentin the
mid-Cenozoicand radiated into species which
now occupy nearly all bodies of water on
the continent. But the minnows are repre-
Cyprinidae
2000
250
200
40
? 28
> 16
Miocene
one or a few
membersof the
subfamilyLeuciscinae from
Eurasia
Centrarchidae
30
9
30
9
> 8
> 6
Eocene (?)-Miocene
formssimilarto
Serranidae (sea
basses)
sented by many more living species than are
the sunfish,and there are more fossilspecies
of minnowsas well. Evidence forthe greater
rate of speciation among the minnowsis discussed in more detail elsewhere (Avise and
Ayala 1976) and is summarizedin Table 1.
Since species of minnows (and sunfish) have
been described primarilyby morphological
criteria,the conclusion of a higher rate of
speciation among minnows is really a statement about a higher perceived rate of diversification
intomorphologically
recognizable
typesof organisms.Biological species criteria
may not be met in all cases. For purposes
of the presentstudy,we must simplyregard
North American Cyprinidae as an unusually
rapidly speciating phylad. This should be
true, even if the group has been "oversplit"
somewhat by classical taxonomistsbecause
there is nearly an order of magnitude more
recognized species of minnows than sunfish.
Using standard techniques of starch-gel
electrophoresis,we have assayed levels of
genic heterozygosity
in populationsbelonging
to a total of 69 species of North American
Cyprinidaeand in populationsof an additional
19 species of Centrarchidae. A finding of
significantlyhigher levels of heterozygosity
among the minnows would support hypotheses causally linking genetic variability to
rate of speciation. Conversely,a finding of
comparable levels of heterozygosityin the
two familieswould be consistentwithmodern
interpretationsthat ecological factors are
primarily responsible for varying rates of
speciation.
424
Materialsand Methods
AVISE
I
II
I
I
Pgi-I
Pgi-2
I
I
Fish were frozen on dry ice immediately
14 aftercapture and stored at -60?C until they
could be processedand run,which was almost
12invariablywithin six monthsof time of collection. The horizontal starch-gel electroEs-3 *
phoreticprocedures are similarto those now
lo _
routinelyemployed by many laboratoriesto
assay levels of genic variation in a wide
8
*Pgm
varietyof organisms(for detailed procedures Q_
see Selanderet al. 1971 and Ayala et al. 1972).
Only thosesystemswere scoredwhichshowed 06
exceptionallyclear banding patterns.
*6pgd
4
Different types of loci characteristically
4 /Ldh-I
*Got-l
differin mean levels of heterozygosity
across
*Mdh
species (Selander 1976). For example, es2
terase loci encode an unusuallyvariable class
Ldh- Idh Es - I
of proteins. In order to obtain unbiased relaI
I
p i
I
I
I
I
tive estimates of heterozygosityin sunfish
2 4 6 8 10 12 14 16
0
versus minnows, an attempt was made to
h, CENTRARCHIDAE
assay analogous (and often presumably hoat homologousloci asmologous) proteins in the two groups. A FIGURE 1. Heterozygosities
total of 1429 specimensof 19 species of Cen- sayed in both the Cyprinidaeand Centrarchidae.
The correlation
is significant(r = 0.69, df= 9, 0.01
trarchidaewas assayed at 11-15 genetic loci: < P < 0.05).
lactate dehydrogenases(Ldh, 2 loci), isocitrate dehydrogenase (Idh), 6-phosphogluconate dehydrogenase(6Pgd), esterases (Es, 2 also has
weaknesses: if a locus is considered
loci), malate dehydrogenase (Mdh), glutapolymorphicwhen two or more alleles are
mate-oxalate transaminases (Got, 2 loci),
present, the proportionof polymorphicloci
tetrazoliumoxidase (To), phosphoglucomu- increases
with increasingsample size; if more
tase (Pgm), phosphoglucoseisomerases (Pgi,
stringentcriteriafor polymorphismare used
2 loci), peptidase (Pep), and one non-enzy(i.e., frequencyof commonallele < 0.95), the
matic protein (Pt). A total of 499 specimens
proportionof polymorphicloci becomes partly
of 69 species of Cyprinidae was assayed at
dependent upon the particular criterion
14-24 genetic loci: Ldh (2 loci), Idh, 6Pgd, chosen.
Average heterozygosity,
for a popuEs (3 loci), Mdh (3 loci), Got (2 loci), To,
lation, H, is defined as
Pgm, Pgi (2 loci), alcohol dehydrogenase
(Adh), a-glycerophosphate dehydrogenase
1 it=
(Gpd), triosephosphateisomerase (Tpi), and
hq,
~(1)
t=1
five loci encoding nonenzymatic proteins.
where hi is the proportion of individuals
For more complete descriptionsof these sysat the ith locus and 1 is the
heterozygous
tems in sunfishsee Avise and Smith (1974),
number
of
loci.
Since estimatesof H are not
and in minnowssee Avise and Ayala (1976).
dependent
upon
arbitrarycriteria and are
Ten to eleven homologous loci were consisunbiased
with respect to sample
relatively
tentlyscored in both the cyprinidsand cenfor
size,
they
provide
purposes of the present
trarchids(Fig. 1).
the most appropriate summaries of
study
Several statisticsmay be employed to sumlevels of genic variability(see Nei 1975).
marize levels of genic variability within a
Only eight genomes per species were expopulation. The mean number of alleles per amined in most of the 60 sampled species
locus suffersthe major bias of being strongly of Cyprinidae inhabitingthe eastern United
dependent upon sample size; as more indi- States, includingmembersof the large genus
viduals are sampled, more rare alleles are Notropis, although sample sizes for most
found. The proportionof polymorphicloci centrarchidsand westerncyprinidswere con-
HETEROZYGOSITY
425
AND SPECIATION
TABLE 2. Estimatesof genic variability
in NorthAmericanCentrarchidae. Polymorphicloci defined by
frequencymostcommonallelle < 0.95. Intra-locusand inter-locusvariancescalculated accordingto pro(1974).
cedure of Nei and Roychoudhury
genus
#loci
sampled
species
Lepomis
auritus
Lepomis
cyanellus
Lepomis
gibbosus
Lepomis
gulosus
Lepomis
humilis
Lepomis
macrochirus
Lepomis
marginatus
Lepomis
megalotis
Lepomis
microlophus
Lepomis
punctatus
Acantharchuspomotis
Ambloplites rupestris
Archoplites interruptus
Centrarchus macropterus
Elassoma
evergladei
okefenokee
Elassoma
Enneacanthus obesus
Micropterus salmoides
Pomoxis
nigromaculatus
14
14
14
14
14
15
14
14
14
14
11
11
11
11
11
11
11
11
11
mean #
alleles
per locus
percent
polymorphic
loci
1.71
1.21
1.14
1.14
1.28
1.40
1.21
1.78
1.28
1.21
1.09
1.36
1.09
1.00
1.00
1.09
1.00
1.18
1.09
36
21
14
14
14
27
14
50
14
21
9
36
0
0
0
9
0
18
9
siderably larger (up to 760 individual bluegill, Lepomis macrochirus). Single locus
heterozygositieswere calculated from allele
frequenciesusing Hardy-Weinbergprobabilities (hi = 1 - ZX2j' where Xj is the frequency
of the jth allele at the locus).
The variance of h, given by
V( h) =
1
i=l_
iH2'
2
(2)
mean ? S.E.
heterozygosity
per locus
0.082 ?
0.083 ?
0.056 ?
0.030 ?
0.046 ?
0.056 ?
0.069 ?
0.122 ?
0.033 ?
0.082 ?
0.029 ?
0.129 ?
0.004 ?
0.000
0.000
0.027 ?
0.000
0.082 ?
0.010 ?
0.036
0.046
0.039
0.022
0.029
0.040
0.049
0.051
0.017
0.045
0.029
0.061
0.004
0.027
0.055
0.010
intralocus
variance
interlocus
variance
.00129
.00049
.00038
.00080
.00056
.00076
.00027
.00175
.00040
.00056
.00186
.00180
.00011
-
.01685
.02913
.02091
.00598
.01122
.02164
.03334
.03467
.00365
.02779
.00739
.03913
.00007
-
.00053
.00750
.00046
.00042
.03281
.00068
-
-
V(h) are reported. For the majorityof species a single population was assayed, so conclusionsof thisstudyapply strictlyto possible
correspondence of rate of speciation with
mean population heterozygosity. Since an
overwhelming result from electrophoretic
studies suggests that conspecificpopulations
are typically very similar in overall allelic
composition,this restrictionis probably not
too severe (Avise 1974; Ayala 1975).
was partitionedinto the inter-locusand intralocus components according to procedures Results
developed by Nei and Roychoudhury(1974).
Levels of genic variabilityin the centrarchid
For 73 of the 80 species exhibitingvariability, species examinedin thisstudyare summarized
the variance due to inter-locusheterogeneity in Table 2. Estimates of heterozygosity(H)
was greater,usually many timesgreater,than range from a low of 0.000 in Enneacanthus
that due to intra-locusvariation attributable obesus, Elassoma evergladei,and Centrarchus
to sample size and allele frequencies (Tables macropterusto a high of 0.129 ? 0.061 in
2 and 3). As emphasized by Fuerst et al. Ambloplites rupestris. Heterozygosityesti(1977) and Nei and Roychoudhury(1974), mates for the 69 minnow species are prefor purposes of estimatingaverage hetero- sented in Table 3 and exhibita similarrange
zygosityper population, it is far preferable from0.000 (in Notropis coccogenis,Notropis
to examinea large numberof loci ratherthan dorsalis,Notropisspilopterus,Hybopsis lineaa large number of individuals.
punctata, and Semotilus atromaculatus) to
Genetic variabilitywithina species has two 0.154 ? 0.052 (in Notropis texanus). Since
components: heterozygosity
withinlocal pop- heterozygosity
levels can be influencedby a
ulations and differencesbetween populations. varietyof evolutionaryforces,both stochastic
For thosespecies representedby samplesfrom and deterministic,the apparent differences
two or more populations (Lepomis auritus, among particularspecies of minnowsor sunL. macrochirus,L. microlophus,and L. gulo- fishmustbe interpretedwith extremecaution.
sus), average values of the statisticsH and The confidencefor any single heterozygosity
426
AVISE
3. Estimatesof genic variabilityin NorthAmericanCyprinidae.Polymorphic
loci definedand variances calculatedas forTable 2.
TABLE
genus
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
Notropis
species
#loci
sampled
mean #
alleles
per locus
17
17
17
17
17
16
16
17
17
17
17
17
16
1.24
1.23
1.12
1.23
1.12
1.25
1.06
1.41
1.29
1.23
1.00
1.18
1.25
atherinoides
atropiculus
baileyi
bellUs
boops
buchanani
callisema
chalybaeus
chrosomus
chrysocephalus
coccogenis
cornutus
cummingsae
dorsalis
16
Notropis
euryzonus
16
fumeus
Notropis
17
Notropis
galacturus
17
gibbsi
Notropis
17
Notropis
greenei
17
Notropis
hudsonius
16
Notropis
hypselopterus 17
Notropis
leedsi
16
Notropis
longirostris
17
Notropis
15
lutipinnis
lutrensis
Notropis
16
Notropis
maculatus
16
niveus
Notropis
16
Notropis
ozarcanus
16
Notropis
petersoni
17
Notropis
17
pilsbryi
rubellus
Notropis
16
16
signipinnis
Notropis
14
spilopterus
Notropis
sp. (undescribed) 17
Notropis
stramineus
17
Notropis
17
telescopus
Notropis
texanus
17
Notropis
17
topeka
Notropis
trichroistius
17
Notropis
umbratilis
17
Notropis
16
uranoscopus
Notropis
venustus
16
Notropis
volucellus
17
Notropis
17
whipplei
Notropis
xaenurus
15
Notropis
zonatus
16
Notropis
zonistius
17
Notropis
16
Campostoma anomalum
nubila
16
Dionda
buccata
16
Ericymba
bicolor
24
Gila
24
Hesperoleucussymmetricus
16
lineapunctata
Hybopsis
Hybopsis
sp. (undescribed) 15
storeriana
15
Hybopsis
exilicauda
24
Lavinia
24
Mylopharodonconocephalus
16
leptocephalus
Nocomis
Nocomis
micropogon
16
1.00
1.06
1.24
1.18
1.06
1.35
1.06
1.06
1.12
1.24
1.20
1.38
1.50
1.12
1.06
1.35
1.12
1.12
1.06
1.00
1.23
1.24
1.18
1.41
1.23
1.12
1.12
1.25
1.19
1.24
1.23
1.07
1.12
1.18
1.06
1.31
1.12
1.25
1.46
1.00
1.20
1.07
1.21
1.04
1.06
1.06
percent
polymorphic
loci
24
18
12
24
12
19
6
35
24
24
O
12
19
0
6
18
18
6
29
6
6
12
18
13
31
38
12
6
29
12
12
6
0
24
35
12
41
24
12
12
25
19
18
24
7
12
18
6
25
12
21
25
0
13
7
17
4
6
6
Mean ? S.E.
heterozygosity
per locus
0.079 ?
0.070 ?
0.026 ?
0.074 ?
0.040 ?
0.080 ?
0.015 ?
0.114 ?
0.101 ?
0.096 ?
0.000
0.044 ?
0.082 ?
0.037
0.041
0.018
0.035
0.029
0.046
0.015
0.045
0.048
0.045
0.029 ?
0.050 ?
0.048 +
0.013 +
0.096 +
0.017 ?
0.013 ?
0.043 ?
0.050 ?
0.058 ?
0.103 ?
0.135 ?
0.041 ?
0.031 ?
0.130 ?
0.059 ?
0.043 +
0.014 ?
0.000
0.092 +
0.130 ?
0.037 ?
0.154 ?
0.093 ?
0.035 +
0.040 ?
0.066 ?
0.078 ?
0.081 +
0.069 ?
0.025 ?
0.028 ?
0.053 ?
0.014 ?
0.076 +
0.043 ?
0.059 ?
0.067 ?
0.000
0.050 ?
0.018 ?
0.048 +
0.006 ?
0.015 ?
0.029 ?
0.029
0.028
0.027
0.013
0.039
0.017
0.013
0.032
0.028
0.045
0.044
0.048
0.030
0.031
0.052
0.040
0.032
0.014
0.000
0.033
0.048
0.044
0.047
0.026
0.052
0.044
0.025
0.030
0.030
0.043
0.045
0.034
0.025
0.019
0.031
0.014
0.036
0.032
0.028
0.025
0.037
0.018
0.024
0.006
0.015
0.029
intralocus
variance
interlocus
variance
.00531
.00323
.00292
.00490
.00312
.00299
.00146
.00720
.00297
.00351
.00309
.00273
.01796
.02535
.00259
.01592
.01118
.03087
.00214
.02722
.03620
.03091
-
.00073
.00473
.00402
.00146
.00781
.00201
.00146
.00228
.00473
.00213
.00670
.00839
.00228
.00146
.00814
.00080
.00228
.00155
.00383
.00626
.00326
.00755
.00383
.00274
.00214
.00803
.00315
.00629
.00479
.00264
.00292
.00361
.00155
.00620
.00228
.00043
.00085
-
.00351
.00214
.00037
.00005
.00155
.00073
-
.01542
.03413
-
.01273
.00860
.00837
.00141
.01805
.00261
.00141
.01410
.00860
.02824
.02428
.02847
.01212
.01392
.03783
.02640
.01410
.00159
.02908
.03129
.00824
.03842
.02908
.00788
.01316
.00637
.02643
.02813
.01486
.00674
.00286
.01273
.00159
.01454
.01410
.01839
.01415
-
.01703
.00272
.01345
.00081
.00205
.01273
HETEROZYGOSITY
TABLE
genus
427
AND SPECIATION
3.-( Continued).
species
#loci
sampled
Notemigonus crysoleucas
Orthodon
microlepidotus
Phoxinus
erythrogaster
Pimephales notatus
Pimephales vigilax
Pogonichthys macrolepidotus
Ptychocheilus
grandis
Rhinichthys cataractae
Richardsoniusegregius
atromaculatus
Semotilus
24
24
14
14
16
24
24
16
24
16
mean #
alleles
per locus
1.29
1.08
1.21
1.21
1.31
1.13
1.13
1.12
1.13
1.00
per cent
polymorphic
loci
21
4
21
14
25
8
4
12
8
0
mean ? S.E.
heterozygosity
per locus
0.068 ?
0.015 ?
0.078 ?
0.045 ?
0.084 ?
0.036 +
0.011 ?
0.015 ?
0.030 ?
0.000
0.029
0.011
0.044
0.032
0.041
0.011
0.011
0.015
0.024
intralocus
variance
interlocus
variance
.00145
.00032
.00382
.00396
.00620
.00020
.00030
.00228
.00034
.01873
.00258
.02328
.01038
.02070
.00270
.00260
.00132
.01348
-
-
estimateis low since (1) relativelyfew loci and 0.049 + 0.009, respectively.The patterns
contributeto the genic variability,(2) the of genetic variability,reflected in the frenumberof individualssampled per species is quency distributionsof single locus heterogenerallysmall, and (3) in most cases only zygositiesacross all assayed species, are also
a single population of a species was sampled. virtually identical in Cyprinidae and CenThe fact that several species of sunfishand trarchidae (Fig. 2). Thus we cannot falsify
minnows displayed no variation at the loci the hypothesisthat there is no relationship
examined does not imply that theirgenomes of within-speciesvariabilityand rate of speciation in these two familiesof fishes.
totally lack genetic variability.
There is considerableheterogeneity
in mean
However, withineach familyan interesting
level of variabilityacross loci in both the patternemerges. Of the 200 species of Cyminnows and sunfish (Table 4). The most prinidae inhabitingNorth America, roughly
placed in a single genus
consistentlyvariable loci in Cyprinidae are one-halfare currently
Es-2, Pgi-1, and Pgi-2, with mean heterozy- Notropis. On the average, the Notropis spegosities across species equal to 0.183, 0.148, cies appear significantlymore heterozygous,
and 0.142, respectively. Among Centrarchi- H = 0.059 + 0.006, than do members of
dae, the Es-3 locus is most variable: mean other cyprinidgenera, H = 0.037 + 0.006
heterozygosityequals 0.157. Eleven of the (t67 = 2.46, P < 0.01, one-tailedtest). Similoci examined in the majorityof Cyprinidae larly, Lepomis is the most diverse of cenand Centrarchidaewere judged likely to be trarchid genera (comprising 11 of the 30
homologous on the basis of zymogrampat- species), and species of Lepomis appear more
terns and tissue specificities.For these loci, heterozygouson the average, H = 0.066 +
there is a significantcorrelation between 0.009, than do representativesof the other
mean level of variabilityin sunfishand minH = 0.031 -+-0.015 (t17 = 2.04, P <
nows (Fig. 1). Selander (1976) has empha- genera,
0.05,
one-tailed
test). If we accept the current
sized the desirabilityof assaying homologous
belief that rates of speciationin Notropisand
enzymes when close comparisons among
Lepomis have been particularly rapid for
species are attempted.
their respective families, then within CenNotwithstandingthe fact that individual
trarchidaeand Cyprinidae a positive correlavalues of H are not precise,overall estimates tion between rate of speciation and heteroof levels of genic variationin Cyprinidaeand zygositymay exist.
Centrarchidaeshould reflectreal differences Members of Lepomis and Notropis are
between the two groups if they do indeed renowned for propensityof interspecieshyexist. Mean heterozygositiesacross species bridization (Hubbs 1955). Thus the higher
(H) in minnowsand sunfishare summarized mean heterozygositiesof species in these
in Table 5. Cyprinidae and Centrarchidae genera could conceivablyresult frompresent
appear remarkablysimilar in amount of ge- or past introgressionof alleles fromone spenetic variability: mean heterozygositiesper cies to another. Roberts (1964) hypothesizes
species in the two groups are 0.052 + 0.004 that hybridizationand introgression
provided
428
AVISE
4. Mean heterozygosities
at various loci in TABLE 5. Summaryof levels of heterozygosity
per
species of Cyprinidaeand Centrarchidae.Note that species in NorthAmericanCyprinidaeand Centrarhomologiesfor Pt-0 locus in minnowsand sunfish chidae.
are uncertain.
TABLE
mean heterozygosityper species
locus
Cyprinidae
( #species)
Centrarchidae
( #species)
Ldh-I
Ldh-2
Mdh
Es-I
Es-3
Idh
Pgi-i
Pgi-2
Pgm
6Pgd
Got-I
Got-2
To
0.032
0.007
0.029
0.000
0.103
0.000
0.148
0.142
0.073
0.047
0.037
0.003
0.122
(69)
(69)
(69)
(69)
(69)
(69)
(69)
(69)
(69)
(69)
(69)
( 9)
( 9)
0.000
0.000
0.011
0.054
0.157
0.000
0.084
0.083
0.076
0.058
0.077
0.056
0.047
Es-2
Adh
Gpd
Tpi
Pt-0
Pt-1
Pt-2
Pt-3
Pt-4
0.183
0.021
0.036
0.002
0.000
0.006
0.000
0.114
0.044
(55)
( 9)
( 9)
( 9)
(69)
(65)
( 9)
( 9)
(69)
0.000 ( 1)
-
Pep
-
(19)
(19)
(19)
(10)
(10)
(19)
(19)
(19)
(19)
(19)
(19)
(19)
(10)
Group
Centrarchidae
Lepomis
othergenera
total
Cyprinidae
Notropis
othergenera
(easternU.S.)
othergenera
(westernU.S.)
total
#species
assayed
Mean ? S.E.
heterozygosity
per species
10
9
19
0.066 ? 0.009
0.031 ? 0.015
0.049 ? 0.009
47
0.059 ? 0.006
13
0.036 ? 0.008
9
69
0.038 ? 0.008
0.052 ? 0.004
0.009 (19)
the genetic and phenotypicvariabilitynecessary for adaptation of many centrarchidsto
changingenvironmental
conditionsduringthe
Pleistocene. However, this thesis should be
reevaluated in view of recent evidence that
despite ability to hybridize,centrarchidsare
very differentin allelic composition (Avise
and Smith1974). The extentand evolutionary
significance,if any, of introgressionamong
species of Notropis, is unknown. Thus in
broad perspective,the most reliable comparison between rate of speciation and genic
heterozygosityinvolves Centrarchidaeversus
Cyprinidae,althoughresultsforthesefamilies
are moderated somewhat by results of comparisons of genera withineach family.
Discussion
A plethoraof hypotheseshas been advanced
to account for apparent differencesin levels
of heterozygosityamong differentorganisms
(Ayala 1976; Lewontin 1974; Berger 1976).
Prominenthave been attemptsto relate the
geneticsystemitselfwith population features
such as patternsof reproduction,recombination, or functionaldiversitywhich might be
favored in particular environmentalregimes
(Allard 1975). For example, environmental
heterogeneityper se may select for greater
variabilityby favoringdifferentgenetic varienvironmental
antsin different
niches (Powell
1971; McDonald and Ayala 1974). Levins'
(1968) theoryof adaptive strategyled to a
suggestion that organisms which perceive
theirenvironment
as patchy (a coarse-grained
environment)respond by maintaininggreater
levels of variability(Selander and Kaufman
1973). More recently,the suggestionhas been
made that trophicresourcestabilityincreases
genic heterozygosityby permittingspecialization to habitats which are then perceived
by the organismsas coarse-grained(Valentine
1976; Valentine and Ayala 1974; Ayala et al.
1974). Althoughsome data exist to support
these hypotheses,the sum of all available
evidence suggests that genetic variabilityis
remarkablysimilar among species, independent of environmentalconsiderationsor lifestyle (Lewontin 1974).
In this paper the hypothesisis proposed
that phylads which speciate more rapidly do
so because of increasedlevel of within-species
genetic variabilitywhich is then available to
conversion to species differencesunder appropriate environmentalcircumstances. This
hypothesishas the corollarythat rate of speciation could be limitedin phylads with low
genetic variability,irrespective of environmental considerations. This hypothesis has
been testedwithrespectto electrophoretically
detectable variationin products of structural
HETEROZYGOSITY
800
>601
L' 40
D
CYPRINIDAE
H =0.052
20
.10 .20 .30 .40 .50 .60 .70 .80
HETEROZYGOSITY
o80
t60
LL40
D
a
CENTRARCH
IDAE
=0.049
W 20
.10 .20 .30 40 .50 .60 .70 .80
HETEROZYGOSITY
FIGURE2. Frequency distributions
of single locus
heterozygosity
acrossall assayedspeciesof Cyprinidae
and Centrarchidae.
genes in two familiesof fishes characterized
by grosslydifferent
rates of speciation. Since
Cyprinidaeand Centrarchidaeexhibiton the
average nearly identical levels of genic variation (H = 0.052 ? 0.004 for 69 species of
cyprinids,and H = 0.049 ? 0.009 for 19 species of centrarchids),the hypothesisthat rate
of speciation and level of heterozygosityare
related is not supported.
These resultsare inconsistentwith the idea
that level of variabilityin structuralgenes
affectsrate of speciation. The levels of heterozygosityin Cyprinidaeand Centrarchidae
are nearly identical to the mean values previouslyreportedforothervertebrates(Selander and Kaufman 1973). "Normal" levels of
heterozygosityin most phylads appear sufficient to supportwidelyvaryingrates of population differentiation
and speciation. Hence
these results are not inconsistentwith prevailing hypothesesrelegatinga major role to
ecological considerationsin determiningrates
of species proliferation.Of course, the minnows and sunfishconstituteonly a single test
of the originalhypothesis.Despite the availability of heterozygosityestimates in more
than 250 species (Powell 1976), I am not
aware of any other appropriatesets of data
AND SPECIATION
429
to examine this question. Because it is possible thata subtlepositivecorrelationbetween
H and speciation rate does exist, but was
overriddenin this test by other phenomena,
additional testswith othertypes of organisms
would be welcome.
One such phenomenonleading to a negative
theoreticalcorrelationbetween rate of speciation and genic heterozygosity
has been proposed by Soule (1971, 1976), who argued
that younger species should be less polymorphic than older species, when speciation
is accomplished by population bottlenecks
and loss of heterozygosity.The time course
of change in heterozygosity
depends not only
on the size of the population bottleneck,but
also on the subsequent rate of population
increase (Nei et al. 1975). Once heterozygosityis reduced to a low level, the number
of generationsnecessaryto reestablishequilibrium heterozygositylevels for neutral alleles
is veryroughlythe reciprocalof the mutation
rate (Nei et al. 1975) or perhaps ten million
years for Centrarchidae or Cyprinidae (assuming ,u = 10-7, generationlength = one
year, heterozygosity
severelyreduced during
speciation). If we assume the NorthAmerican
centrarchids and cyprinids are 50 million
yearsold, and speciationeventshave occurred
at regular time intervalsin all lineages, the
average durationsof centrarchidand cyprinid
species are about 10 million and 6 million
years, respectively; mean heterozygosity
among cyprinids should be reduced below
equilibriumlevels. If ? 10-8, H should be
below equilibrium in both families, and if
__ 10-6,
H shouldbe at equilibrium
in both
families. Gross uncertaintiesabout t, equilibrium heterozygositylevels, and specific
evolutionaryhistoriesof minnowsand sunfish
preclude more definitiveanalysisof the "timedivergence"model. However, the findingof
increased H in Notropisand Lepomis (many
of whose members are thought to be particularlyyoung compared to most species in
othercyprinidand centrarchidgenera-Avise
and Smith 1977; Avise, in prep.) is exactly
the opposite of what would be predicted if
age of species is positivelyrelated to heterozygosity. Either speciationsin Centrarchidae
and Cyprinidaedo not normallyentail severe
loss of heterozygosity,
and/or mean durations
of species are sufficientto permit recovery
of lost variability.
430
AVISE
.050
LL
LLJ(
0>
X
_
V
X
a
I
.050
I
CENTRARCHIDAE
CYPRINIDAE
0
0 >-
N.025 -
21
N.025
>-
.05
.10
HETEROZYGOSITY
.15
.05
.10
HETEROZYGOSITY
.15
FIGURE 3.
and interlocus
variMean heterozygosities
ances of heterozygosity
forspeciesof NorthAmerican
Cyprinidae.The solid line indicatesthe theoretical
relationshipunder the neutralityhypothesisfor an
infiniteallele model (the theoreticalcurve for a
stepwise mutationmodel is very similarover the
rangeof heterozygosities
plotted).
4. Mean heterozygosities
and interlocus
variances of heterozygosity
for species of NorthAmerican Centrarchidae.The solid line indicates the
theoretical
relationship
underthe neutrality
hypothesis
foran infiniteallele model (the theoreticalcurvefor
a stepwisemutationmodel is very similarover the
range of heterozygosities
plotted).
Another related attempt to account for
observed heterozygosity
levels in naturalpopulations constitutes the mutation-drifthypothesis, which holds that the majority of
protein variation is neutral with respect to
fitness (Kimura and Ohta 1971). This hypothesisis particularlyvaluable, since neutral
mutationrates and population sizes account
forchanges in genic composition,and specific
predictions can be generated about characteristicsof genetic variation in populations.
Nei (1975) and Fuerst et al. (1977) determined the theoretical relationship between
mean and variance of heterozygosity,and
found a close fit with empirical data taken
from 129 species of vertebratesand invertebrates. Figures 3 and 4 indicate that the fit
of mean and interlocusvariance of heterozygosities to the theoreticalrelationshipunder
the neutralmutationmodel is also very good
for both the Cyprinidae and Centrarchidae.
If variation in the structuralgenes assayed
in this study is indeed invisible to natural
selection, by definitionit could not play a
role in determiningrates of speciation. In
the future it will be valuable to propose
additional hypothesesabout possible genetic
factorsaffectingspeciation and to test these
hypotheseswith a broader class of genes.
One finalpossibilityconcerningthe lack of
apparent correlationbetween H and speciation rate should be considered. Although
improbable,it is conceivable that the difference in numbersof living minnowsand sun-
fish is attributablesolely to differentialextinctionratherthan to differential
speciation.
In this case, our presentstudy would in fact
have tested the null hypothesisthat there is
no correlationbetween rate of extinctionand
mean level of heterozygosity
in an evolutionary phylad. Neithercould this hypothesisbe
falsifiedwith the present data. However, a
strongbias mightbe operatingagainst a falsification-those sunfishspecies which did survive to be assayed were those with higher
heterozygosities.
Results of this study complementand extend those of two earlier reports which attempted to relate genic heterozygosityto
evolutionaryrates. Ayala et al. (1973) concluded froma studyof Tridacna maximathat
the massive marine extinctionsregisteredin
the fossil record were not due to a general
scarcityof genetic variabilityin populations
inhabiting stable environments. Similarly,
Selander et al. (1970) found that the slow
rate of morphological evolution in "living
fossils"such as Limulus polyphemuswas not
due to a lack of variationat the genic level.
The present study shows that the slow rate
of speciation in sunfishrelative to minnows
is not attributableto a lower level of withinspecies variationin structuralgenes. Attempts
to find a positive relationshipbetween rate
of evolutionarychange (either anagenetic or
cladogenetic) and level of genic variationas
measured electrophoreticallyhave been unsuccessful. The wealth of electrophoretic
FIGuRE
HETEROZYGOSITY
variation present in virtuallyall outcrossing
organismsappears to reflectlevels of genome
variabilitysufficientto account forwide flexibility in rate and pattern of evolutionary
response to environmentalchallenges. Explanations for differingrates of speciation
apparentlymustbe soughtin termsof specific
relationships, and/or
organism-environment
othertypes of genetic influences.
431
AND SPECIATION
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