Genetic divergence of Anadara trapezia populations 
Table 4 Statistical description of pair-wise comparisons of allelic similarities within and between populations 
125 
WA / 
WA 
VIC/ 
VIC 
NSW/ 
NSW 
WA/ 
VIC 
WA/ 
NSW 
VIC/ 
NSW 
Mean 
Standard error 
24.81 
0.39 
25.76 
0.30 
26.52 
0.24 
24.69 
0.19 
25.41 
0.24 
25.47 
0.20 
species. Moreover, populations that have been 
isolated for considerable time usually show some 
degree of divergence, at least in allozymes 
(Richardson et al. 1986) and the lack of any 
evidence of divergence for populations that have 
presumably been separated for at least 10,000 years 
is very surprising. 
The results obtained give rise to several 
hypotheses. It is possible that (1) the population in 
Albany has been recently introduced rather than 
the remains of a relict of the Western Australian 
population seen in the fossil record. Cotton 
(1957,1961) reported an unsuccessful attempt to 
reintroduce A. trapezia to St Vincents' Gulf in South 
Australia. While no evidence is known for such an 
attempt in Western Australia, the limited 
distribution of the species does leave such a 
possibility open. Alternatively, a set of well 
adapted gene complexes exists in the species 
resulting in very slow rates of evolution. 
(2) If the observed allozyme variation is adaptive, 
rather than selectively neutral (Kimura and Ohta 
1973) then, as long as the environment at Oyster 
Harbour is similar to those of A. trapezia habitats in 
eastern Australia, then there is no ground for 
expecting changes in the genotypes. 
(3) The evolutionary literature is full of 
discussion of issues related to the rate of evolution. 
Is it gradual (e.g., Wright 1931; Ayala et al. 1975; 
Provine 1986; Wake, Yanev and Freelow 1989) or is 
it punctuated (Eldredge and Gould 1972; Stanley 
1975). It has been much debated that most well- 
established species evolve at a very slow rate (e.g., 
Mayr 1982; Grant 1963; Barton and Charlesworth 
1984). Mayr (1982) relates this to what he termed 
"the unity of the genotype" i.e., individuals within 
a species carry within their genetic make-up a well- 
integrated set of adapted genes (Dobzhansky 1951). 
Restructuring of the genotype is less likely to 
happen in large populations as gene flow will tend 
to counteract the effect of mutations in a stable 
environment. Isolated and small populations are 
thus most vulnerable to adaptive and selective 
changes. However most isolated populations either 
reestablish contact with the parent population or 
become extinct. Mayr (1982), while acknowledging 
that evolutionary events such as speciation are 
usually linked with swift dramatic changes in 
environmental conditions, i.e., changes in the 
adaptive landscape, identified two different 
aspects of allopatric speciation. Firstly, widely 
discontinuous portions of a species often fail to 
diverge, and secondly, highly isolated populations 
are sometimes very drastically different from the 
parent population. Thus, it appears that every 
founder population does not speciate. Moreover, 
homeostatic mechanisms prevent well established 
species from undergoing rapid evolutionary 
change. As Carson (1975) pointed out, "the 
loosening up of the cohesion of the genotype is an 
important and perhaps the decisive component in 
much of speciation". 
(4). Alternatively, Eldredge and Gould (1972) and 
Gould and Eldredge (1977) proposed the concept 
of punctuated equilibrium. This concept postulates 
that evolution does not happen gradually but 
rather, it appears that large wide-ranging species 
remain static for a very long time and new forms 
arise in peripheral small isolated populations at a 
very rapid rate. However the influences of founder 
events on the magnitude and pattern of speciation 
are in theory directly related to the size and 
duration of such events and also on the resulting 
rate of population recovery (Nei, Maruyama and 
Chakraborty 1975). In addition, as discussed by 
Avise (1994), the survival of particular lineages 
following a bottleneck may be a purely stochastic 
event. 
In the eastern regions of Australia A. trapezia has 
survived for a relatively long time in most 
estuaries even though it may have undergone 
drastic population crashes in various places in 
periods due to catastrophes such as major floods. 
Recolonisations have been observed in some places 
on the east coast due to the ability of the organism 
to disperse by pelagic larvae and also the presence 
of ocean currents that enable connectedness among 
geographically separated populations. The 
population in Oyster Harbour, near Albany, 
however requires special management strategies as 
there are no adjacent populations to replenish the 
genetic pool in the event of a major catastrophe. 
Indiscriminate exploitation by commercial fishers 
may also result in the extinction of this species 
from the southwestern region of Australia. 
ACKNOWLEDGEMENTS 
We wish to thank Shirley Slack-Smith (Western 
Australia Museum) and Anthony Forster (Fisheries 
Department of Western Australia) for providing 
specimens of A. trapezia from Oyster Harbour, 
