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Consanguinity avoidance and mate choice in Sottunga Finland.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 79235-246 (1989)
Consanguinity Avoidance and Mate Choice in
Sottunga, Finland
ELIZABETH O’BRIEN, L.B. JORDE, BJORN RONNLOF,
JOHAN 0. FELLMAN, AND ALDUR W. ERIKSSON
Department of Human Genetics, University of Utah School of Medicine,
Salt Lake City, Utah 84132 (E.O., L.B.J.);Samfundet Folkhalsans Genetiska
Institut, 00101 Helsinki 10, Finland (B.R.,J.O.F., A. W.E.); Institute of
Human Genetics, Free University, 1007 MC Amsterdam, The
Netherlands (A.W.E.)
KEY WORDS
Inbreeding, Potential mates analysis
ABSTRACT
Potential mates analysis is used to determine some of the
social and demographic characteristics that influence mate choice in a small
island population. Potential mate pools are defined for males in this population;
characteristics such as population size and composition with respect to consanguinity are specified. Determinants of mate choice are examined in light of
mate availability and potential mate pool characteristics for endogamous maters, exogamous maters, nonmaters, and males of various occupations. Random
kinship is assessed from potential mate pools and compared to kinship between
actual mates.
The island community approximated a random mating population from 1700
to 1900 with some evidence for consanguinity avoidance intensifying in the
period 1900-1950. Despite the island’s small population size, kinship coefficients between random mates and actual mates are not high because of relatively high immigration rates. Having considered the contributions of various
factors that influence mate choice, the significance of the island mating structure for genetic variation and the distributions of certain genetic disorders is
discussed.
Regular mating systems have been the
basis for much that falls within the scope of
population genetics theory. To take one obvious example, the “fixation index,” F, was
originally intended a s a measure of the
effects of mating systems on heterozygosis
(Wright, 1921, 1922). Many theoretical treatments of the effects of mating systems on
genetic variability have been developed since
the beginning of the century (Kimura and
Crow, 1963; Robertson, 1964; Wright, 1965).
Less easy to articulate are systems of mating
in natural populations. Human populations
can prove especially elusive in this regard,
lacking strict regularity in mate selection.
However, certain demographic and structural
characteristics of human populations influence mate availability and, therefore, mate
selection. These characteristics, such as the
size of a population, its age and pedigree
structure, the geographic distribution of indi-
@ 1989 ALAN R. LISS. INC.
viduals, social stratification, or migration
patterns, can all impose constraints on mate
choice. These constraints, acting in concert,
produce mating systems that are far more
complex than those studied by theoreticians.
In previous studies we have analyzed
inbreeding and the pedigree structure of
Sottunga in order to understand the factors
underlying the genetic composition of the
population. A high frequency of tapetoretinal
degeneration, a rare autosomal recessive disease, cannot be attributed to high inbreeding
rates (O’Brien et al., 1988a). High frequencies
of both tapetoretinal degeneration and von
Willebrand disease, a n otherwise rare autosoma1 dominant disorder, are more likely
explained by “founder effect,’’ the legacy of
unusual gene frequencies originally estabReceived April 15, 1988 revision accepted August 18, 1988.
236
E. O’BRIEN ET AL.
lished by a small founding group (O’Brien et
al., 1988b). Previous analysis of this population leads to the prediction that any nonrandom mating in Sottunga should be in the
direction of consanguinity avoidance.
Here mate choice in Sottunga is analyzed
in order to examine its effect on genetic
variability in this small, remote island
community in the &and archipelago. Potential mates analysis (PMA) is used to define
Sottunga’s mating structure. Mate choice
within human populations occurs within a
context of social and demographic constraints. The PMA technique considers the
effects of various population characteristics
that influence mate availability, mate choice,
and ultimately the genetic structure of the
population (Dyke, 1971; Leslie, 1985). Of
primary interest to this study are factors that
affect variation in mate choice through time,
such as the size and composition of potential
mate pools. Levels of consanguinity for random pairings of mates compared to actual
mates will show the extent to which mate
selection causes divergence from random
mating in Sottunga. Changes in age at marriage and age differences between spouses
will depict changes in population composition, which might influence mate choice.
Regional migration patterns were known to
have had an impact on genetic variability
among the islands (Mielke et al., 1976; Jorde
et al., 1982); the role of mate selection with
respect to migration will therefore be considered. Social stratification within the
island, which h a s been studied very little,
will be considered for its potential role in
mate choice.
BACKGROUND
Sottunga is one of 16 municipalities (or
Lutheran parishes) of the &and archipelago
located in the Baltic Sea between Sweden
and Finland. The island is one of five very
small municipalities situated in the more
remote eastern reaches of &and. The founding population of the contemporary Rlanders
is considered to be those settlers who reinhabited the islands after the Great Northern
War of 1700-1721. During the war h a n d was
occupied by Russian troops, and a large proportion of the population of more than 10,000
individuals evacuated the islands. Approximately 6,000 individuals, including many of
the previous inhabitants and others from the
Swedish mainland, resettled the islands following the war. (See Mead and Jaatinen,
1975; Mielke et al., 1976; Eriksson, 1980 for
brief summaries of regional history and settlement.) Most Alanders trace their early
origins to the east coast of Sweden. Genetic
distance studies have demonstrated high
affinity between h a n d e r s and Swedes (Jorde
et al., 1982).
Until the present century Alland remained
isolated from all but a small amount of
immigration from Sweden, Finland, and
nearby islands. Estimates suggest that until
about 1900, only 2.5% of all spouses in h a n d
came from outside the population, and parish endogamy was as high as 86% (Workman and Jorde, 1980). Genetic studies have
shown larger genetic distances and greater
drift potential concentrated in the outer
islands (Workman and Jorde, 1980; Jorde et
al., 1982). Three generations of gene frequency data from Alanders showed Sottunga to have the largest average genetic
distance from other parishes owing to unusual frequencies of ABO and Rh alleles
(Eriksson et al., 1973; Carmelli and Jorde,
1982).
The turn of this century marks the beginning of the breakdown of Aland’s insularity
(Mielke et al., 1976). The effects of this process were demonstrated in a previous study
of inbreeding in Sottunga (O’Brien et al.,
1988a) where it was shown that, after 200
years of gradual increase, inbreeding declined
precipitously after 1900. However, increased
migration between islands and with outside
populations h a s not been a uniform trend
throughout Aland since 1900. Sottunga’s
population size, which h a s always been
small (300-400), h a s declined in this century
because of emigration (Jorde et al., 1982;
Mielke et al., 1976; O’Brien et al., 1988a).
MATERIALS AND METHODS
According to the Swedish ecclesiastical
law of 1686, parish ministers were required to
record all births, marriages, and deaths
among parishioners. The parish registries
contain vital data that are quite complete for
individuals born in &and since the early
1700s (Mielke et al., 1976,1987).This study is
based upon genealogies reconstructed by one
of us (B.R.) from information contained in
Sottunga’s parish registry. The genealogies
currently consist of 3,292 individuals and
over 800 nuclear families. The pedigree information spans three centuries and up to 15
generations for some individuals. Over onehalf of those included in our records (1,860
CONSANGUINITY AND MATE CHOICE
individuals) were born in Sottunga between
1690 and 1986.
This study is based on the first marriages
of 573 males from Sottunga’s genealogies.
The age distribution of males and females at
the time of marriage and the distribution of
age differences between spouses were evaluated for these marriages.
Potential mate pools were constructed using
the following criteria for mate selection. All
potential mate analyses include only individuals from the data base who 1) have a
known birth year and 2) lived 24 or more
years (the cutoff for the lower quartile of the
distribution of age at marriage). The potential mate pool for a given male was first
drawn from all females in the population 520
years his age, excluding sibs, mother, grandmothers, daughters, aunts, nieces, and first
cousins; 99.8%of actual marriages fall within
this interval of age difference a t marriage.
first-cousin marriages were prohibited without dispensation until 1872 (Norio et al., 1973)
and were, therefore, excluded. (Certain other
types of marriages, such as a male marrying
his brother’s widow, also required dispensation but these implicate second marriages
which were excluded from this analysis.)
Potential mate pools were established for
each male in the genealogy. Mate pools were
redrawn for Sottunga-born males paired
with Sottunga-born females using the 520
year age difference criterion and excluding
the same relatives. Coefficients of kinship
were estimated for each 1) male in the
genealogy paired with his potential mates,
2) Sottunga-bornmale paired with Sottungaborn potential mates, and 3) pair of actual
mates.
A second set of kinship coefficients was
calculated for the Sottunga-born sample
weighting each coefficient by the proportional amount of time t h a t a given male and
female were actually available to one another a s potential mates. An individual was
considered available to another as a mate
from age 16 (youngest age at marriage) until
marriage or death, whichever came first.
Exposure was calculated as the proportion
of the male’s period of availability during
which a given female was also available as
a mate. This proportion h a s in the denominator the male years from age 16 to marriage or death, whichever came first, and in
the numerator the number of female years
from age 16 to marriage or death t h a t overlap the male years. If both individuals had
237
missing marriage dates and death dates,
including those who did not marry and had
not died, then age 50 was assigned the upper
age limit of exposure. The calculated exposures, expressed as proportions, were then
multiplied by a pair’s coefficient of kinship.
Thus, kinship coefficients between individuals who did not overlap i n their years of
availability were given weights of “0.” Those
who overlapped completely on their dates of
availability were given weights of “1,” and
all others were given weights between 0 and
1.Mean kinship coefficients for males paired
with their potential mates were standardized by dividing the sum of the weighted
coefficients by the mean weight (Leslie
1985).
Individuals of a cohort spanning a n interval of time do not experience life history
events simultaneously; one’s potential mate
pool changes constantly as members of the
cohort enter and leave potential mate status
(Leslie, 1983a). Weighting kinship coefficients in the fashion described above incorporates the probability of choosing for a
mate each female who is selected as a potential mate by simple age restriction. This
weighting scheme, therefore, more accurately represents a male’s average kinship
with his pool of potential mates. Furthermore, consanguineous individuals are agecorrelated (Barrai et al., 1962; Hajnal, 1963;
Cavalli-Sforza eta]., 1966; Leslie, 1983b), so
that without weighting, consanguineous relationships might bias kinship with potential
mates.
Average kinship and average size of potential mate pools were determined for males
divided into various marriage and migration categories. For each 50-year birth cohort
in the interval 1700-1950, potential mate
pools were defined for males who were born
in Sottunga a n d 1)married in Sottunga, 2)
never married, 3) emigrated at marriage.
In a previous study it was shown that
farmers had larger families, had more descendants through time, and made larger
contributions to Sottunga’s gene pool t h a n
did nonfarmers (O’Brien et al., 198813).Given
t h a t farmers might have had distinct pedigree affiliations, and perhaps social stature,
Sottunga’s males were divided into two
groups by the broad occupational distinction, farmers vs. nonfarmers. Although individuals might have had more than one
occupation in their lifetime, classifications
were based on activities of greatest duration
238
E. O’BRIEN ET AL.
and economic consequence. Potential mate
pools were established for the two groups,
and differences between them were evaluated.
The average number of first through third
cousins, including half-cousins, were calculated for potential mate pools. The number
of actual cousin matings at each level of
relationship was compared to the number of
cousin matings expected under random mating using a chi-square goodness-of-fit test.
Potential mates were selected again using a
reduced age difference criterion of H O years,
and new distributions of observed and expected cousin matings were generated.
Ninety percent of actual marriages were
between mates meeting this age difference
criterion. The two expected distributions of
cousin matings demonstrate the amount of
age correlation between cousins (Leslie,
1983b). The expected proportions of cousin
matings were not adjusted to account for
variation in the amount of time t h a t pairs
were available to one another as mates.
RESULTS
The distributions of age at marriage and
age differences between spouses are given in
Figures 1and 2, respectively. The mean ages
a t marriage are 28.5 years for males and 26.3
years for females. The youngest person married was 16, and the oldest was 97.
Temporal trends in the size of potential
mate pools and average kinship with poten-
T
250
I
150
Frequency
”
“;I1 1
*
\
*
- Females
0
<20
tial mates are shown in Figures 3 and 4. In
Figure 3 the number of potential mates is
shown to increase continuously until 1900
and decline thereafter. This trend is a direct
result of changes in Sottunga’s population
size. Birth cohort sizes grew until 1850 and
declined after 1900. The greater number of
potential mates shown for the entire genealogy (“All”) as opposed to island-born males
demonstrates the considerable number of
migrants in the genealogy.
In Figure 4 average kinship between Sottunga-born males and their potential mates
is shown to increase from 1700 to 1900 and
decline somewhat thereafter. The general
increase in kinship until 1900 reflects population growth with better ascertainment of
ancestors as pedigrees gained depth. The
average coefficients for those born in Sottunga, unweighted and weighted, using a 10or 20-year selection criterion, are very similar. The average values for Sottunga-born
males and their spouses (nonrandom kinship) are notably lower than for island-born
males and their potential mates (random
kinship). The difference between these two
components of kinship demonstrates evidence
of consanguinity avoidance.
Mean values for the number of potential
mates for Sottunga-bornmales through time
are shown in Figure 5. The labels “20 yrs.”
and “10 yrs.” refer to the age difference
criteria used to select potential mates. The
label “all” refers to all females who met the
25
30
35
40
45
+ : ;
X
”
50
55
Age
at Marriage
60
65
70
75
80
x
85
F
90
Fig. 1. Distribution of ages at mamage for males and females.
%
95
-*
100
239
CONSANGUINITY AND MATE CHOICE
140
1
Frequency
<-20
-16
-12
-8
-4
0
4
8
12
>20
16
Age D i f f e r e n c e
Fig. 2. Distribution of age difference (male age - female
age) between males and females at marriage.
who never married, and who emigrated at
marriage are shown in Figure 6. These
values represent no significant differences
among groups except in the most recent time
period when kinship with potential mates
for those who never married is lower than
for endogamous maters and emigrants. Kinship values for island-born males and their
spouses are shown for comparison. Kinship
between mates is lower at all times t h a n
average kinship with potential mates. This
again suggests persistent consanguinity
age criterion. “Non-zero” refers to those who
were available as mates, i.e., those with nonzero weights according to the weighting
scheme described above. Substantial differences in the size of potential mate pools occurs with changes in selection criteria. The
number of available females within 10 years
of a given male’s age is less than one-half
the number of all females within 20 years of
his age.
Average kinship with potential mates for
males born in Sottunga who married there,
~otentia~
Mates
1
,50
*0°‘
/i
x
*
*
Sottunga-
1
0
I-.+
1700
---
1750
1800
1850
1900
Cohort
Fig. 3. Average number of potential mates for males of
each birth cohort. Potential mates are females aged f20
years of male age.
240
E. O’BRIEN ET AL
0.014
0.012
--
20 yrs.
unneighted
0.010
+ 20
yrs.
weighted
0.008
0.006
x 10 yrs.
weignted
0.004
0 Mates
Kinship
0.002
/
_ ~ _ _
0.000 I
1700
----I
1750
1850
1800
1900
Cohort
Fig. 4. Average kinship coefficients with potential
mates for males of each cohort and for three selection
criteria. Values are for Sottunga-born males paired with
potential mates and actual mates.
avoidance. The large discrepancy between
kinship with potential and actual mates
suggests that consanguinity avoidance was
strongest in the 1900-1950 period. The decline in kinship with potential mates for
nonmaters during this period suggests that
these individuals are less connected in a
pedigree sense to the rest of the population.
Males of all three marriage statuses born
in Sottunga are similar in terms of the size
of their potential mate pools. Variation in
the number of sibs and first cousins is the
only factor that might have caused differ-
ences among the three groups. Any group
having more sibs and/or first cousins would
have comparatively smaller mate pools,
while contributing more mates to others’
pools.
Farmers as a group remained very stable
in number through time in Sottunga because
the mandated number of farms changed
only once during the island’s history. The
nonfarmer group followed the island‘s general pattern of growth and decline. A previous study showed that farmers had higher
mean family sizes, more descendants, and
180
1201
,A.
,
,
\
\
140
, , , f i
100
Po t e n t l a 1
- - - All
20 yrs.
Mates
*
80
’
,
^^
0
1700
*
i
-
*
x Nun-zero
10 y r s .
I
t
1800
1750
Non-zero
20 yrs.
i+
1850
1900
Cohort
Fig. 5. Average number of potential mates for “all”
males in the genealogy, and for Sottunga-bornmales fol-
lowing 10- and 20-year selection criteria with weighting
are shown.
241
CONSANGUINITY AND MATE CHOICE
o'016
T
0.014
0.012
0.010
Avg
Kinshio
- Stga-married
0.008
x Unmarried
0.006
+- Emigrated
0.004
0.002
0.000
LI
1700
1750
1850
1800
I
1900
Cohort
Fig. 6. Average kinship with potential mates for
Sottunga-born males who married in Sottunga, who never
mamed, and who emigrated a t marriage are plotted for
each birth cohort. Kinship values are unweighted, and the
age criterion is f20 years. Average kinship between actual
mates is shown for comparison.
made larger genetic contributions to the
population on average than nonfarmers
(O'Brien et al., 1988b). I n addition, it was
shown that farmers were the sons and
grandsons of farmers more often than expected by random assignment. The higher
average kinship values among farmers compared to nonfarmers (1750-1950) demonstrated in Figure 7 reflects the bias in favor
of lineal pairs (father-son and father-grandson) in the farmer category.
Farmers and nonfarmers show no significant differences in the size of their potential
mate pools or in average kinship with them
(ANOVA results are not illustrated). No
consistent trends through time suggest a
distinction between farmers and nonfarmers
with respect to potential mates. Farmers do
show slightly higher kinship with actual
mates through time than do nonfarmers,
but the difference is not significant.
The average kinship coefficients reported
0'06
0.05
I\\\
Avg .
Kinshio
1700
1750
1 BOO
1850
1900
Cohort
Fig. 7. Average kinship between farmers (F/J?), between farmers and nonfarmers (F/NF), and between non-
farmers (NF/NF) are plotted for each birth cohort. Only
males born in Sottunga are included.
242
E. O’BRIEN ET AL.
in Figures 4, 6, and 7 reflect a substantial
contribution from remote consanguinity.
Since mating couples would not usually be
aware of consanguinity at a remote level,
consanguinity avoidance was further evaluated in terms of close cousin (first through
third) relationships. Table 1reports the proportion of potential mates, together with the
observed and expected number of matings,
for each cousin relationship. The results are
subdivided by birth cohort and are given for
the 20-year age-restricted potential mate
sample. First through third cousin categories are designated in the column headings;
“other” consists of potential mates less
related than the third cousin level. “N” is
the number of marriages for the cohort. The
chi-square goodness-of-fit test was used to
evaluate the differences between observed
and expected numbers of cousin matings.
The chi-square values are statistically significant (P 5 .05) only for the 1900-1950
cohort. Combining all categories of cousin
marriage also shows no significant difference between the observed and expected
number of cousin marriages over the five
cohorts (x2 = 4.28, df = 4) in Table 1. However, the trend toward avoidance of cousin
matings through time is clear.
The same test was applied to observed
and expected cousin matings using the
smaller 10-year age criterion for potential
TABLE 1. Chi-square test for differences between
observed and expected numbers of cousin matings by
degree of relationship (first through third cousins)’
Cohort
N
1
2
1%
70 ,025 ,014 ,030
0
0
2
1.77 1.00 2.09
98 ,024 ,018 ,041
1
2
4
2.42 1.82 4.03
18004
135 .021 ,018 .038
Observed
0
1
7
Expected
2.80 2.43 5.16
18505
138 ,028 ,026 ,052
Observed
0
5
8
Expected
3.84 3.65 7.29
19006
132 ,017 ,016 ,048
Observed
1
3
0
2.32 2.17 6.34
Expected
17002
Observed
Expected
17503
Observed
Expected
2%
,012
0
86
,033
1
3.29
,035
3
4.79
,047
6
6.51
.044
3
5.83
3
Other
,007
,904
0
68
.53 63.75
,049
,828
3
87
4.82 81.62
.083
,801
8
116
11.20 108.61
,071
,770
6
113
9.86 106.83
,074
.796
4
121
9.88 105.45
~
‘The proportions are average proportions of potentid mates in
each cousin category for the cohort. The age criterion for potential
mate selection is i 2 0 years.
‘x2 = 1.61. P 5 .51.
3x2 = 2.09. 5 .64.
4x2 = 4.02. P 5 28.
= 4.56. P 5 29.
6x2 = 8.86. P 5 .04.
5iL
mate selection. These results are reported in
Table 2 where the same general pattern of
consanguinity avoidance in nearly every
cousin category through time, significant
only in the recent period, is repeated.
DISCUSSION
Figure 5 demonstrated how changes in the
inclusion criteria for potential mate selection
caused large reductions in the size of potential mate pools. Under the 20-year age criterion, weighting the coefficients by availability
caused a 35% reduction in the size of potential
mate pools. Restricting the age selection criterion more, to +lo years, decreased the size
of the potential mate pools by >55% (except
in the 1850-1900 cohort where the reduction
is 53%) compared to the unweighted 20-year
sample. Despite the large reductions in the
number of mates caused by varying the age
criterion, the average kinship values change
very little. This is because remote consanguinity, which is not strongly affected by
changes in age restriction, contributes much
more to total kinship in this population than
does close consanguinity (O’Brien et al.,
1988a). In the island population of St. Thomas in the Virgin Islands, larger than Sottunga, yet a small isolate, similarly large
reductions in mate pool size resulted from differences in selection criteria (Dyke, 1971).
Again, despite these differences in mate pool
size, the general effect on kinship with potential mates was minimal.
TABLE 2. Chi-square test for differences between
observed and expected numbers of cousin matings by
degree of relationship (first through third cousins)‘
Cohort
17002
Observed
Expected
17503
Observed
Expected
18004
Observed
Expected
18505
Observed
Expected
19006
Observed
Expected
N
1 ___
1% __
2 __
2% __
3 ___
Other
70 .033 .007 ,030 .011 .009
.908
0
0
0
2
0
68
2.32 .54 2.10 .so
.65 63.59
98 ,029 ,012 ,050 ,028 ,053
331
1
2
4
1
3
87
2.87 1.19 4.57 2.74 5.16 81.47
135 ,027 ,012 ,046 ,028 ,091
,797
0
1
8
7
3
116
3.61 1.58 6.22 3.79 12.23 107.57
138 ,034 .021 ,060 ,039 ,079
.765
0
5
8
6
6
113
4.76 2.94 8.36 5.38 10.93 105.63
132 ,020 ,012 .053 .032 ,087
,796
1
4
0
3
3
121
11.48 105.04
4.26 __
2.68 1.55 6.98 -
‘The age criterion for potential mate selection is +lo years
2x2 = 1.89. P 5 .46.
3x2 = 2.14. P 5 .52.
4x2 = 4.44. P 5 29.
5x2 = 6.89. P 5 .1L
6x2 = 9.62. P 5 .03.
CONSANGUINITY AND MATE CHOICE
The redistribution of cousin proportions
caused by changing Sottunga’s age criterion for potential mates from +20 years to
+ l o years (Tables 1, 2) is as expected (Hajnal, 1963): cousins in even “steps” (first,
second, and third) increase, while cousins of
“half-step” decrease. The difference in the
distribution of cousin relationships, however, is very small given that the age interval was halved. Leslie (1983a,b) has shown
theoretically and empirically the conditions
under which age restrictions on potential
mates cause negligible differences in consanguinity among married individuals.
When pedigrees are deep, the age correlation
between relatives declines a s relationships
become more distant. Furthermore, in a
growing population as Sottunga was until
this century, a n individual accumulates a
larger number of potential mates among
distant relatives through time t h a n among
relatives of close consanguineous relationship (Leslie, 1983b). This is because close
relatives are the offspring of a smaller range
of ancestors removed by a limited range of
generations from one’s own.
Temporal changes in mate choice patterns in Sottunga occur in the last two birth
cohorts, 1850-1900 and 1900-1950. Changes
in the demographic characteristics of the
population in these two time periods help to
explain the differences in mate choice patterns. I n 1850-1900 average kinship coefficients between actual mates and between
potential mates reached their maximum
values, as did the number of potential mates
for island-born males. These trends can be
attributed to gains in pedigree depth and
population growth, respectively. Emigration
among males a t marriage declined somewhat, and female immigration was down to
9.5% from 15.5%in the previous cohort.
I n the 1900-1950 cohort, kinship with
potential mates leveled off, and kinship
between actual mates fell to its lowest level
since 1750. Sottunga’s effective size dropped
in 1900-1950 because emigration rose to
>16%, immigration fell almost to lo%, and
the proportion of island-born males who
never married reached its maximum of
>45%. The decline in population size is also
reflected by reduced numbers of potential
mates and cousins. Given Sottunga’s reduced size, one might have expected kinship
between mates to increase as a result of
higher random kinship, but this did not
occur.
243
Similar changes in kinship between actual
and potential mates accompanied the breakup
of isolation beginning at the turn of this
century in the Sanday population of the
Orkney Islands (Brennan, 1981; Brennan
and Boyce, 1980). I n both Sanday and Sottunga lower levels of consanguinity between
mates reflects the changed character of
potential mate pools as migration patterns,
exposure to off-island individuals, and perhaps attitudes about marriage preferences
changed.
Other studies have shown differences in
kinship with potential mates among endogamous maters, exogamous maters, and
nonmaters. I n the Virgin Island populations of St. Bart and St. Thomas, those who
emigrated or never married were shown to
be more closely related to their available
mates than were endogamous maters (Dyke,
1971; Leslie, 1980; Leslie et al., 1981). In Sottunga differences in mate availability are not
found among exogamous, endogamous, and
nonmater groups. Sottunga-born males who
emigrated at marriage are not more closely
related to their potential mates than others,
even if groups are compared in terms of
weighted kinship values. Nonmaters are
slightly less related to their potential mates
than endogamous or exogamous maters and
show a slight lack of close cousins among
potential mates.
Sottunga-born males who never married
possess a distinguishing characteristic t h a t
might account for their marital status. Sixtythree percent of nonmaters have no recorded
occupation. (Nonmaters account for 73% of
all males without occupations.) Given t h a t
these individuals were born in Sottunga and
have pedigree information no less complete
than males with recorded occupations, their
lack does not appear to signify mere gaps in
the data. Furthermore, 30% of those lacking
a n occupation were the offspring of fathers
born elsewhere as opposed to 12% of those
with occupations. These data suggest t h a t
individuals with limited economic prospects
are at a mating disadvantage. Although a n
individual who lacks a recorded occupation
does not necessarily lack a means of subsistence, the implication is one of a marginal economic situation aggravated by “outsider” status.
It h a s been remarked that farmers (i.e.,
landowners) constituted an “upper class” in
h a n d (Eriksson, 1980), but mate choice does
not show marked variation according to this
244
E. O’BRIEN ET AL.
occupational distinction. Farmers are slightly
more closely related to their mates than are
nonfarmers and somewhat more closely related among themselves than nonfarmers.
These patterns might reflect some effect of
land inheritance on mating structure; alternatively, farmers’ pedigrees tend to have
greater depth owing to the fact that the
farming occupation is older than any other
on the island. As a class, farmers are not
significantly different from nonfarmers in
mate availability or choice.
Because Sottunga’s population size was
never much larger than approximately 300
individuals, one might have expected potential mate pools to be small, contain large
proportions of related individuals, and give
relatively high kinship coefficients. However, kinship with potential mates in Sottunga in this century (<.012) is not much
higher t h a n in the larger (661) Northside,
Virgin Islands population (.007) for the
period 1890 to 1966 (Dyke, 1971). Furthermore, it is lower in Sottunga than in the St.
Bart population, which is larger still (2,000).
St. Bart contains 12 geographic subdivisions (Leslie, 1980, 1983b), each of which is
roughly the size of Sottunga. The average
kinship coefficient for random pairs within
subdivisions was .026 for the period 19451969.
Leslie (1983a) h a s shown that, in a growing isolated population, the frequency of
remote consanguineous matings increases
a s proportions of potential mates in that
category increase. Under these conditions,
and with close consanguinity avoidance,
remote consanguinity becomes the much
larger component of random kinship and
has a greater effect t h a n close consanguinity on inbreeding levels. This effect has been
shown in the Northside and St. Bart populations (Leslie et al., 1981), as well a s in Sottunga (O’Brien et al., 1988a).
Migration, however, limits the effects of
remote consanguinity (Leslie, 1983a). Kinship between spouses was lower in Sottunga
in all time periods than kinship with potential mates for island-born individuals. I n
part this was due to close consanguinity
avoidance and in part to marriage with
individuals from outside Sottunga. The effects of migration were also demonstrated in
a previous study, which showed a striking
build-up of remote consanguinity within
particular pedigrees, yet the proportion of
such pedigrees diminished rapidly in this
century as migration patterns changed
(O’Brien et al., 1988a).
Finally, the effects of Sottunga’s pedigree
structure a n d mate choice patterns are considered for their effects on homozygosity
levels in the population. Random mating is,
of course, expected to produce genotypes in
Hardy-Weinberg proportions among offspring. Departures from this expectation can
occur in either direction, depending upon
such factors as population structure, size,
and mating preferences (Workman, 1969).
I n the small island population of Tristan da
Cunha, for example, incest avoidance produced a n excess of heterozygotes (nonsignificant) at some loci despite a comparatively
high inbreeding rate (.040) (Thompson and
Roberts, 1980; Jenkins et al., 1985). Consanguinity avoidance slowed the rate of increasing homozygosity resulting from the accumulation of random inbreeding in the Northside population (Leslie et al., 1978). In Sottunga, genotype frequencies for six codominant systems show no departure from
Hardy-Weinberg proportions. This result is
not inconsistent with the opposing effects of
1) patterns of mate choice including close
consanguinity avoidance and marriage with
individuals from outside the population and
2) the build-up of remote consanguinity over
many generations in a small island community.
Two cautionary notes concerning the data
used in this analysis should be considered.
First, these estimates of kinship, like all
such estimates from genealogical data, are
somewhat undervalued. Pedigree information cannot be complete in a n absolute sense
for a given individual or to the same extent
for all individuals. High migration rates in
particular cause gaps in pedigree information. For this reason, the bulk of this analysis was restricted to individuals born in Sottunga who, it appears, have pedigree information of equal quality. Second, Sottunga is
a small island of approximately 16 km2 and
might therefore constitute a homogeneous
population with respect to mate choice.
Nonetheless, geographic characteristics important to group structure might have been
important for mate choice but are unmeasurable with the data at hand.
CONCLUSIONS
An analysis of mate choice in Sottunga demonstrates consanguinity avoidance through-
CONSANGUINITY AND MATE CHOICE
out the period of analysis, 1700-1950. A
nearly consistent lack of close cousin (first
through third) marriages compared to the
number expected under random mating was
observed in all time periods, although the
discrepancy is not significant until the most
recent period, 1900-1950. (Incestuous matings were not considered in this analysis;
these were certainly avoided and would have
elevated the avoidance effect for relationships of close consanguinity.) Mates chosen
from outside Sottunga contributed to both
close and remote consanguinity avoidance.
These marriage trends in Sottunga produce
consistently higher random kinship compared
to nonrandom kinship through time.
Occupational data for this population
proved useful to characterize further the factors that influenced mate choice on the
island. Sottunga’s emigrants and nonmaters
do not appear distinct in the number of
potential mates available to them nor in their
relatedness to potential mates. However, evidence for economic status a s a condition of
marriageability is apparent from the very
high prevalence (73%) of nonmaters among
those with no designated occupation. The
marginal economic status of these individuals is also associated with a n immigrant
background. Whereas landholders in Sottunga constitute a group slightly more related
among themselves and slightly more related
to their spouses than nonfarmers, their potential mate pools and mate choices are not characteristically distinct.
This study supports results from a previous
investigation of inbreeding, where it was determined that the random and nonrandom
components of inbreeding together did not
account for the high frequencies of recessive
genetic diseases in this population (O’Brien
et al., 1988a). Although the population remained very small through time, comparatively high migration rates and population
growth into the late 1800s limited the potentially more dramatic effects of the build-up of
consanguinity that might have developed
were the island more isolated. Here we show
evidence for persistent consanguinity avoidance, which should increase heterozygosity
above that expected by purely random mating in a closed population. Nevertheless, the
number of heterozygotes in Sottunga does
not diverge significantly from that expected
under random mating. This is not surprising
in that goodness-of-fit tests for such differences are insensitive, particularly when in-
245
breeding levels are low and opposing forces
have influenced genotypic proportions.
ACKNOWLEDGMENTS
This research was supported by NSF grant
BNS-8319448 and by grants from the Sigrid
Jusblius Foundation, Helsinki. We thank
Kari Pitkanen, Sarah Williams-Blangero,
James Mielke, Margaret Gradie, and two
anonymous reviewers for their helpful suggestions during the preparation of this manuscript.
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