Biological Journal afthe Linnean SocieQ (1991), 44: 65-80. With 6 figures
High levels of genetic variation in natural
populations of marine lower invertebrates
A. M. SOLE-CAVA* AND J. P. THORPE
Department of Marine Biology, University of Liverpool, Port Erin Marine Laboratory,
Port Erin, Isle of Man
Received 5 M a y 1969, accepted for publication 25 October 1990
The predictions of neutralist and selectionist hypotheses have been tested many times in the past,
but mostly using data only from organisms such as vertebrates, with generally low to average
heterozygosities. The more recent discovery of particularly high levels of genetic variation in marine
sponges and coelenterates provides an opportunity to use data from such species to contribute
further to the understanding of the determinants of heterozygosity in natural populations.
Therefore, 23 species of sponges and coelenterates from temperate, tropical and boreal waters were
analysed by gel electrophoresis for an average of 14.3 enzyme loci per species.
Mean heterozygosity values for each species were unusually high, ranging between 0.106 and
0.401. The means and variances of the heterozygosity estimates showed reasonable correlation with
neutralist predictions (with both the stepwise mutation and the infinite alleles models). Population
sizes were generally difficult to estimate with any confidence, but, for one sponge species for which
this was possible, levels of heterozygosity again were similar to neutralist predictions, although the
same was not apparently true for three species of sea anemone. No differences were found between
heterozygosity levels of tropical and temperate species of sponges and coelenterates, thus apparently
contradicting the selectionist ‘trophic resource stability’ and ‘temporal environmental variation’
hypotheses. Conversely, however, the consistently high levels of genetic variation found in
coelenterates and sponges may be argued to be related to common biological characteristics, such as
sessile life, great evolutionary ‘age’, limited ability to disperse and probable low homoeostatic
capability.
Our results seem, overall, to agree well with neutralist expectations for species with large, stable
population sizes. Also, the mean heterozygosities, their variances and the observed and expected
proportions of polymorphic loci seem to fit well with predictions based on the neutralist hypothesis.
However, the selectionist ‘environmental grain’ and the ‘shifting balance’ hypotheses fit the data
equally well. As with much earlier work, the problems in distinguishing between the various
predictions of selectionist or neutralist ideas make it both difficult and unwise to draw definite
conclusions.
KEY WORDS:-Heterozygosity
invertebrates.
-
genetic variation
-
coelenterata - porifera - isozymes -
CONTENTS
Introduction . . . . . . .
. . . .
Materials and methods
Results
. . . . . . . .
Discussion.
. . . . . . .
Population size and heterozygosity .
Other neutralist predictions . .
. . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
. 66
.
67
67
69
71
. . . . . . . . . . . . .
. . . . . . . . . . . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
66
.
.
.
.
* Present address-Dep. Biol. Geral. Instituto de Biologia, Universidade Federal Fluminense, C P 100.183,
24000-Niteroi, Rio de Janeiro, Brazil.
0024-4066/91/090065
+ 16 $03.00/0
65
0 1991 The Linnean Society of London
66
A. M. SOLE-CAVA AND J. P. THORPE
Environment and heterozygosity . . . . .
Heterozygosity and phylogrnetic age of the group .
Conclusions . . . . . . . . . .
Acknowledgements . . . . . . . . .
References
. . . . . . . . . . .
. . . . . . . . . 74
. . . . . . . . . 75
. . . . . . . . . 75
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. . 77
.
77
INTRODUCTION
Data concerning levels of genetic variation in natural populations of many
animal and plant species have been available for a number of years (for reviews
see e.g. Powell, 1975; Selander, 1976; Nevo, 1978; Burton, 1983; Nevo, Beiles &
Ben-Schlomo, 1984) and, therefore, estimates of heterozygosity or of proportion
of loci polymorphic are not, per se, generally of great interest. From the very large
body of data now available it is clear that in the great majority of animal and
plant populations, the proportion of polymorphic loci (P) falls within the range
0.104.50 whilst mean heterozygosity per locus (H) varies from about 0.02 to
about 0.15. Very low H values are uncommon and are (predictably) frequently
linked to very low population size (e.g. Bonnell & Selander, 1974; Selander &
Kaufman, 1973a), whilst heterozygosity values above 0.30 are considerably
rarer. The comprehensive survey by Nevo (1978) covering 277 populations
spread over 243 species of vertebrates, invertebrates and plants, records only four
heterozygosity estimates over 0.25 with the highest at 0.309. I n the subsequent
even larger survey by Nevo el al. ( 1984) covering 1 1 1 1 species, only 2 1 (of which
eight were parthenogenetic) were recorded as having mean heterozygosity values
over 0.25. The available data are highly biased towards terrestrial species and
fish.
Several recent studies (e.g. Manchenko & Balakirev, 1984; Bucklin, 1985;
Sole-Cava, Thorpe & Kaye, 1985; Soli-Cava & Thorpe, 1986, 1990) have
provided evidence for high or unusually high levels of genetic variation in
marine coelenterates and sponges. In the present work data are presented
showing generally very high levels of genetic variation in 23 species of sponges
and coelenterates, analysed for a total of 330 gene loci, and the results are
compared with those which might be expected based on predictions from various
hypotheses.
MATERIALS AND METHODS
The species studied and sampling areas from which these were collected are
given in Table 1. All work was carried out using only live specimens, with the
exception of samples from the Red Sea, which were transported frozen and
analysed within seven days of collection. Sample sizes were between 10 and 50
individuals, a range which is considered to be appropriate for the estimation of
mean heterozygosity (Gorman & Renzi, 1979).
All species were analysed by horizontal starch gel electrophoresis (see
Sole-Cava el al., 1985 and SolC-Cava, 1986, for a description of the technique).
Staining of the gels was by standard procedures (Brewer, 1970; Shaw & Prasad,
1970; Harris & Hopkinson, 1978). Enzyme nomenclature follows that of Harris
& Hopkinson (1978).
GENETIC VARIATION IN LOWER INVERTEBRATES
67
TABLE
I . Species of Porifera and Cnidaria studied in this paper. Fleshwick, Calf of Man, Chicken
Rocks, Douglas, and Port St. Mary are collection sites around the Isle of Man (Irish Sea). Eilat is a
collection site at the north of the Gulf of Aqaba (Red Sea). Paraggi is a collection site in the
Ligurian Sea (Mediterranean Sea)
Species
Taxon
Location
Actinia equina
Actinia prasina
Actinia sp.
Actinothoe sphyrodela
Adamsia carciniopados
Agelas oroides
Anemonia viridis
Axinella damicornis
Axinella verrucosa
Cassiopea andromeda
Chondrilla nucula
Chondrosia reniJormis
Halichondria panicea
Metridium senile
Mycale macilenla
Petrosia JiciJormis
Sarcodicgon roseum
Sarcophyton ehrenbergi
Suberites luridus
Suberites pagurorum
Subtrites rubrus
Urticina eques
Urticina felina
Hexacorallia
Hexacorallia
Hexacorallia
Hexacorallia
Hexacorallia
Demospongiae
Hexacorallia
Demospongiae
Demospongiae
Scyphozoa
Demospongiae
Demospongiae
Demospongiae
Hexacorallia
Demospongiae
Demospongiae
Octocorallia
Octocorallia
Demospongiae
Demospongiae
Demospongiae
Hexacorallia
Hexacorallia
Fleshwick
Fleshwick
Fleshwick
Calf of Man
Douglas
Paraggi
Port Erin
Paraggi
Paraggi
Eilat
Paraggi
Paraggi
Port St. Mary
Port Erin
Chicken rocks
Paraggi
Chicken rocks
Eilat
Chicken rocks
Chicken rocks
Chicken rocks
Port Erin
Port Erin
Depth
(m)
0
0
0
20
30
15
10
15
15
12
15
15
0
10
40
15
40
18
40
40
40
5
5
Collection
Intertidal
Intertidal
Intertidal
SCUBA
Dredge
SCUBA
SCUBA
SCUBA
SCUBA
SCUBA
SCUBA
SCUBA
Intertidal
SCUBA
Dredge
SCUBA
Dredge
SCUBA
Dredge
Dredge
Dredge
SCUBA
SCUBA
RESULTS
Values for observed and expected heterozygosities, proportion of loci
polymorphic and mean observed and effective numbers of alleles for each species
studied are given in Table 2. All the species showed high levels of variation
(range of values for mean expected heterozygosity per locus ranging from 0.106
to 0.401). The values compare well with those obtained by other authors for
other species of coelenterates and sponges (Table 2). The species called Actinia
sp. is an uncommon cryptic, but morphologically distinguishable, species similar
to A. equina, from which it shows clear genetic differences even when sympatric.
DISCUSSION
From the values obtained so far for levels of genetic variation in sponges and
cnidarians (Table 2), it is clear that these animals are among the most
consistently variable groups of organisms known (see Fig. 1). The values
obtained from our work agree well with those indicated by previously published
data for these groups (for references see Table 2). If parthenogenetic species are
excluded, Nevo el al. (1984) listsjust six (one amphibian and five molluscs) out of
11 11 species studied giving heterozygosity values above 0.30. From Table 2 it
can be seen that of the 23 species of coelenterates and sponges for which data are
A. M. SOLE-CAVA AND J. P. THORPE
68
TABLE
2. Measures of genetic variation in natural populations of sponges and coelenterates nl,
Number of loci analysed; H,,,
mean observed heterozygosity; H,, mean expected heterozygosity
(unbiased estimate); P(,,,,,, proportion of polymorphic loci; n-al, mean number of alleles per locus;
en-al, mean effective number of alleles; N,V,, product of effective population size and mutation
rate (see text); EP,,,,,, estimated level of polymorphism (see text).
n-a1
en-a1
NcV,
EPo,,
1.235
1.176
I .944
1.760
1.333
1.786
2.500
1.667
1.545
3.290
1.500
3.440
1.762
1.091
1.089
I .500
1.120
1.496
1.495
1,101
1.318
1.438
1.374
1.224
1.600
1.384
1.490
1.302
1.070
0.015
0.088
0.016
0.083
0.062
0.018
0.051
0.083
0.068
0.040
0.150
0.059
0.122
0.042
0.01 1
0. I70
0.649
0. I70
0.630
0.518
0.193
0.458
0.631
0.560
0.382
0.834
0.498
0.768
0.399
0.116
0.120
0.189
0.336
0.227
0.1 10
0.250
0.250
0.222
0.455
0.500
0.580
0.420
0.600
0.563
0.833
0.688
0.188
2.610
1.375
1.250
1.222
2.545
1.917
1.750
1.833
I .600
I .625
2.000
1.813
1.313
1.140
I .095
1.143
1.152
I .388
1.494
I .369
1.405
1.133
1.320
1.649
1.430
1.153
0.035
0.024
0.036
0.025
0.062
0.082
0.063
0.048
0.030
0.057
0.126
0.076
0.024
0.343
0.250
0.350
0.259
0.520
0.63 I
0.534
0.428
0.300
0.498
0.779
0.604
0.249
5
2,7
2,J
4
4
4
8
8
2
2
2
2,9
0.164
0.145
0.850
2.461
1.500
0.092
0.670
11
16
0.268
0.217
0.500
2.000
1.282
0.049
0.444
10
19
18
9
10
13
13
28
18
16
18
14
18
14
18
0.200
0.189
0.312
0.125
0.199
0.254
0.137
0.195
0.335
0.167
0.436
0.401
0.410
0.363
0.257
0.246
0.305
0.526
0.500
0.778
0.300
0.462
0.6 15
0.344
0.611
0.750
0.667
0.860
0.833
0.786
0.833
1.737
I .556
2.333
1.600
I .538
2.077
1.571
1.778
2.375
1.722
2.929
2.727
2.929
2.667
I .363
1.330
1.550
1.204
I .360
1.467
1.230
1.320
1.731
1.259
2.043
I .947
I .96 I
I .838
0.063
0.058
0.113
0.036
0.062
0.085
0.040
0.061
0.126
0.050
0.193
0.167
0.174
0.143
0.531
0.502
0.743
0.347
0.520
0.641
0.378
0.522
0.780
0.452
0.900
0.868
0.880
0.823
2
2,9
Species
nl
H,
Ho
p0.w
Actinia equina
Actinia equina
Actinia prasina
Actinia prasina
Actinia sp.
Actinia tenebrosa
Actinothoe sphyrodela
Adamsia carciniopados
Agelas oroides
Anemonia uiridis
Anthopleura artemisia
Anthopleura carneola
Anthopleura elegantissima
Anthopleura orientalis
Anlhopleura s l e h l a
Anthopleura xanthogrammica
Axinella damicornis
Axinella verrucosa
Bunodactis stelloides
Bunodactis texaensis
Bunodosoma californica
Bunodosoma cavernata
Bunodosoma granulifera
Cassiopea andromeda
Chondrdla nucula
Chondrosia reniformis
Halichondria panicca
Metridium exilis
Metridium senile
(clonal)
Metridium senile
(solitary)
Metridium senile
(solitary)
Mycale macilenta
Petrosia j c iformis
Phyllactis conquilcga
Sarcodicpon roseum
Sarcophyton ehrenbergi
Subentes domuncula
Subentes luridus
Subentes pagurorum
Subentes rubrus
Urticina eques
Urticina eques
Urticina felina
Urticina felina
17
0.067
0.245
0.068
0.278
0.227
11
0.057
0.260
0.059
0.250
0.198
0.067
0.169
0.249
0.215
0.139
0.375
0.190
0.328
0.144
0.040
-
0.176
0.778
0.1 18
0.61 1
0.41 1
0.250
0.429
0.750
0.611
0.455
0.941
0.41 7
0.889
0.381
0.091
18
8
8
9
11
12
12
12
5
16
12
15
16
0.122
0.087
0.125
0.090
0.199
0.247
0.200
0.160
0.106
0.187
0.335
0.234
0.086
0.063
0.944
15
18
17
18
18
12
14
16
18
22
17
12
18
42
-
0.157
0.261
0.192
0.148
0.151
-
0.154
-
~
~
-
0.232
0.267
-
0.215
0.365
0. I75
0.424
0.389
0.348
0.323
2.222
Ref.*
i
2,3
1
2,3
2
4
2
2
2
2
5
4
5
6
4
10
2
4
2
2
12
2,13
2,13
2,13
14
2
14
2
*References: 1, Haylor et al. (1984); 2, this paper; 3, Solt-Cava & Thorpe (1987);4, McCommas (1982);5,
Smith & Potts (1987); 6, Manchenko & Balakirev (1984); 7, Sole-Cava & Thorpe (1991); 8, McCommas &
Lester (1980); 9, SolC-Cava & Thorpe (1990); 10, Bucklin & Hedgecock (1982); 1 1 , Bucklin (1986); 12,
Balakirev & Manchenko (1985); 13, Solt-Cava & Thorpe (1986); 14, Solt-Cava et al. (1985).
GENETIC VARIATION IN LOWER INVERTEBRATES
69
*3*0
Figure I . Average levels of heterozygosity per locus for various higher taxa. Data for coelenterates
and sponges are from this paper (see Table Z), other data from Nevo cf al. (1984). Cross hatched
area for 'reptiles' indicates contribution of parthenogenetic species.
available, seven (four sea anemones and three sponges) have heterozygosity
estimates above 0.30.
Population size and heterozygosity
According to neutralist expectation, high levels of genetic variation should be
associated with unusually high mutation rates or high and temporally stable
population sizes (Kimura, 1983). These predictions follow from the basic
equation:
H = 4XV0
1 4.Nev,
where N, is the effective population size, and V, is the mutation rate (Kimura &
Crow, 1964).
There is no a priori reason to assume that for cnidarians and sponges as a whole
levels of mutation will be especially high. For practically all organisms, V, is
assumed to be of the order of lo-' for electrophoretically detectable mutations
(Kimura, 1983). It may be suggested, therefore, that if the bulk of the variation
is indeed the product of stochastic phenomena, differences in effective
population size should be primarily responsible for the observed level of
heterozygosity found in any particular population. If so, then the expected
effective population sizes of the species studied here should range from about
2 x lo5 (in Cassiopea andromeda) to 2 x lo6 (in Urticina egues).
Total population sizes in marine animals are generally poorly known
particularly for organisms such as sponges and cnidarians. An additional
problem is that, because of the unusual reproductive strategies of many lower
invertebrates, even if the population numbers were known, they would still be
+
70
A. M.SOLE-CAVA AND J. P. THORPE
difficult to translate into effective population sizes. Some sea anemones (e.g.
Urticina spp., Actinia fragacea) reproduce mainly, if not exclusively, sexually
(Chia, 1976; Carter & Thorpe, 1981) and, because of cross-fertilizing
hermaphroditism and temporal stability, have effective population sizes which
are probably similar to the actual population size of the species. However, other
cnidarians and sponges (e.g. most Actinia spp., Halichondria spp.) employ both
sexual and asexual reproduction (Sara & Vacelet, 1973; Chia, 1976), which may
result in the colonization of particular areas by large numbers of asexually
reproduced‘ genetically identical clonemates (e.g. Anthopleura, Francis, 1973).
This ‘genotype amplification’ phenomenon will ultimately result in big
discrepancies between actual and effective population sizes (Kimura, 1983).
There is, however, a possibility of making approximate estimates of
population size, in two of the species studied because these occur as epizoonts of a
commercially exploited bivalve species for which limited population data are
available. The sponges Suben’tes luridus and S. rubrus grow only as encrustations
on Chlamys operculuris (queen scallop) (see SolC-Cava & Thorpe, 1986), a species
of which there is a large, but more or less geographically discrete, population in
the northern Irish Sea. From available statistics for the fishery for C. opercufaris
and studies of recruitment and mortality of the sponges over a number of years it
may be estimated that the actual population size of Suberites luridus and S. rubrus
in the Irish Sea is at least 3.5 x 10’.
The effective population size, again, is difficult to estimate, but these sponges
reproduce sexually and, possibly, asexually by dispersal of gemmule (Sara &
Vacelet, 1973; Simpson,,1980). It is expected, therefore, that N, for these species
will be smaller than the actual population size, depending, amongst other things,
upon the proportion of the population which has been produced asexually. A
rough estimate of this proportion can be obtained from the electrophoretic
analysis itself. No two individuals of either species were identical over all loci.
This suggests asexual reproduction is not important in Suberites spp. The highest
expected frequency of clonemates for each species, given the sample sizes used, is
lower than 5%. If we use these numbers, we can calculate a conservative
effective population size for both species of about 3.2 x lo7. This number may be
compared with the neutralist estimate of 1.1 x lo6 for S. luridus and S. rubrus
taken together. It seems, therefore, that the high levels of genetic variation in
those two sponge species can be explained by the neutralist theory alone,
although, of course, selection effects cannot be excluded.
For species like Actiniu eguinu, A . prusinu and Actiniu sp., however, the situation
may be quite different. These species can reach high densities in the intertidal
zone (Haylor, Thorpe & Carter, 1984), but the levels of genetic differentiation
between local populations can be very high (SolC-Cava & Thorpe, 1987).
Subdivisions of natural populations into more or less isolated units produces a
decrease in effective population size (Kimura, 1983). I t would seem that if the
probable effects of asexual reproduction and temporal reductions in population
size are included, in Actiniu species effective population sizes are likely to be very
small, probably no bigger than lo4 for Actiniu equina and probably considerably
lower in the comparatively scarce Actinia prusina and Actinia sp. For these
populations, according to neutralist predictions (for .Ne= lo4, He z 0.038)
expected heterozygosity levels at equilibrium should be far lower than those
observed and hence for these three species observed levels of genetic variability
are not apparently easily reconciled with neutralist predictions.
GENETIC VARIATION IN LOWER INVERTEBRATES
0*06
0
71
t
0.1
0.2
0.3
0.4
0.5
Mean heterozygosity
Figure 2. Variances against mean for heterozygosity values for different species of coelenterates and
sponges. The lines are curves based on neutralist predictions for the stepwise mutation (A) and the
infinite alleles (B) models (Kimura & Crow, 1964; Kimura & Ohta, 1973).
Other neutralist predictions
Several other predictions of the neutralist theory have been tested so far by
selectionists and neutralists, with (unsurprisingly) conflicting results and
interpretations (e.g. Hartl, 1980; Kimura, 1983; Nevo, 1983). Neutralist theory
predicts that mean heterozygosities in different populations should be positively
correlated with the amount of genetic divergence between them (Nei & Tateno,
1975; Chakraborty, Fuerst & Nei, 1978; Skibinski & Ward, 1981, 1982; Ward &
Skibinski, 1985), and with the variance of the heterozygosity (Nei, 1975; Nei,
Fuerst & Chakraborty, 1976; Fuerst, Chakraborty & Nei, 1977; Nevo, 1983).
Such studies have generally involved the compilation and reviewing of
extensive data from the literature and plotting and comparing observed values
from natural populations with those predicted by the neutralist theory (e.g.
Fuerst et al., 1977; Nevo, 1983; Ward & Skibinski, 1985). The number of species
studied in the present work is smaller than those used in the aforementioned
reviews; however, levels of heterozygosity are mostly far higher than the majority
of the species studied previously and are, therefore, important, because they
provide substantial additional data for heterozygosity levels which were
previously very scarce.
If the mean heterozygosity observed for the sponge and sea anemone
populations are plotted against their variances (Fig. 2) the distribution of points
on the graph can be compared with those expected under the infinite alleles
model (Kimura & Crow, 1964) and to the stepwise mutation model (Kimura &
Ohta, 1973). Nevo (1983) found significant differences between the number of
points above and below these curves for his set of data, and suggested that
selection was responsible for the differences found. In the limited set of data used
here the number of points above and below the curves are not significantly
A. M. SOLE-CAVA AND J. P. THORPE
72
0.66
0.05
..
t
0.04
x
u0
g 0.03
.-0
0.02
0.01
0.1
0
0.2
0.3
0.4
0.5
Heteroz ygosit y
Figure 3. Variances against mean for heterozygosity values for different enzyme loci in coelenterates
and sponges. The lines are neutralist predictions, as in Fig. 2.
different from either the infinite alleles model (7 below, 14 above; x2 = 2.33;
P > 0.10) or the stepwise mutation model (13 below, 7 above (one on the line);
x2 = 1.80; P > 0.10).
The means and the variances of heterozygosities for homologous loci can be
analysed in the same way (Gojobori, 1982). In this case, the main determinants
0.8
-
-8
cd
0
0.I
a2
a3
0.4
,5
Hetcrotygo8ity
Figure 4. Relationship between percentage of polymorphic loci
and level of heterozygosity
(H) in the species ofcoelenterates and sponges from Table 2. The line drawn is that expected for the
infinite alleles model (as in Fuerst ct al., 1977).
GENETIC VARIATION IN LOWER INVERTEBRATES
73
P(O.9Sl
Figure 5. Relationship between observed proportions of polymorphic loci (EP0,95)and neutralist
expectations for the infinite alleles model. R = Pearson’s correlation coefficient (P < 0.0001).
of mean heterozygosity for each enzyme will be sampling errors and the different
functional constraints for each enzyme, rather than differences in population size
(Gojobori, 1982; Kimura, 1983; SolC-Cava & Thorpe, 1989). T h e genetic
variation observed at different loci in the species studied here is not dissimilar to
neutralist expectations (Fig. 3), although in coelenterates and sponges the
enzymes showing the highest heterozygosity
values
(hexokinase,
phosphoglucomu tase and esterases) are different from those found by Gojobori
(1982) using data from higher animals.
Finally, the observed proportions of polymorphic loci can also be compared
with neutralist expectations (Fuerst et al., 1977). The probability of protein
polymorphism (P,,,)can be calculated, for the infinite alleles model, as:
PPOlY = I - @
where q is the minimum polymorphic level (normally 0.05 or 0.01) and u is given
by u = 4N,V0, where Ne is the effective population size and V, is the mutation
rate (Kimura & Ohta, 1971). The distribution of the data for coelenterates and
sponges again shows a reasonable fit to that predicted by the neutralist theory
(Figs 4,5). Thus results from the highly polymorphic animals studied here show
moderate agreement with the predictions of the neutralist theory of molecular
evolution. However, as pointed out by various authors (e.g. Lewontin, 1974;
Hartl, 1980; Nevo, 1983), the major problem with all the tests mentioned above
is that a number of other curves could fit the data equally well. Confidence limits
of variance/mean heterozygosity curves have been estimated empirically (Fuerst
et al., 1977) and are generally large ( z 50% of the variance for heterozygosities
above 0.25). Overall the interpretation of results is made more difficult since
neutralist and selectionist predictions can be very similar, although derived from
apparently opposing points of view (Kimura, 1983; Nevo, 1983).
74
A. M. SOLE-CAVA AND J. P. THORPE
Environment and heterotygosity
A number of hypotheses have been proposed linking the amount of genetic
variation in natural populations to environmental factors (Ayala & Campbell,
1974; Hedrick, Ginevan & Ewing, 1976; Gillespie, 1978; Hartl, 1980; Smith &
Fujio, 1982; Nevo, 1983). A very simple suggestion is overdominance, based on
the idea that, if different alleles produce proteins with different physiological
optima in relation to some key environmental factor (such as temperature), then
heterozygotes for those alleles would be more fit than either of the homozygotes
in environments which had large ranges for that environmental factor (see
reviews, e.g. Zouros & Foltz, 1987). Some allozymes appear to differ in
biochemical properties (Watt, 1977; Danford & Beardmore, 1979; Burton &
Feldman, 1983; Hoffman, 1985) and have been suggested as being responsible
for relationships between overall heterozygosity and parameters such as survival
or growth rate in a variety of organisms (e.g. Beardmore & Ward, 1977; Ledig,
Guries & Bonefield, 1983; Koehn & Gaffney, 1984; Koehn, 1985). There are,
however, often conflicting results from similar organisms studied under similar
experimental conditions (e.g. Beaumont el al., 1985; Pierce & Mitton, 1982).
Furthermore, the existence of high genetic variation in haploid organisms
(Milkman, 1973; Selander & Levin, 1980; Yamazaki, 1981) indicates that high
levels of heterozygosity are not necessarily maintained by heterozygote
advantage alone (see Kimura, 1983).
An alternative view is that the spatial heterogeneity of the environment may
promote genetic diversity without the need for an increase in the fitness of the
heterozygote. If the habitat of a population includes a number of sub-divisions,
with characteristic micro-climatic, chemical and biological conditions, and if
different alleles have different associated selection factors in those microenvironments, then the population as a whole may be expected to show
increased genetic variation (Selander & Kaufman, 1973; Hedrick el al., 1976;
Allard, Miller & Kahler, 1978; Nevo, 1978, 1983; Smith & Fujio, 1982). Thus
the level of gene variation may be expected to depend on the number of subdivisions which are noticeable in the environment of a given species (cf.
‘environmental grain’, Levins, 1968).
The amount of homeostatic control of an organism influences the way it
experiences the environment. Organisms with high homeostasis will tend to ‘see’
the environment as more fine grained (Levins, 1968). Body size and vagility are
also related to the environmental grain of a given organism and may, therefore,
be correlated with genetic variation (Selander & Kaufman, 1973b). A series of
papers and reviews support directly or indirectly the environmental grain
hypothesis: large mammals, for example, have very low genetic variation (Nevo,
1978, 1983; Wright, 1978), although this could be due alternatively to their
small population sizes (Kimura, 1983). Most pelagic elasmobranchs have low
levels of genetic variation (Smith, 1986) whereas territorial demersal sharks, such
as Sguatina spp. show high heterozygosities (SolC-Cava et al., 1983; Smith, 1986).
In teleosts, more specialist pleuronectid flatfish generally appear to show higher
levels of heterozygosity than roundfish (Smith & Fujio, 1982). Also in molluscs,
data for highly mobile pelagic squid species indicate that these frequently have
very low levels of genetic variation (e.g. Ally & Keck, 1978; Christofferson el al.,
1978; Thorpe, Havenhand & Patterson, 1986), whereas less mobile molluscs
GENETIC VARIATION IN LOWER INVERTEBRATES
75
mostly have much higher levels of heterozygosity (see e.g. Selander & Kaufman,
1975; Beaumont & Beveridge, 1984). In crustaceans there is evidence of a higher
level of genetic variation in barnacles, which are sessile (Nevo, 1978, but see
Flowerdew, 1983) than in the bigger, more vagile decapods (Nelson &
Hedgecock, 1980).
Benthic, sessile marine invertebrates are typical representatives of animals
with a coarse grain environmental strategy: they very often present a patchy
distribution due to recruitment (Johnson & Black, 1982, 1984), clustered or
philopatric occupation of substrata (Shields, 1982; Grosberg & Quinn, 1986),
substrate selection by larvae (Giesel, 1970; Schroeder & Hermans, 1975) or postsettlement selection (Fell, 1974; Quicke el al., 1985).
The predominance of a ‘coarse grain’ strategy among sessile marine
invertebrates seems, thus, to correlate well with the high level of heterozygosity
that they present. Other selectionist hypotheses, such as the ‘trophic resource
stability’ (Valentine & Ayala, 1974) or the ‘temporal environmental variation’
(Karlin & Levikson, 1974; Turelli, 1977) do not seem to be supported by the
present data, because levels of heterozygosity appear equally high in both
temperate and tropical species (Tables 1, 2; Fig. 5).
Heterougosity and phylogenetic age of the group
The organisms studied in the present work show many ecological similarities:
they are sessile, specialist and may be argued to see the environment as coarse
grained. However, they also belong to groups generally considered to be the
most primitive and, therefore, presumably the ‘oldest’ amongst extant animals
(Clarkson, 1986).
A far from recent suggestion is that levels of heterozygosity may be related to
the phylogenetic ‘age’ (Soule, 1972) or to the ‘conservativeness’ (Gorman &
Kim, 1977) of the organisms. These authors have proposed the idea that old and
conservative groups should possess higher genetic variation than younger or
more speciose ones. Any such relationship could, however, be interpreted in both
neutralist (older and less speciose groups have had more time to accumulate gene
variation, keeping at the same time stable population sizes) and selectionist
(more ‘primitive’ groups have a lower homeostatic efficiency) ways. The
proportion of ‘primitive’ species is higher in the sea than on land, and it is
difficult to dissociate the different effects on levels of genetic variation that each
theory may predict. Possibly the higher proportion of genetically highly variable
species found in the sea is related to the high diversity of that environment or to
particular ecological conditions. However, it is equally plausible to say that the
higher genetic variation found in the sea is mainly due to the fact that the
majority of the more ‘primitive’ (old, conservative) species live in that
environment.
Conclusions
The search for the causes of genetic variation in natural populations, whether
they be adaptive necessity (selection) or chance (drift), is a complex task that
will probably not be solved by any simplistic theory alone (Lewontin, 1974;
Nevo, 1983). In fact, the differences between the two schools are sometimes so
76
A. M. SOLE-CAVA AND J. P. THORPE
Neutralist hypotheses
Selectionist hypotheses
Figure 6. Diagrammatic representation of the relationship between various neutralist and
selectionist hypotheses (indicated as thick walled boxes). Nc = Effective population size.
subtle that the same evidence may be used to support one or another according
to personal bias (Fig. 6). Consequently, the theories of both schools of thought
have, since their creation, themselves evolved to best fit the large amounts of
data being produced in attempts to support or contradict various hypotheses.
Neutralists have drifted from the original concepts of 'actually neutral"
mutations (Kimura, 1968; Kimura & Ohta, 197 1) to 'effectively neutral' ones
(Ohta, 1973; Kimura, 1979), whereas selectionists adapted the initially simplistic
overdominance hypothesis (discussed at length Dobzhansky, 1970 & Lewontin ,
1974) into the more sophisticated ecological hypotheses.
The main argument of the neutralists is that if random events are sufficient to
explain the genetic structure of a population, then selectionist, deterministic
arguments are unnecessary (Kimura, 1979). Selectionists, on the other side,
claim that the interaction between environment and organisms should not be
ignored, and that neutralists have a too dichotomized view of the natural
world-selection at the phenotypic level and drift at the molecular level (Nevo,
1983). Selectionists have problems in building models because of the complexity
of nature and the populations they are studying. Neutralists have a
mathematically more coherent theory, but one that has to rely on cetere paribus
conditions, in an ideally simplified world which may not have much to do with
reality (see Cartwright, 1980 and Levins & Lewontin, 1985 for a discussion of
the problems of using this kind of approach in science). Selectionists very often
lack precision; neutralists may lack realism (Lewontin, 1974).
GENETIC VARIATION IN LOWER INVERTEBRATES
71
Genetic drift has been underestimated in the past as ‘evolutionary noise’
(Fisher, 1930; Mayr, 1963). More recently, however, it has become to most
geneticists an important factor in the evolution of populations (e.g. Lewontin,
1974; Wright, 1978; Hartl, 1980). It can be argued that the problem about the
importance of stochastic and selectionist factors now seems to have shifted from
‘which one’ to ‘how much of each’ (see e.g. Nevo, 1978). The closest attempts to
integrate both processes in a general theory of evolution may have been that of
Wright ( 1978 and references therein). His shifting-balance theory includes
phases of intense stabilizing selection, which both selectionists and neutralists
accept as predominant, followed by shifts in the fitness of the organisms caused
by either genetic drift away from adaptive peaks or by changes in the
environment. The landscape of the adaptive plane is itself probably changed by
the species which live in it, and the whole process is likely to be very dynamic
(Hartl, 1980; Nevo, 1983). It is possible that drift and selection will have
different relative importances in different times for a given group (Gillespie,
1986); neutral processes can predominate in some more stable periods, whilst
selection can act more intensely in others.
ACKNOWLEDGEMENTS
The authors thank Professor J. A. Beardmore for helpful discussions and
Professor T. A. Norton for provision of facilities. A. M. Sole-Cava was supported
by grants from CAPES and CNPQ, Brazil.
REFERENCES
ALLARD, R. W., MILLER, R. D. & KAHLER, A. L., 1978. The relationship between degree of
environmental heterogeneity and genetic polymorphism. Verslagen der Koninklijke Nederlandse Akademie van
Wetenschappen, 70: 49-73.
ALLY, J. R. R. & KECK, S. C., 1978. A biochemical genetic population structure study of market squid.
Loligo opalescens, along the California coast. California Fish and Game Fisheries Bulletin, 169: 1 13-1 2 I .
AYALA, F. J. & CAMPBELL, C. A., 1974. Frequency-dependent selection. Annual Review of Ecology and
Systematics, 5: 115-138.
BALAKIREV, E. S. & MANCHENKO, G. P., 1985. Intraspecies genetic variation in the sponge Suberites
domuncula. Biologia Moyra, 5: 36-40.
BEARDMORE, J. A. & WARD, R. D., 1977. Polymorphism, selection and multi-locus heterozygosity in the
plaice, Pleuronectes platessa L. In F. B. Christiansen & T . M. Fenchel (Eds), Measuring Selection in Natural
Populations: 207-222. Berlin: Springer-Verlag.
BEAUMONT, A. R. & BEVERIDGE, C. M., 1984. Electrophoretic survey of genetic variation in Pecten
maximu, Chlamys opercularis, C. varia and C. distorta from the Irish Sea. Marine Biology, BI: 299-306.
BEAUMONT, A. R., GOSLING, E. M., BEVERIDGE, C. M., BUDD, M., D. & BURNELL, G. M., 1985.
Studies on heterozygosity and size in the scallop, Pcctm marimus. In P. E. Gibbs (Ed.), Proceedings of the
Nineteenth European Marine Biologv Symposium: 443-454. Cambridge: Cambridge University Press.
BONNELL, M. A. & SELANDER, R. K., 1974. Elephant seals: genetic variation and near extinction. Science,
184: 908-909.
BREWER, G. J., 1970. An Introduction to Isovme Techniques. New York: Academic Press.
BUCKLIN, A,, 1985. Biochemical genetic variation, growth and regeneration of the sea anemone, Mefridium,
of British shores. Journal ofthe Marine Biological Association of the United Kingdom, 65: 141-157.
BUCKLIN, A. & HEDGECOCK, D., 1982. Biochemical genetic evidence for a third species of Metridium
(Coelenterata: Actiniaria). Marine Bioloo, 66: 1-7.
BURTON, R. S., 1983. Protein polymorphism and genetic differentiation of marine invertebrate populations.
Marine Biology Lctters, 4: 193-206.
BURTON, R. S. & FELDMAN, M. W., 1983. Physiological effects of an allozyme polymorphism.
Glutamate-pyruvate transaminase and response to hyperosmotic stress in the copepod Tigriopur cal~ornicus.
Biochemical Genetics, 21: 239-25 I .
A. M. SOLE-CAVA AND J. P. THORPE
78
CARTER, M. A. & THORPE, J. P., 1981. Reproductive, genetic and ecological evidence that Actinia equina
var. mesembryanthnnum and var. fragacea are not conspecific. Journal of the Marine Biological Association of the
United Kingdom, 61: 79-83.
CARTWRIGHT, N., 1980. The truth doesn’t explain much. American Philosophical Quarterb, 17: 159-163.
CHAKRABORTY, R., FUERST, P. A. & NEI, M., 1978. Statistical studies on protein polymorphism in
natural populations. 11. Gene differentiation between populations. Genetics, 88: 367-390.
CHIA, F.4. 1976. Sea anemone reproduction: patterns and adaptive radiation., In G. 0. Mackie (Ed.),
Coelenterate Ecology and Behuviour: 261-270. New York: Plenum Press.
CHRISTOFFERSON, J. P., FOSS, A., LAMBERT, W. E. & WELCH, B.,, 1978. An electrophoretic study of
select proteins from the market squid, Oligo opalescens, along the California coast. California Fish and Game
Fisheries Bulletin, 169: 123-1 33.
CLARKSON, E. N. K., 1986. Znvertebratc Palaeontology and Evolution. London: Allen and Unwin.
DANFORD, N. D. & BEARDMORE, J. A., 1979. Biochemical properties of esterase-6 in Drosophila
melanogaster. Biochemical Genetics, 17: 1-22.
DOBZHANSKY, T., 1970. Genetics OJ the Evolutionary Process. New York: Columbia University Press.
FELL, P. E., 1974. Porifera. In A. C. Giese & J. S. Pearse (Eds), Reproduction of Marine Invertebrates, Vol. I:
1-131. London: Academic Press.
FISHER, R. A., 1930. The Genetical Theory of Natural Selection. Oxford: Oxford University Press.
FLOWERDEW, M. W., 1983. The circumboreal barnacle. Balanus balanoides (L.) and its subpopulations. In
G. S. Oxford & D. Rollinson (Eds), Profein Polymorphism: Adaptive and Taxonomic Sign$cancc: 75-84. London:
Academic Press.
FRANCIS, L., 1973. Clone-specific segregation in the sea anemone Anthopleura slegantissima. Biological Bulletin,
144: 64-72.
FUERST, P. A., CHAKRABORTY, R. & NEI, M., 1977. Statistical studies on protein polymorphism in
natural populations. I. Distribution of single locus heterozygosity. Genetics,86: 455483.
GIESEL, J. T., 1970. On the maintenance of shell pattern and behavioral polymorphism in Acmaea digitalis, a
limpet. Evolution, 24: 98-1 19.
GILLESPIE, J. H., 1978. A general model to account for enzyme variation in natural populations: V. The
SAS.CFF model. Theoretical Population Biology, 14: 1-45.
GILLESPIE, J. H.,1986. Natural selection and the molecular clock. Molccular Biology and Evolution, 3: 138-155.
GOJOBORI, T., 1982. Means and variances of heterozygosity and protein tinction. In M. Kimura (Ed.),
Molecular Evolution, Protein Polymorphism and the Neutral Theory: 137-1 48. Berlin: Springer-Verlag.
GORMAN, G. C. & KIM, Y. J., 1977. Genotypic evolution in the face of phenotypic conservativeness.
Abudefdf (Pomacentridae) from the Atlantic and Pacific sides of Panama. Copcia, 1977: 694497.
GORMAN, G. C. & RENZI, J., 1979. Genetic distance and heterozygosity estimates in electrophoretic
studies: effects of sample size. Copcia, 1979: 242-249.
GROSBERG, R. K. & QUINN, J. F., 1986. The genetic control and consequences of kin recognition by the
larvae of a colonial marine invertebrate. Nafure, 322: 456459.
HARRIS, H. & HOPKINSON, D. A,, 1978. Handbook o f E n . p e Electrophoresis in Human Genetics. Amsterdam:
North-Holland.
HARTL, D. L., 1980. Principles of Population Genetics. Sunderland, Massachusetts: Sinauer Associates.
HAYLOR, G. S., THOPRE, J. P. & CARTER, M. A., 1984. Genetical and ecological differentiation between
sympatric colour morphs of the common intertidal sea anemone Actinia equina. Marine Ecology Progrcss Series,
16: 281-290.
HEDRICK, P. W., 1986. Genetic polymorphism in heterogeneous environments: a decade later. Annual Revicm
of Ecology and Systematics, 17: 535-566.
HEDRICK, P. W., GINEVAN, M. E. & EWING, E. P., 1976. Genetic polymorphism in heterogeneous
environments. Annual Remew OJEcology and Systematics, 7: 1-32.
HOFFMAN, R. J., 1985. Thermal adaptation and the properties of phosphoglucose isomerase from a sea
anemone. In P. E. Gibbs (Ed.), Proceedings of the Nineteenth European Marine Biology Symposium: 505-514.
Cambridge: Cambridge University Press.
JOHNSON, M. S. & BLACK, R., 1982. Chaotic genetic patchiness in an intertidal limpet, Siphonaria sp.
Marine Biology, 70: 157-164.
JOHNSON, M. S. & BLACK, R., 1984. Pattern beneath the chaos: the effect of recruitqent on genetic
patchiness in an intertidal limpet. Evolution, 38: 1371-1383.
KARLIN, S. LEVIKSON, B., 1974. Temporal fluctuations in selection intensities. Case of small population
size. Thcorctical Population Biology, 6: 383-412.
KIMURA, M., 1968. Evolutionary rate at the molecular level. Nature, 217: 624-626.
KIMURA, M., 1979. The neutral theory of molecular evolution. ScienfiJicAmerican, 241: 94-104.
KIMURA, M., 1983. The Neutral Theory of Molecular Evolution. London: Cambridge University Press.
KIMURA, M. & CROW, J. F., 1964. The number of alleles that can be maintained in a finite population.
Genetics. 49: 725-738.
KIMURA, M. & OHTA, T., 1971. TheorcticaI Aspects of Population Genetics. Princeton: Princeton University
Press.
KIMURA, M. & OHTA, T., 1973. Mutation and evolution at the molecular level. Genetics, 73: 19-35.
GENETIC VARIATION IN LOWER INVERTEBRATES
79
KOEHN, R. K., 1985. Adaptive aspects of biochemical and physiological variability. In P. E. Gibbs (Ed.),
Proceedings of the Nineteenth European Marine Biology Symposium: 425-44 I . Cambridge: Cambridge University
Press.
KOEHN, R. K. & GAFFNEY, P. M., 1984. Genetic heterozygosity and growth rate in Mytilus edulis. Marine
Biology, 82: 1-7.
LEDIG, F. T., GURIES, R. P. & BONEFIELD, B. A., 1983. The relationship ofgrowth to heterozygosity in
pitch pine. Evolution, 37: 1227-1238.
LEVINS, R., 1968. Evolution in Changing Environments. Princeton, New Jersey: Princeton University Press.
LEVINS, R. & LEWONTIN, R. C., 1985. The Dialectical Biologist. London: Harvard University Press.
LEWONTIN, R. C., 1974. The Genetic Basis of Evolutionary Change. New York: Columbia University Press.
MANCHENKO, G. P. & BALAKIREV, E. S., 1984. Allozymic variation in Anthoplrura orientalis from Peter
the Great Bay of the Sea of Jqpan. Genetika. 10: 2072-2074.
MAYR, E., 1963. Animal Species and Evolution. New York: Harvard University Press.
McCOMMAS, S. A,, 1982. Application of electrophoretic techniques to some systematic problems in the family Actiniidae.
Unpublished Ph.D. Thesis, University of Houston, Texas.
McCOMMAS, S. A. & LESTER, L. J., 1980. Electrophoretic evaluation of the taxonomic status of two
species of sea anemone. Biochemical Systematics and Ecology, 8: 289-292.
MILKMAN, R., 1973. Electrophoretic variation in Escherichia coli from natural sources. Science, 182:
1024- 1026.
NEI, M., 1975. Molecular Population Genetics and Evolution. North-Holland, Amsterdam.
NEI, M. & TATENO, Y., 1975. Interlocus variation of genetic distance and the neutral mutation theory.
Proceedings of the National Academy of Sciences, 77: 2806-2809.
NEI, M., FUERST, P. A. & CHAKRABORTY, R., 1976. Testing the neutral mutation hypothesis by
distribution of single locus heterozygosity. Nature, 262: 491493.
NELSON, K. & HEDGECOCK, D., 1980. Enzyme polymorphism and adaptive strategy in the decapod
crustacea. American Naturalist, 116: 238-280.
NEVO, E., 1978. Genetic variation in natural populations: patterns and theory. Theoretical Population Biology,
13: 121-177.
NEVO, E., 1983. Adaptive significance of protein variation. In G. S. Oxford & D. Rollinson (Eds), Protein
Polymorphism: Adaptive and Taxonomic SigniJicance: 239-282. London: Academic Press.
NEVO, E., BEILES, A. & BEN-SCHLOMO, R., 1984. The evolutionary significance of genetic diversity:
ecological demographic and life-history correlates. I n G. S. Mani (Ed.), Evolutionary Dynamics of Genetic
Diversip: 13-2 12. Heidelberg: Springer-Verlag.
OHTA, T., 1973. Slightly deleterious mutant substitutions in evolution. Nature, 246: 96-98.
PIERCE, B. A. & MITTON, J. B., 1982. Allozyme heterozygosity and growth rate in the tiger salamander.
Ambystoma tigrinurn. Journal of Heredip, 73: 250-253.
POWELL, J. R., 1975. Protein variation in natural populations of animals. Evolutionary Biology, 8:
79- 1 19.
QUICKE, D. L. J.? DONOGHUE, A. M., KEELING, ‘I. F. & BRACE, R. C., 1985. Littoral distributions
and evidence for differential post-settlement selection of the morphs of Actinia equina. Journal of the Marine
Biological Association of the United Kingdom, 65: 1-20.
SARA, M. & VACELET, J., 1973. Ecologie des demosponges. In P.-P. Grass6 (Ed.), Traiti de <oologie, III.
Spongiaires: 462-576. Paris: Grasse, Masson et cie.
SCHROEDER, P. S. & HERMANS, C. O., 1975. Annelida: Polychaeta. In A. C. Giese & J. S. Pearse (Eds),
Reproduction of Marine Invertebrates, I l l : 1-2 14. London: Academic Press.
SELANDER, R. K., 1976. Genetic variation in natural populations. In F. J. Ayala (Ed.), Molecular Evolution:
21-45. Sunderland, Massachusetts: Sinauer Associates.
SELANDER, R. K. & KAUFMAN, D. W., 1973a. Self fertilisation and genetic population structure in a
colonising land snail. Proceedings of the National Academy of Sciences, 70: 1 1 8 6 1 190.
SELANDER, R. K. & KAUFMAN, D. W., 1973b. Genetic variability and strategies ofadaptation in animals.
Proceedings of the National Academy of Sciences, 70: 1875-1877.
SELANDER, R. K. & KAUFMAN, D. W., 1975. Genetic structure of populations of the brown snail (Helix
aspersa). I-Microgeographic variation. Evolution, 29: 385-401.
SELANDER, R. K. & LEVIN, B. R., 1980. Genetic diversity and structure in Escherichia coli populations.
Science, 210: 545-547.
SHAW, P. R. & PRASAD, R., 1970. Starch gel electrophoresis of enzymes-a compilation of recipes.
Biochemical Genetics, 4: 297-320.
SHIELDS, W. M., 1982. Philopatry, Inbreeding, and the Evolution of Sex. New York: State University of New York
Press.
SIMPSON, T. L., 1980. Reproductive processes in sponges: a critical evaluation of current data and views.
International Journal of Invertebrate Reproduction, 2: 25 1-269.
SKIBINSKI, D. 0. F. & WARD, R. D., 1981. Relationship between allozyme heterozygosity and rates of
divergence. Genetics Research, 38: 65-73.
SKIBINSKI, D. 0. F. & WARD, R. D., 1982. Correlation between heterozygosity and evolutionary rate of
proteins. Nature, 298: 490492.
A. M. SOLE-CAVA AND J. P. THORPE
80
SMITH, B. L. & POTTS, D. C., 1987. Clonal and solitary anemones (Anthopleura) of western North America:
population genetics and systematics. Marine Biology, 94: 537-546.
SMITH, P. J., 1986. Low genetic variation in sharks (Chondrichthyes). Copeia, 1986: 202-206.
SMITH, P. J. & FUJIO, Y., 1982. Genetic variation in marine teleosts: high variability in habitat specialists
and low variability in habitat generalists. Marine Biology, 69: 7-20.
SOLE-CAVA, A. M., 1986. Studies of biochemical genelics and taxonomy in coelenterates and sponges. Unpublished
Ph.D. Thesis, University of Liverpool: Port Erin, Isle of Man.
SOLE-CAVA, A. M. & THORPE, J. P., 1986. Genetic differentiation between morphotypes of the marine
sponge Suberitcs jcus (Demospongiae: Hadromerida). Marine Biology, 93: 247-253.
SOLE-CAVA, A. M. & THORPE, J. P., 1987. Further genetic evidence for the reproductive isolation of the
green sea anemone Actinia prasina Gosse from the common intertidal beadlet anemone Actinia equina (L.).
Marine Ecology Progress Series, 38: 225-229.
SOLE-CAVA, A. M. & THORPE, J. P., 1989. Biochemical correlates of genetic variation in marine lower
invertebrates. Biochemical Genetics, 27: 303-3 12.
SOLE-CAVA, A. M. & THORPE, J. P., 1990a. High levels of genetic variation in marine sponges. In
K. Rutzler, (Ed.), New Perspecliucs in Sponge Biology: 332-337. Washington: Smithsonian Institution Press.
SOLE-CAVA, A. M. & THORPE, J. P., 1990b. A new Mediterranean species of Awinella detected by
biochemical genetic methods. In H. Keupp & J. Reitner (Eds), Fossil and Recent Sponges. Brrlin:
Springer-Verlag.
SOLE-CAVA, A. M., THORPE, J. P. & KAYE, J. G., 1985. Reproductive isolation with little genetic
divergence between Urticina ( = Tealia) felina and U . cques (Anthozoa: Actiniaria). Marine Biology, 85:
279-284.
SOLE-CAVA, A. M., VOOREN, C. M. & LEVY, J. A., 1983. Isozymic differentiation of two sibling species
of Sqwtina (Chondrichthyes) in South Brazil. Comparatiue Biochemistv and Physiology, 75B: 355-358.
SOULE, M., 1972. Phenetics of natural populations 111. Variations in insular populations of a lizard. American
Naturalist, 106: 429-446.
THORPE, J. P., HAVENHAND, J. N. & PATTERSON, K. R., 1986. Repor6 ofthe Univcrsily of Liverpool
(Department of Marine Biology) lo the Falkland Islandr Development Corporation on Stock and Stock and Species
Iden6ilies of Paragoninn Shelf Illex. Port Stanley, Falkland Islands: Falkland Islands Development
Corporation.
TURELLI, M., 1977. Random environments and stochastic calculus. lheoretical Population Biology, 12:
140-178.
VALENTINE, J. W. & AYALA, F. J., 1974. Genetic variation in Frieleia halli, a deep-sea brachiopod. Deep-sea
Research, 22: 37-44.
WARD, R. D. &' SKIBINSKI, D. 0. F., 1985. Observed relationships between protein heterozygosity and
protein genetic distance, and comparisons with neutral expectations. Genetic Research, 45 3 15-340.
WATT, W. B., 1977. Adaptation at specific loci. I. Natural selection of phosphoglucose isomerase in Colias
Buttedies: biochemical and population aspects. Genefics, 87: 177-194.
WRIGHT, S., 1978. Euolution and the Genetics of Populations. 4. Variabilib Within and Among Natural Populations.
Chicago: University of Chicago Press.
YAMAZAKI, T., 1981. Genetic variabilities in natural populations of the haploid plant, Gonocephalum conicurn.
I. The amount of heterozygosity. japanese Journal oJGcnetics, 56: 373-383.
ZOUROS, E. & FOLTZ, D. W., 1987. The use of allelic isozyme variation for the study of heterosis. In M. C.
:
Topics in Biological and Medical Research,
Rattazzi, J. G . Scandalios & G. S. Whitt (Eds), I s o ~ s Current
Vol. 13: 1-59. New York: Alan R. Liss, Inc.