A striking example of the founder effect
in the mollusc Littorina saxatilis
A. J. KNIGHT*, R. N. HUGHES? AND R. D. WARD*
*Environmental Biology Unit, Department o f Human Sciences, Loughborough University of
Technology, Loughborough, Leicestershire L E I 1 3357, ?School o f Animal Biology,
University College of North Wales, Bangor, Gwynedd LL57 2UW
Received 5 June 1987, accepted f o r publication 16 June 1987
Two South African populations of Littorina saxatilis were examined by starch-gel electrophoresis at
16 enzyme loci and compared with 13 populations of North Atlantic saxatilis from both American
and European coasts, and with six British populations of the closely related species Littorim urcana.
The South African animals showed a severely reduced heterozygosity ( R= 0.052) compared with
Atlantic populations of saxatills ( H = 0.181), and the mean genetic distance between the two areas
was high ( D = 0.203) compared with distances within the North Atlantic suxatilis populations
( B = 0.034). In fact, the suxatilis from South Africa were genetically more distant from the North
Atlantic samples of L. saxatilis than were the arcana from British shores. The reduced genetic
heterozygosity and genetic divergence of the South African populations is attributed to founder
effects following a postulated recent introduction by man.
KEY WORDS:-Liltorina
genetic distance.
saxatilzs
-
Littorina arcana - founder effect - allozymes - polymorphism
CONTENTS
Introduction . . . . . . . . . . . . . . .
Materials and methods.
. . . . . . . . . . . .
Results . . . . . . . . . . . . . . . . .
Allozyme analysis . . . . . . . . . . . . .
Colour variation in the South African samples . . . . . .
Discussion. . . . . . . . . . . . . . . .
Acknowledgements
. . . . . . . . . . . . .
References. . . . . . . . . . . . . . . .
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41 7
418
418
4 18
423
424
425
425
INTRODUCTION
Littorina saxatilis is a marine intertidal gastropod commonly found on rocky
shores, boulder shores and saltmarshes of the North Atlantic. The taxonomy of
this ‘species’ remains a somewhat contentious issue, although most workers in
this area currently recognize L. saxatilis to be a complex of four species: viz.:
L. saxatilis sensu stricto Olivi (= L. rudis Maton), L. neglecta Bean, L. nigrolineata
Gray and L. arcana Hannaford Ellis. The first two of these species reproduce
ovoviviparously (giving birth to miniature adults, ‘crawlaways’) and the second
417
0024-4066/87/120417
+ 10 $03.00/0
01987 The Linnean Society of London
418
A. J. KNIGHT 87 AL.
two oviparously (laying egg masses). Despite this difference in reproductive
mode, the four species are closely related to one another genetically (Ward &
Warwick, 1980; Ward & Janson, 1985; Knight & Ward, unpublished). The
saxatilis complex is essentially restricted to the North Atlantic, with one or
possibly two populations in the Mediterranean, but recently two isolated
populations of L. saxatilis have been reported from South Africa.
Day (1974) first reported Littorina saxatilis in South Africa, from Langebaan
and Knysna lagoons, Cape Province. Later, Hughes (1979) described the
morphological characteristics of these two populations, and noted that they bore
a close resemblance to European L. saxatilis from saltmarsh habitats. Intensive
surveys (Hughes, 1979) failed to discover the species elsewhere in the Cape
Province and this, together with the long stretches of unsuitable habitat
(tropical sand beaches) along the West African coast, suggested that L. saxatilis
had been accidentally transported from Europe by ships using Langebaan and
Knysna lagoons as harbours. If so, these populations are likely to have been
founded from a few snails, and effective population sizes are likely to have
remained low for a number of generations. As a consequence of these bottleneck
events, the new populations should show marked allele frequency differentiation
from their parental populations, and the degree of genetic variation within them
should be reduced.
The present paper tests these predictions by examining, through starch gel
electrophoresis, patterns of genetic variation in the South African snails and
then compares these patterns with those of European and North American
populations of saxatilis. We have also included in the comparison samples of
L. arcana.
MATERIALS AND METHODS
Fifteen populations of L. saxatilis were sampled, two from South Africa, eight
from the United Kingdom, one each from Ireland and Italy, and three from
North America. The six samples of arcana assayed were all from the U.K.
Localities of the samples are given in Table 1. Fourteen loci were screened in
each population using previously published methods (Ward & Warwick, 1980;
Janson & Ward, 1984; Knight & Ward, 1986). Variants a t these loci are known
to segregate in a Mendelian manner (Ward, Warwick & Knight, 1986; Knight,
Warwick & Ward, unpublished). Two further loci were scored for the first time,
Aco-1 and Aco-2. Heterozygotes at each of these aconitase loci were two-banded,
indicating the customary monomeric structure of this enzyme. Most of the data
analysis was performed using the BIOSYS-1 software package (Swofford &
Selander, 1981) .
RESULTS
Altozyme analysis
Allele frequencies for the 21 populations are given in Table 2. It is striking
that the South African populations are monomorphic for a number of loci (Mpi,
Pgm-2 and Np) which are rarely found to be monomorphic in European or
North American populations. This, together with reduced variability at a
FOUNDER EFFECT IN L I T T O R I N A
419
Table 1. Localities of the sampling sites
Site no.
Country
Location
Littorina saxatilis
1
South Africa
2
South Africa
3
Scotland
4
Scotland
5
England
6
England
7
England
8
Channel Isles
9
Wales
10
N. Ireland
11
Eire
12
Italy
13
United States
14
United States
15
Canada
Langebaan lagoon, W coast, Cape Province
Knysna lagoon, S coast, Cape Province
North Bewick harbour wall, E Lothian
Logan road, Isle of May, Fife
Rock shelf, Swanage, Dorset
Rock spit, Bude, Cornwall
Rock shelf, Watchet, Somerset
Cliff face, Nez des Pas, Jersey
Under pebbles, Bangor, Menai Straits, Gwynedd
Grand Causeway, Giant’s Causeway, Derry
Limestone pavement, Doolin, Clare
Mdell Orte Canal wall, Venice
Unknown location in state of Maine
Breakwater, Stonnington, New Hampshire
Rocks, Churchill, Hudson’s Bay, Manitoba
Littorina arcana
16
Scotland
17
Scotland
18
Scotland
19
England
20
Wales
21
Channel Isles
North Berwick harbour wall, E Lothian
Roxborough hotel seawall, Dunbar, E Lothian
Cliff face, Milsey bay, E Lothian
Rock face, Hartland Quay, Devon
Rock face, Porth Maria, Anglesey
Slipway, Le petit Etacquerel, Jersey
number of other loci (Pgm-1 and Odh), causes a marked reduction in the mean
heterozygosity per locus (r7) of the South African samples in comparison with
the other L. saxatilis (Table 3 ) . The mean A of the two South African
populations is 5.2% compared with 18.1yo for the remaining 13 saxatilis. Indeed,
the South African animals are substantially less variable than those from
Venice, Italy, whose own reduced variability in comparison with Atlantic
samples had earlier been attributed to a past bottleneck in population size
(Janson, 1985). Over the 16 loci we screened, the Venice population had an A
of 13.1% compared with the mean A of the other European populations of
19.6%. Note that the Venice population we sampled came from a canal within
Venice itself whereas that of Janson came from the southern part of the Venice
lagoon (Chioggia); this spatial differentiation probably accounts for the
(generally small) allele frequency differences between our Venice sample and
that of Janson.
Perhaps less expected than the reduction in heterozygosity of the South
African populations was the finding that at Aat-I, the two South African
populations were very nearly fixed for different alleles, and that at two further
loci the two populations had significantly different allele frequencies (Pgi,
P < 0.001; Pgm-1. However, it should be remembered that populations of
European saxatilis separated by only a few metres can show significant
differentiation (Janson & Ward, 1984): such differentiation can arise from
either stochastic or deterministic forces. It is notable that all the South African
alleles have been detected in European populations of saxatilis (Pgm-1’ 5 , not
represented in Table 2, has been recorded from Swedish populations).
-
1.00
1.00
-
100
85
75
Idh-2
0.78
0.22
-
-
1.00
1.00
105 0.99
100 85 75 0.01
--
.-..
1.00
-
100
100
85
70
20
110
105
100
90
80
Pgm-1
Pgm-2
Odh
-
~-
~~~
--
-
-
1.00
-
-
-
0.98
0.02
_
0.33
0.37
0.30
0.16
0.50
0.34
1.00
-
-
0.56
0.44
0.02
0.67
0.31
0.02
0.98
0.94
0.10 0.06
-
0.90
0.50
0.48
0.03
0.08
0.68
0.24
0.03
0.97
0.01
0.94
0.05
0.41
0.42
0.17
0.19
0.60
0.21
1.00
-
0.47 0.54 0.17
0.53 0.46 0.83
_
-
-
0.98
0.03
0.69
0.31
0.16
0.84
_
-
1.00
-
0.58
0.42
_
0.64 0.68
0.36 0.32
0.10
-
0.90
0.65
0.35
_
0.80
0.20
0.11
_
0.89
0.77
0.23
_
0.83
0.17
3
~-
_
0.02
0.94
0.04
0.10
0.52
0.39
0.88
0.12
-
1.00
-
_
1.00
__
-
1.00
0.42
0.52
0.06
0.67
0.33
-
_
-
0.62
0.36
0.02
0.65
0.35
-
-
0.80
0.13
0.07
0.71
0.29
_
-
-
_
-
0.80
0.21
-
_
-
0.98
0.02
0.01
_
0.93
0.05
-
0.47
0.47
0.07
_
-
_
1.00
0.78
0.74
0.24
0.03
0.26
0.20
0.54
_
_
0.72
0.28
0.22
-
-
-.
0.38
0.59
0.03
0.22
0.03
0.75
0.07
0.57
0.37
0.18
0.63
0.18
0.04
0.78
0.17
1.00
1.00
_
-
0.10
0.62
0.29
0.50
0.50
_
_
1.00
-
1.00
_
_
0.61
0.39
-
0.61
0.39
_
-
1.00
1.00
_
1.00
-
1 .oo
-
_
_
-
ITA
IRE
0.03
0.74
0.24
12
11
_
_
1.00 1.00 1.00
-
0.36
0.64
1.00 1.00 1.00
_
_
_
-
0.57
0.38
0.05
0.70
0.28
-
0.02
7
8
9
1
0
4
5
6
SCO SCO ENG ENG ENG JER WAL NIR
-
1.00
1.00
-
150
Aat-2
-
0.03
0.97
-
1.00
120
100
60
-
-
Aat-1
-
-
-
1.00
-
-
1.00
0.29
0.71
---
-
120
100
75
110
0.06
100 0.56
90 0.39
140
SA
SA
Mpi
Pgi
Locus
2
1
L. saxatilis
14
-
1.00
-
-
0.73
0.08
0.19
0.02
0.98
0.1 1
0.77
0.11
-
-
0.76
0.24
0.77
0.23
-
0.04
0.96
0.52
0.28
0.19
0.01
0.70
0.29
1.00
1.00
1.00
0.19
0.79
-
0.27
0.73
-
0.10
0.70
0.20
0.36
0.47
0.17
0.06
0.94
1.00
-
0.25
0.75
-
0.74
0.21
0.05
0.14
0.64
0.21
1.00
-
0.23
0.77
0.98 0.98 0.89 1.00 1.00
_
_
_
0.02 0.02 0.11
-
-
0.91
0.09
-
-
-
0.69
0.31
0.86
0.07
0.07
0.05
0.95
0.29
0.71
0.27
0.74
-
0.71
0.26
0.03
0.22
0.73
0.05
1.00
-
0.21
0.79
-
-
0.93
0.07
-
-
-
0.73
0.27
-
0.46
0.55
1.00
-
0.36
0.64
-
1.00
-
0.98
0.02
-
-
0.47
0.53
0.08
0.70
0.23
1.00
-
0.82
0.18
-
0.54
0.46
0.40
0.60
1.00
-
0.01
0.84
0.16
0.28
0.70
0.25 0.37
0.75 0.63
_
-
0.83
0.17
0.87
0.13
0.94
0.06
-
-
0.11
0.89
-
1.00
_
_
0.89
0.11
0.69
0.31
0.83
0.17
.-
0.87
0.14
-
0.75
0.25
-
0.74
0.26
-
-
0.65
0.35
-
-
1.00
-
0.62
0.39
-
0.80
0.21
-
0.85
0.15
16
17
18
20
21
19
15
USA USA CAN SCO SCO SCO ENG WAL JER
13
L. arcana
Table 2. Allele frequencies of 15 populations of Littorina saxatilis and 6 populations of L. arcana
b
h
Y
5i;
5
?
0
.-
-
110
~
1.00
-
0.02
0.98
105
100
-
1.00
-
1.00
1.00
0.03
0.97
1.00
160
100
-
-
-
9 5 7 5 -
0.19
0.81
-
1.00
0.10
0.41
0.53
0.06
100 0.99
-
-
-.
-
.-
1.00
-
-
-
1.00
1.00
1.00
1.00
-
1.00
__
1.00
-
-
0.10
0.90
-
0.40
0.60
1.00
-
1.00
-
-
0.96
0.04
-
0.35
0.52
0.14
-
-
-
-
-
-
-
-
140
100
65
35
-
180
1 4 5 100 1.00
6 0 -
1.00
-
1.00
-
0.01
0.90
0.06
0.03
0.22
0.33
0.45
0.03
0.40
0.49
0.09
-
-
1.00
1.00
1.00
-
-
0.09
0.84
0.06
0.15
0.74
0.11
-
-
1.00
__
1.00
L
-
0.92
0.08
-
-
0.98
0.02
-
0.99
0.01
-
-
-
-
1.00
-
1.00
-
-
-
1.00
-
1.00
-
-
0.02
0.93
0.05
1.00
0.05
0.30
0.55
0.10
-
0.04
0.96
0.02
0.39
0.20
0.39
-
1.00
-
-
-
1.00
1.00
1.00
-
1.00
-
-
-
1.00
-
1.00
-
-
-
0.31
0.69
-
-
1.00
-
-
0.03
0.32
0.58
0.08
-
0.03
0.97
1.00
-
1.00
-
-
0.06
0.94
-
0.75
0.15
0.10
-
-
1.00
-
1.00
-
1.00
-
-
0.96
0.05
-
0.06
-
0.94
---
-
1.00
-
-
-
0.09
0.91
1.00
-
-
0.01
0.93
0.06
0.01
0.65
0.14
0.20
-
1.00
-
1.00
-
1.00
-
-
1.00
-
-
-
0.06
0.94
--
0.30
0.20
0.50
1.00
1.00
-
-
1.00
-
-
0.06
0.94
-
0.10
0.15
0.75
1.00
-
1.00
-
-
-
1.00
-
0.02
-
0.03
0.96
-
0.12
0.17
0.71
-
24.3
1.3
12.5
5.5
24.5
1.8
50.0
20.9
16.6
1.8
56.3
21.5
22.1
1.7
43.8
20.5
43.8
17.4
1.8
23.1
24.1
1.9
50.0
22.2
19.8
1.7
31.3
16.6
27.2
1.9
43.8
17.9
15.9
1.8
37.5
18.8
18.0
1.4
31.3
13.1
12
ITA
11
IRE
10
9
WAL NIR
all./locus = mean number of alleles per locus.
% polym. = percentage of loci which are polymorphic (0.95 criterion).
% fl/locus = mean (Hardy-Weinberg) percentage of heterozygosity per locus.
35.9
2.1
50.0
20.9
4
5
6
8
3
7
SCO SCO ENG ENG ENG JER
R = mean number of individuals screened per locus.
24.6
1.3
12.5
4.9
2
SA
1
SA
L. saxatilis
13
14
15
26.0
1.6
43.8
15.8
19.3
1.6
37.5
11.0
40.1
1.9
50.0
17.5
USA USA CAN
17
18
19
L. arcana
20
-
1.00
-
1.00
-
-
1.00
-
-
0.07
0.90
0.03
-
-.
21
1.00
..-
1.00
-
-
1.00
-
-
0.09
0.82
0.09
0.92
0.08
-
14.1
1.8
50.0
20.5
20.4
1.8
50.0
18.7
27.8
1.9
50.0
19.0
15.1
1.6
50.0
16.0
18.1
1.6
43.8
15.7
12.4
1.6
50.0
17.7
SCO SCO SCO ENG WAL JER
16
-
0.15
0.85
1.00
1.00
-
-
0.93
0.07
-
-
1.00
-
0.17
0.83
-.
Table 3. Levels of genetic variation in 15 populations of Littorina saxatilis and six populations of L. arcana
Three loci, Mdh-1, Mdh-2 and Idh-I are fixed identically in all populations.
all./locus
Yo P_olYm.
yo H/locus
R
~~
Ed-1
AGO-2
Aca-I
Hdh
A. J. KNIGHT E T AL.
422
Inspection of Table 2 confirms the close genetic relationship between
L. saxatilis and L. arcana (Ward & Warwick, 1980; Ward & Janson, 1985). No
locus is diagnostic. Perhaps the only allele present in appreciable frequencies in
one species and not the other is Hdh'80,and that allele is restricted to Scottish
populations of arcana.
By comparing allele frequencies at the 16 loci and drawing up matrices of
genetic distance, it is possible to examine the overall relationships between these
taxa. A phenetic tree constructed using the UPGMA algorithm (Sokal &
Sneath, 1963) and Nei's unbiased genetic distances (Nei, 1978) is given in
Fig. 1. Given that the standard errors of the branch points are in the
neighbourhood of 0.06, not much weight should be put on the precise placement
of most of these populations. However, it is striking that the South African
populations cluster together and appear quite distinct from the remaining
samples. There is no other obvious geographic structuring of the data. In fact,
the South African L. saxatilis appear to be genetically less closely related to the
North Atlantic saxatilis than are the L. arcana (Fig. 1 and Table 4).The latter
populations fall into two groups, a northern, Scottish, group, and a more
southern group. These two groups are primarily distinguished by a switch in the
frequency of the most common allele at the Odh locus, from Odh'OO in the
northern animals to 0 d h l o 5in the southern animals.
An unrooted Wagner tree, constructed from Rogers genetic distance (Rogers,
1972), is given in Fig. 2. This method of tree construction, unlike the UPGMA
arcana
arcana
arcana
w
0.25
0.20
0.15
0.10
0.05
I
I
1
1
1
Genetic distance
0
I
16 arcana
17 arcana
Orcana
Figure 1 . UPGMA tree of genetic distances (Nei, 1978) among populations. Littarzna arcana
populations are identified; all others are L. saxatilis.
FOUNDER EFFECT IN LITTORINA
423
Table 4. Mean genetic distances (Nei, 1978) between populations of Littorina
saxatilis and L. arcana from different areas
Genetic distance
Comparison
1 S Africa saxatilis us. S Africa saxatilk
2 S Africa saxatilis us. N Atlantic saxatilis
3 S Africa saxatilis us. N Atlantic arcana
4 N Atlantic saxatilis us. N Atlantic saxatilis
5 N Atlantic saxatilis us. N Atlantic arcana
6 N Atlantic arcana us. N Atlantic arcana
~~
Meanfs.E.
Range
N
0.073
0.203f0.011
0.21 5 f0.013
0.034 f0.003
0.072f0.004
0.044 fO.008
-
0,138-0.350
0.160-0.306
0.001-0.120
0.012-0.200
0.001-0.108
I
24
12
63
75
15
~
N
= number
of pairwise comparisons.
The North Atlantic samples of saxatilis exclude the Venice population.
method, does not assume equal rates of evolution following a branch point. The
resulting network is rather different from the UPGMA tree. The two South
African populations still cluster together and remain well separated from the
remaining samples, but now all six arcana populations cluster together and away
from the saxatilis samples.
Colour variation in the South African samples
Not only has there been divergence a t the allozyme level between the South
African populations, but there has also been divergence in shell colour. The
Langebaan population is characterized by a tesselated pattern superimposed on
I
14
L. arcono I L. saxotitis
I
Figure 2. Unrooted Wagner tree of genetic distances (Rogers, 1972) among populations.
424
A. J. KNIGHT E T AL.
background colours ranging from white, grey, fawn, brown to black, with
occasional orange individuals. The Knysna animals are characterized by the
tesselated pattern superimposed on an amber or, less frequently, on a fawn or
dark brown background. T h e tesselations are very faint on amber individuals. It
may be assumed that this variation is under genetic control, since saxatilis shell
colour morphs that have been investigated in breeding experiments have been
shown to be inherited (Warwick, Knight & Ward, unpublished).
DISCUSSION
It seems probable that in the South African populations visual predators have
selected for cryptic or disruptive colouration. I n Langebaan, the snails rest on
the closely packed stems of Spartina, where they are difficult to see, even in
bright sunlight. The Knysna snails live among pebbles of a light brown hue, the
shells matching the colour of the stone very closely. Both sites are frequented by
numerous waders, egrets and passerines. Relatively brief observations, however,
indicated that the crab Cyclograpsus punctatus was the most important predator.
Contrary to earlier assertions (Hughes, 1979), this crab was shown in laboratory
trials to be capable of opening the largest shells of L. saxatilis and it is abundant
at both sites. Predation of L. saxatilis may be minimized not only by the cryptic
nature of shell colouration but also by behavioural patterns. At Langebaan, the
crab was never seen climbing the Spartina stems, where the saxatilis may find
sanctuary. At Knysna, the pebble ridge colonized by saxatilis provides protective
crevices.
The South African saxatilis show a severe erosion of genetic variation and the
mean genetic distance of the two South African populations to North Atlantic
samples of saxutilis (D= 0.203, Table 4) is substantially greater than that of the
North Atlantic populations to one another (D= 0.034). These features of the
South African samples are consistent with past bottlenecks in population size
(Chakraborty & Nei, 1977), and it seems likely that these populations
originated from a small number of European founders accidentally transported
by shipping to the natural harbours a t Langebaan and Knysna.
Interestingly, another intertidal mollusc introduced into South African
waters, perhaps within the last two decades, does not show such effects. This is
the bivalve Mytilus galloprovincialis, where there has been relatively little
differentiation between a European sample (from Spain) and samples from the
western Cape coast of South Africa (Grant & Cherry, 1985). The genetic
distance (over 23 loci), of the European and South African samples is very low
(D= 0.010) and heterozygosities are very similar. This suggests that the
introduction of L. saxatilis into South Africa involved much more severe
bottlenecks than was the case for M . galloprovincialis.
The degree of genetic differentiation of the South African populations is such
that the North Atlantic populations of L. saxatilis and L. arcana are genetically
more similar to each other than they are to the South Africa saxatilis (Table 4).
Thus some intra-specific distances are considerably greater than the mean interspecific distance. Such a phenomenon has been reported before, from the closely
related members of the Partulu complex on Moorea, French Polynesia (Johnson,
Murray & Clarke, 1986a). Parallelling the Littorina, the Partula show rather little
inter-specific differentiation and rather high intra-specific differentiation. The
FOUNDER EFFECT IN L I T l O R I N A
425
Partula species on Moorea appear to have evolved on the island since it was
formed about 1.5 million years ago, and it has been proposed that in Partula
Nei’s genetic distance increases by about 0.13 units every million years
(Johnson, Murray & Clarke, 1986b). We do not yet know when or where the
saxatilis complex underwent their ratiation, but the mean distance between the
North Atlantic populations of L. saxatilis and L. arcana over the 16 loci screened
here is 0.072. This compared with an earlier estimate of 0.044, based on 21 loci
scored in six English and Scottish populations of saxatilis and three Scottish
populations of arcana (Ward & Warwick, 1980). Use of the Partula molecular
clock would predict a divergence time of the two saxatilids of between 550 000
and 340000 years ago. This is in the same area as the earlier estimates of
between 220000 and 792000 years (Ward & Warwick, 1980), using the
molecular clock formulae of Nei (1975) and Maxson & Wilson (1974),
respectively.
These dates must be regarded as speculative, but when considered alongside
other unpublished data showing that there is little genetic differentiation
between all four members of the saxatilis complex, it seems very likely that
speciation within this complex has been a recent event.
One caveat must be raised here. Injudicious use of genetic distance figures
and molecular clocks can in certain circumstances lead to conclusions that are
likely to be erroneous. For example, in the present study the mean genetic
distance between the South African and North Atlantic populations of saxatilis,
0.203, would indicate a divergence time using the Partula molecular clock of
about 1.5 million years. However, we are proposing here a recent human
introduction of L. saxatilis into South Africa. How can we reconcile these two
estimates? I t has been shown that in the presence of population bottlenecks,
genetic distance rapidly increases in early generations as a consequence of a
reduction in heterozygosity in the populations being compared (Chakraborty &
Nei, 1977). The low heterozygosity of the South African populations is
indicative of recent bottlenecks, and thus the comparatively high genetic
distance between them and North Atlantic populations is to be expected: it does
not reflect an ancient origin for the South African populations.
ACKNOWLEDGEMENTS
R.N.H. is indebted to the Royal Society of London for a travel grant and to
Andre Genade for sending a supplementary sample of snails from Knysna. We
also wish to thank Tom Warwick, John Perry, Julie Knight, Ian Munro, John
Hope, Neil Billington and Emanuele Rodino for assistance in collecting other
samples, and the Natural Environment Research Council for a research grant to
R.D.W.
REFERENCES
CHAKRABORTY, R. & NEI, M., 1977. Bottleneck effects on average heterozygosity and genetic distance
with the stepwise mutation model. Evolution, 31: 347-356.
DAY, J. H . , 1974. A Guide to Marine Lije on South Afiican Shores. 2nd edition. Cape Town, Rotterdam: A. A.
Balkema for University of Cape Town.
GRANT, W. S. & CHERRY, M. I., 1985. M y t h s galloprovincialis Lmk. in Southern Africa. Journal of
Experimental Marine Biology and Ecology, 90: 179-19 I .
426
A. J. K N I G H T E'T AL.
HL'GHES, R. N.. 1979. South African populations of Littorina rudis. zoological Journal uf the I,innean . S o c i f ~ ,65:
119-126.
JANSON, K., 1985. A morphologic and genetic analysis of Littorina saxatilis (Prosobranchia) from Venice, and
on the problem of saxatilis-rudis nomenclature. Biological Journal of the Linnean Society, 24: 5 1-59.
JANSON, K. & WARD, R. D., 1984. Microgeographic variation in allozyme and shell characters in Littorina
saxatilis Olivi (Prosobranchia: Littorinidae). Biological Journal of the Linnean Society, 14: 41 7-428.
JOHNSON, M. S., MURRAY, J. & CLARKE, B., 1986a. Allozymic similarities among species of Partula on
Moorea. Heredity, 56: 319-327.
JOHSSON, M . S., MURRAY, J. & CLARKE, B., 1986b. An electrophoretic analysis of phylogeny and
evolutionary rates in the genus Partula from the Society Islands. Proceedings of the Royal Society qf London,
Series B, 227: 161-177.
KNIGHT, A. J. & WARD, R . D., 1986. Purine nucleoside phosphorylase polymorphism in the genus Littorina
(Prosobranchia: Mollusca) . Biochemical Genetics, 24: 405-41 3.
MAXSON, L. R. & WILSON, C. A,, 1974. Convergent morphological evolution detected by studying the
proteins of the tree frogs of the Hyla exirnia species group. Science, 185: 66-68.
NEI, M . 1975. Molecular Population Genetics and Euolution. Amsterdam: North-Holland.
NEI, M., 1978. Estimation of average heterozygosity and genetic distance from a small number of individuals.
Genetics, 89: 583-590.
ROGERS, J. S., 1972. Measures of genetic similarity and genetic distance. Studies in Genetics VII. Uniniuersity OJ
Texas Publication, 7213: 145-153.
SOKAL, R. R. & SNEA'I'H, P. H. A., 1963. Principles of Numerical TaxonornJ). San Francisco: Freeman.
SWOFFORD, D. L. & SELANDER, R. B., 1981. BIOSYS-I: a FORTRAN program for the comprehensive
analysis of electrophoretic data in population genetics and systematics. Journal of Heredip, 72: 281 -283.
W,4RD, R. D. & JANSON, K., 1985. A genetic analysis of sympatric subpopulations of the sibling species
Littorina saxatilis (Olivi) and Littorina arcana Hannaford Ellis. J o u of ~Molluscan
~
Studies, 51: 86-94.
WARD, R . D. & WARWICK, T . , 1980. Genetic differentiation in the molluscan species Littorina rudis and
Littorina arcana (Prosobranchia: Littorinidae). Biological Journal of the Linnean Society, 14: 41 7-428.
WARD, R . D., WARWICK, 1'.& KNIGHT, A. J., 1986. Genetic analysis of ten polymorphic enzyme loci in
Littorina saxatilis (Prosobranchia: Mollusca). Heredity, 57: 233-241.