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POPULATION GENETICS AND POPULATION SIZE
I N PREHISTORIC TASMANIA
Colin Pardoe
T h e results of our investigation encourage us to think that a bridge can
be established between subjects as seenlirrgly diverse as archaeology and
genetics. We find t h a t it is possible to interpret patterns of the geographic
distribution of genes, something t h a t has long puzzled geneticists. on the
basis of events observed in the archaeological record. At the same time,
insight into developments of archaeological interest ... can be obtained by
analyzing genetic d a t a and using conceptual frarr~eworksdrawn from the
study of population biology [Ammerman and Cavalli-Sforza 1984:133].
Population size, density and distribution are crucial elements in the archaeological
interpretation of material remains. T h e problem is of such great importance that a
continuing controversy exists over the timing and nature of population growth
throughout t h e world. Witness the recent writings, in Australia alone, that not only
postulate palaeodemographic models (Beaton 1985; Lourandos 1980, 1983; Butlin
1983; Webb 1984) but also use those models a s background to the archaeological
interpretations (Hughes and Lampert 1982; Rowland 1983; Ross 1985; Williams
1985).
It is important t h a t when independent information bearing on the subject appears, it
be properly interpreted for applicability and congruence with other work. For these
reasons 1 wish t o discuss the implications of an investigation with seemingly little
relation t o population size and palaeodemography.
In studying t h e prehistory of genetic systems (or the genetics of prehistoric systems,
Pardoe 1984) by the use of non-metric cranial traits in a continent-wide survey, one
of t h e results was the placement of Tasmania very close t o mainland southeastern
Australia; a somewhat predictable result. In fact, it is really only of interest when we
realise the dichotomy in biological and archaeological assessments of Tasmanian
morphological and cultural evolution. T h e problem is t h a t some researchers have
said t h e two groups are similar, others t h a t t h e two areas a r e different. But t h e only
meaningful question t o be asked is one of context: 'how different should they be?'.
'How different, or how similar, a r e the Tasmanians from southern mainland
Australians?' begs the question of in relation t o what.
By using quantitative models of I s d a t i o n by Distance (Pardoe 1986) 1 was able t o
partially assess how different they are compared wit h expectations from population
genetic theory. The results showed how well t h e model worked, with approximately
50% of observed morphological variation explained within the Isolation by D i s t a n c e
framework.
Suddenly, however, we are now faced with a completely different
problem. Should the Tasmanians not have diverged more t h a n was observed? They
were, after all, the most isolated of human populations, subject t o environmental
selection first as the southernmost. part of a continuous eastern seaboard population
(clinal variation) and then for the last 13,000 years or more a s a n isolate in a n
environment substantially different from their nearest neighbours in southern
Victoria.
THE CONTROL OF MORPHOLOGICAL DIFFERENTIATION
The processes t h a t structure the way in which populations diverge are well defined in
evolutionary theory. These are the 'four forces' of evolution: mutation, selection,
gene flow (intermarriage and migration) and genetic drift (founder effects,
bottlenecks and variation in population size generally).
Mutation of phenetic or phenotypic structures is poorly understood, but unlikely t o
be of any significance for this reconstruction. Because most morphological features
result from the action of a large but undetermined number of genes, mutation of any
single gene should not have a large impact on the expression of a feature. Mutations
are also rare events in this time scale. A t any rate, there is no need t o invoke
mutations as the basis for observed differences, for there is no feature which is
exclusively Tasmanian. Furthermore, there is more than enough diversity within
populations t o provide the raw material for differentiation. Lewon tin ( 1972) and
Latter (1980) estimate this t o be in excess of 80% of the total pool of human
diversity.
Selection can be the most powerful of evolutionary forces and is certainly the reason
for the discrepancy between metric and non-metric skeletal studies. Size a n d shape
are extremely sensitive t o selection, showing the effects of systematic environmental
variation in such features as overall size, body proportions and head shape. Nonmetric traits on the other hand are likely t o be transparent overall t o t h e systematic
effects of selection. I have argued elsewhere (Pardoe 1984) t h a t selection, if present
in a measurable degree, is not likely to be systematic over all t h e traits examined.
Gene flow is extremely important in structuring population affinities both on a large
scale. say between Tasmania and Victoria, and on a smaller scale within a region
such as Tasmania itself. It is gene flow t h a t is behind Isolation by Distance, b u t not
Isolation by Barrier, and given the presence of Bass Strait over the last 13,000 years,
gene flow is no explanation of the observed differentiation. In fact, here is one
instance where one of the variables is controlled.
T h a t leaves genetic drift, the random statistical effects t h a t can a1ter population
affinities through events a s opposed to processes.
If populations are small,
subsequent generations are more likely to drift, or change in genetic make-up,
because not all individuals contribute t o the following generations equally. Trait
frequencies are susceptible t o wild fluctuations in small groups because:
... even a slight change in the actual number of people carrying a gene
causes a large change in the percentage of the population endowed with t h a t
trait. In a larger population a change in gene frequency would affect a
smaller percentage of the people. and thus drift would be less pronounced
;Cavalli-Sforza 1969:31].
For example, a trait present a t 5% will be seen in 500 people in a population of
10,000 while in a group of 100, it will be seen in five people. A small change of say
five people is enough t o delete or double the trait in the smaller group, but changes
the proportion by only 0.05% in the larger group.
The size of a population is one factor in the occurrence of genetic drift. Density and
migration are related factors t h a t can also radically alter the amount of drift
(Cavalli-Sforza 1963, 1969).
This brings us back t o the original problem: population size. Since there is now good
evidence t h a t Tasmanians have diverged less than expected for a set of skeletal
features, and since the main cause of differentiation should be genetic drift and not
mutation: selection or gene flow, then it is apparent that genetic drift has been
operating less than expected.
T h e only way t o d a m p genetic drift is to have large population sizes. This can be
achieved in two ways; large absolute sizes of each breeding population or extremely
high levels of gene flow between breeding populations. In effect Tasmania must have
had a very large overall population, very high levels of exogamy or some combination
of both. But what is true for Tasmania must also hold for southern Victoria.
Populations there must also have been subject to less genetic drift than w e might
expect.
T o reiterate, the biological relationship of Tasmanian t o mainland populat ions was
initially structured by Isolation by Dlstonce. operating between first colonisation of
the Tasmanian Peninsula a t about 23.000 gears ago and final separation at about
13,000 years ago. This relationship was rrlainly nlediated bjr gene flow. Howei~er,we
also know t h a t Isolation by Barrier has been operative during the last 13.000 years,
iiited
~ ~ randomly from all the mainland
so Tasmanian populations should have d i f f e r e ~
populations since the structuring effect of gene flow was absent. The random
differentiation should be the result of gerletic drift.
HOW MANY?
The Aboriginal Tasmanian population has been estimated at 3000-5000 by Jones
(1974) and this was supported by Stockton's work on site densities (1982, 1983).
More recent work by archaeologists tends to show denser occupation in many areas.
In fact, one analysis by Stockton (1983:68) results in a population of the northern
half of Tasmania's west coast of 3000-6000; equal t o his own and Jones's (1974)
estimates for the whole island. In the light of other evidence from historical sources,
quantitative assessments of midden deposits and from the uncertain nature of the
archaeological household data, Stockton rejected this higher figure.
The estimation of actual population numbers is not possible by extrapolation from
the model. As far as I am aware, no evidence pertains to the relationship between
population size and genetic drift for poly genic quantitative traits. However, the
scenario of 5000 individuals, say l000 mating pairs, divided between nine tribes or
perhaps even more, further subdivided into many bands, leads inexorably t o the
conclusion that Tasmania should then be subject t o intense genetic drift.
BIRDSELL'S RAINFALL A N D TRIBAL
AREA CORRELATION
,
Birdsell's study of the relationship between tribal size and rainfall (1953) did not
include Tasmania, but even with all the problems of that research (Birdsell discusses
these a t length), the extrapolation of his analysis t o Tasmania suggests a minimum of
284 tribes and as many as 839. That is, according to the mainland data, the amount
of rainfall that would support tribal areas of a t most 160 and more likely 80 km2.
I do not wish t o suggest that the Aboriginal Tasmanian populat.ion was as high as
419,500. On the other hand I do not wish t o reject out of hand the results of carrying
capacity data. There is scope in ecological and evolutionary terms, a t least t o
question the conservative figure of 3000-5000 suggested by Jones (1974) and Stockton
(1983). The independent data from population genetics and ecology are also in
keeping with new notions on mainland pre-contact population size.
CONCLUSIONS
Population estimates for Tasmania will have to be revised upwards. The evidence
discussed here is not sufficient to make those estimates. Palaeodemographic study,
incorporating archaeological data, may provide greater direction.
The size of population must have been large. not just through natural increase a t
some time after colonisation, but from before separation. This must have been true
for southern Victoria as well as Tasmania.
Many researchers have argued for mid- t o late Holocene population increase in
Tasmania.
In the southeast of A ~ s t ~ r a l i aothers
,
have suggested the same
phenomenon. If 3000-5000 souls is the estimate for Tasmanian population prior t o
contact, what would the numbers have been in the Pleistocene? Would such small
groups have been viable?
From the results of biological modelling, not only do I conclude that population sizes
were large, but also that given the time depth of the Isolation by Distance model,
population sizes in Tasmania have been large since separation, if not from
colonisation. At this point I think it is advisable to declare that I am neither in the
'Pleistocene expansion' camp of Birdsell, nor the 'Holocene expansion' camp of
Beaton. What can be concluded about population size from an examination of an
evolutionary process of differentiation is that populations must have been larger than
current estimates. At the very least, they must have been large by separation from
the mainland in the Pleistocene. Any increase afterwards would be invisible to these
techniques.
Anne Ross has stated:
... there are three main types of archaeological data which have been used
by archaeologists around the world to argue the case for population change:
carrying capacity models; changes in artifact counts; and variation in the
number of sites occupied over time [1985:81].
To this should be added models from population biology. The results are not
quantitative, but do have the important quality of being derived independently of
archaeological data and in this manner lend support to hypotheses of population size
and increase in the past.
REFERENCES
Ammerman, A.J. and L.L. Cavalli-Sforza 1984 The Neolithic Transition and the
Genetics of Populations i n Europe. Princeton University Press: New Jersey
Beaton, J.M. 1985 Evidence for a coastal occupation time-lag a t Princess Charlotte
Bay (north Queensland) and implications for coastal colonization and
population growth theories for Aboriginal Australia.
Archaeology in
Oceania 20: 1-20
Birdsell, J .B.
1953 Some environmental and cultural factors influencing the
structuring of Australian Aboriginal populations. American Naturalist
87: 17 1-207
Butlin. N. 1983 Our Original Aggression.
Canberra
Australian National University Press:
Cavalli-Sforza, L.L. 1963 Genetic drift for blood groups. In E . Goldschmidt (ed.)
The Genetics of Migrant and Isdate Populations, pp.34-39. Williams and
Wilkins Co.: New York
Cavalli-Sforza, L.L. 1969 'Genetic
American 22 1:30-37
driftn in an Italian population.
Scientific
Hughes, P.J. and R.J. Lampert 1982 Prehistoric population change in southern
coastal New South Wales. I n S. Bowdler (ed.) Coastal Archaeology i n
Eastern Australia: proceedings of the 1980 Valla conference o n Australian
prehistory, pp. 16-28. Department of Prehistory, Research School of Pacific
Studies, Australian National University: Canberra
Jones, R. 1974 Tasmanian tribes. In N.B. Tindale (ed.) Aboriginal Tribes of
Australia, pp.3 17-54. Australian National University Press: Canberra
Latter, B.D.H. 1980 Genetic differences within and between populations of t h e major
human subgroups. American Naturalist 116:220-37
Lewontin, R.C. 1972 The apportionment of human diversity. Evolutionary Biology
6:381-98
Lourandos, H. 1980 Change or stability?: hydraulics, hunter-gatherers and population
in t,emperate Australia. World Archaeology 11:245-66
Lourarrdos, H. 1983 Intensification: a late Pleistocerie-Holocer~c archaeological
sequence from southwestern Victoria. Archaeology i n Oceania 18:81-94
Pardoe.
C. 1984 Prehistoric human morphological variation in Australia.
Unpublished P h D thesis. Australian National University: Canberra
Pardoe, C. 1986 The application of population genetic models t o the question of
prehistoric Tasmanian biological differentiation. Submitted for publication
Ross, A. 1985 Archaeological evidence for population change in the middle t o late
Holocene in southeastern Australia. -4rchaeology i n Oceania 2O:8 1-89
Rowland, M.J. 1983 Aborigines and environment in Holocene Australia:
paradigms. Australian Aboriginal Studies 2:62-77
Stockton, J. 1982 The prehistoric geography of nort'hwest Tasmania.
PhD thesis. .Australian National University: Canberra
Stockton, J. 1983 The prehistoric population of northwest Tasmania.
Archaeology 17:67-78
Webb,
changing
Unpublished
Australian
S. 1984 Intensification, population and social change in southeastern
Australia: the skeletal evidence. Aboriginal History 8:154-72
Williams, E. 1985 Estimation of prehistoric populations of archaeological sites in
southwestern Victoria: some problems. Archaeology i n Oceania 20:73-80
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