A.M. ZOOLOGIST, 10:53-06 (1970).
Behavior and Genetic Variation in Natural Populations
ROBERT K. SELANDER
Department of Zoology, University of Texas,
Austin, Texas 78712
SYNOPSIS. An analysis of allelic variation at genetic loci controlling several esterases and
hemoglobin, as demonstrated by electrophoresis. indicates that wild populations of the house
mouse (Mus musculus) are characterized by fine-scale genetic subdivision, which, through the
territorial behavior of family groups (tribes), is achieved even in the absence of physical or
ecological barriers to migration.
Heterogeneity in allele frequencies among samples from farms in the same region and from
barns on the same farm was demonstrated. Spatial variation in allele frequencies within single
barns, involving a clustering of like genotypes, was shown by grid-trapping, thus providing
direct evidence of tribal subdivision in continuously distributed populations.
For two loci, Es-3 and Hbb, an excess of heterozygotes appeared in samples from small
populations, while a deficit characterized samples from large populations.
The evolutionary significance of subdivision and consequent drift in house mouse populations
cannot properly be evaluated at this time. Although stochastic processes may play the dominant
role in determining, at a given locus, the genotypes of individuals and frequencies of alleles in
small populations, geographic patterns of variation, as studied in Texas, are characterized by
uniformity of allelic frequency in major physiographic or climatic regions, as would be expected
if selection is determining the frequencies.
Unlike hypothetical panmictic populations of infinite size in which gametic combination occurs at random, natural populations are subject to disturbances of random mating relations through subdivision
and assortative mating, sampling fluctuations in gene frequencies, and the division
of the total population range into ecologically diverse subranges with unique selective effects and limited interrange migration (Yasuda and Morton, 1967).
While the major genetic consequences of
deviations from panmixia have long been
understood on a theoretical level as a result of the mathematical formulations of
R. A. Fisher, S. Wright, and others, until
This research was supported by XIH grant GM15769 and NSF grant CB-6662.
Important contributions to this investigation
were made by S. Y. Yang, who supervised the
laboratory work; by W. G. Hunt, who supervised
the grid trapping at Hildreth Farm and participated in other phases of field work in Texas; and
by S. Stewart, who is responsible for the computer
programs used in analyzing the data. Several of my
colleagues, including R. C. Lewontin, R. D. Milkman, and R. H. Richardson, contributed to this research through critical discussion, and R. Ralin
provided effective assistance in preparing the
manuscript.
I am indebted to R. Hildrelh for his interest in
my work and permission to trap on his farm.
very recently the tools necessary for the
empirical analysis of the genetic effects of
population structure have not been available. Although population geneticists,
ecologists, and, more recently, ethologists
and behavior-geneticists (King, 1967),
have emphasized the potential significance
of behavior in relation to migration, dispersion, and mating systems, the "ecological genetics of behavior," as King (1967)
has designated the study of the relationships between behavior and changes in
gene frequency, can at present be discussed
only in generalities. It is fair to say that
we know almost nothing concerning the
actual importance of populational structure in the evolution of natural populations.
The major objective of my contribution
to this symposium is to examine certain
aspects of the genetic structure of wild
populations of the house mouse (Mus
musculus), especially as they are affected
by the social system of the species, and, in
particular, the behavioral mechanism of
territoriality. Employing the techniques of
gel electrophoresis and selective histochemical staining to demonstrate allelic
53
54
ROBERT K. SELANDER
variation at a number of genetic loci controlling the structure of hemoglobin and
certain enzymatic proteins, it has been possible to show that natural populations of
this species are characterized by extremely fine-scale genetic subdivision, which,
through territorial behavior, is achieved
even in the absence of physical or ecological bai"riers to migration. But despite
marked heterogeneity in allele frequencies
resulting from subdivision at the level of
the local population, geographic patterns
of frequencies are clinal, thus providing
strong circumstantial evidence that selection is the dominant determining factor at
the regional level of populational structure.
I hope this work will contribute to a
better understanding of the interaction of
genetics and behavior in natural populations, and, at the same time, suggest other
potentially beneficial effects of interaction
between these two disciplines.
FIG. ]. Sample localities ami regions for house
mice collected in Texas.
omorphic and polymorphic erythrocytic
antigens in the English human population.
Extrapolating results of these surveys of a
relatively small number of loci to the entire genomes of the organisms, it is obvious
BACKGROUND AND METHODS
that, as Lewontin and Hubby (1966)
The present analysis of genetic subdivi- have emphasized, natural populations possion is part of a comprehensive investiga- sess almost incredible amounts of genetic
tion of protein polymorphism in the house heterozygosity. Elucidating the selective
mouse that has been in progress in my and other bases for maintaining this abunlaboratory since 1967. Other work includes dance of polymorphism constitutes a major
a survey of electrophoretic variation in 35 task of contemporary population genetics
enzymes and non-enzymatic proteins con- (Lewontin, 1967fr) .
trolled by 40 loci in a population occupyThe data bearing on subdivision in poping 10 barns at Hallowell Farm, Ramona, ulations of house mice reported in the
California (Selander and Yang, 1969b; present paper pertain largely to polymorsee also Selander, Hunt, and Yang, 1969). phism at three loci, Esterase 2 (Es-2), EsThis study, which was undertaken for terase 3 (Es-3), and hemoglobin (Hbb),
the dual purpose of detecting poly- as demonstrated by starch-gel electrophoremorphic proteins useful in research on sis of blood plasma and hemolysate from
population genetics and assessing the ex- mice collected in Texas and other parts of
tent of genie polymorphism in the species, North America. For each locus, phenodemonstrated that 30% of the loci are types have been determined for some 10,segregating for two or more alleles and 000 mice collected in over 300 barns or
that the average individual is heterozygous fields. Collecting localities in Texas,
at 11% of the 40 loci. These proportions grouped into sample regions for the purare remarkably similar to those obtained pose of analyzing geographic variation, are
in a survey of proteins controlled by 18 shown in Figure 1. Methods of preparing
loci in the fruit fly, Drosophila pseudo- tissue, electrophoresis, and staining, toobscura, by Lewontin and Hubby (1966) gether with evidence of the genetic deterand to those derived by Lewon tin's mination of the polymorphisms and com(1967rt) analysis of the proportion of mon- plete data on the phenotypic proportions
BEHAVIOR AND GENETICS OF MILS
55
TABLE 1. Allelic variation at three biochemical loci in the house mouse
Allele
Phenotype
Es-2b
Es-2C
Es-2d
Esterasc
Band absent or very faint, with mobility
similar to Es-2"
Fast-migrating band
Band with medium-mobility
Slow-migrating band
Es-2'
Band migrating faster than Es-2b
Es-2'
Band with mobility intermediate between Es-2C and Es-2d
Es-2"
Es-3"
Es-3b
Es-3"
Es-3d
Es-3'
Hbb'
Hbbd
Hbb'
Occurrence
2 (Plasma)
Wild populations
Wild populations; inbred strains
Wild populations
Wild populations in Rio Grande Valley region, Texas,
and in Ohio
Wild population in a single barn at Mayhard Farm,
near Dallas, Texas
Wild populations in Denmark
Esterase 3 (Hemolysate)
Faint band with mobility intermediate
C57BL, BALB/cJ, and some other inbred strains; unbetween Es-3b and Es-3"
recorded in wild populations
Band with mobility intermediate beWild populations and RF, YBR/He, and RFM/Un
tween Es-B" and Es-3d
inbred strains
Slow-migrating band
Wild populations and most inbred strains
Fast-migrating band
Wild populations in Rio Grande Valley, Illinois, California, and Hawaii
Band absent ("silent" allele)
Fiji Island
Hemoglobin {Hbb locus)
Wild populations and C57BL and some other inbred
strains
Fast- and slow-migrating bands
Wild populations and most inbred strains
Similar to Hbbd
Au/Ss inbred strain
Single fast-migrating band
and allele frequencies in the samples, are
presented elsewhere (Selander and Yang,
1969a; Selander, Yang, and Hunt, 1969).
The known alleles at the three loci are
listed in Table 1. Variation in these polymorphic characters is independent of sex
and age (beyond the weanling stage), and
the loci are unlinked. The special value of
these characters for population genetics is
that, in the absence of demonstrable environmental components of variance, the
phenotypes may, in most cases, be translated directly to genotypes and the gene
frequencies of samples determined by direct count of electrophoretic bands of differing mobilities. The house mouse is especially well suited for work on the genetics
of natural populations because wild-caught
individuals breed readily in laboratory
cages and extensive work on the domesticated strains has made the species better
known genetically than any other vertebrate.
SUBDIVISION OF POPULATIONS
Ecologic
G
and Behavioral
Evidence
Theoretical considerations developed by
Lewontin and Dunn (1960) in connection with studies of frequencies of the
«-alleles, together with a number of behav"oral and ecological studies of individuals
in enclosures and under natural conditions, suggest that wild populations are
subdivided into units (tribes, family
groups, or endogamous households) with
an effective breeding size well below that
at which random fluctuations and inbreeding play a major role in determining gene frequencies (see Petras, 1967a, for
a recent review of the literature). Moreover, we have evidence from several studies
(Crowcroft and Rowe, 1963; Anderson
and Hill, 1965; Reimer and Petras, 1967)
that subdivision is achieved in part by territorial behavior of tribal members, especially males, which severely limits mi-
56
ROBERT K. SELANDER
gration among tribes. The general picture
of social organization emerging from the
various studies, especially those of Reimer
and Petras (1967, 1968), is as follows.
Populations of house mice inhabiting
barns, sheds, corn ricks, or other structures
in which food and cover are available for
relatively long periods, become divided
into tribes composed of a dominant
male, several breeding females (including
daughters of the dominant male), and
several subordinate males that presumably
do not contribute to the gene pool. Estimates of the effective breeding size of these
groups range from 5 to 80 individuals
(Petras, 1967a, 19676), but the best estimates place the number at 10 or less. The
formation of these units is due primarily to
male territoriality, but females also contribute to the defense of the tribal territory. Where food and cover are continuously
available, the tribes are stable over long
periods (at least several generations), even
when densities are high, and, in older
tribes, the dominant male is replaced by
one of his offspring. Intertribal migration
is rare and involves only females, there
being effectively no movement of males
between tribes. While neither the actual
sizes of these tribal demes nor the areal
extent of their distributions is adequately
known, there is reason to believe that they
may be very small. One line of evidence
comes from measurements of the distances
moved by marked individuals within
rooms or barns and the frequency of movement between barns or corn ricks. For
example, Young, Strecker, and Emlen
(1950), studying two populations in a
room 170 by 100 feet, found that the average distance moved by a mouse between
trappings was 12 feet. Reimer and Petras
(1968) studied a barn in which the average distance between trappings was 18.6 feet
and a corn crib in which the distance was
only 2.9 feet. The estimated home range
of the mouse in corn ricks in England is
50 square feet on a single level, with little
vertical movement (Southern and Laurie,
1946). Surveying the ecological literature,
Petras (1967a, p. 269) concluded that the
best estimate of physical migration between barns or other buildings is 5%, but
the level of genetically effective migration
is probably much lower.
Territoriality as a factor producing an
insular pattern of populational subdivision
has received relatively little attention
from population geneticists, who generally
think of insularity in terms of pockets of
habitat separated by areas of unsuitable
habitat. But the genetic consequences are
the same, whether migration is restricted
by physical or ecological barriers or by
behavioral "barriers" in the form of territoriality. Where animals occupy isolated
pockets of suitable habitat, both types of
barriers may be involved, together with
isolation by distance. However, the effects
of territoriality alone may be observed
within continuously distributed populations.
Given the polygamous, tribal, territorial
social system postulated for wild house
mouse populations, heterogeneity in allele
frequencies among tribes is inevitable due
to genetic drift, even in large populations
that are continuously distributed. Additionally, because there appears to be little
migration between barns on the same
farm, perhaps owing to a "saturation" of
available space by tribal territories, a pattern of interbarn heterogeneity may be expected, even among large barns housing
many tribes, if barn populations are
founded by a small number of dispersing
individuals. With this background, we may
now examine the genetic evidence for
subdivision at these levels.
Genetic Evidence
Interbarn Variation. Heterogeneity of
allele frequencies among samples collected
in different barns on the same farm is one
of the consistent patterns of variation revealed in the present study of the house
mouse. Typical interbarn variation is
shown in Table 2, in which data for the
Es-3 locus are presented for samples from
four large chicken barns at Pickard Farm,
Raymondville, Texas. The close -corre-
BEHAVIOR AND GENETICS OF MUS
IT. CM CM —
C © O C
AS
5^- M GO —
O C: C: C:
cr: p *^. c
O — O CO
p
o
CM p
— o
cr
o
8
qioqoq
o 00 o
o
OO "fl- CM OC
meet
— !C OC "
O
q-cp
0000
•3
0000
• tc t c
to
— co -*
ir,
-_:_:-".-
57
spondence of observed and predicted proportions of the phenotypic classes seen in
these samples is characteristic of all samples of the species examined.1 A second
example, showing interbarn variation at
three loci among 10 chicken barns at
Nieschwietz Farm, Ensinal, Texas, is
shown in Table 3. A third example is
provided by samples from barns at Hildreth
Farm (Table 6), and additional examples may be found in Selander and Yang
(1969a).
To obtain estimates of average degrees
of variation in frequency of alleles among
barns within farms, I computed for the
Es-3 and Hbb loci weighted variances of
arc sin transformations of frequencies for
each of 26 farms in Texas, then pooled
these variances, weighted by degrees of
freedom (Table 4). A similar analysis of
pooled frequencies for farms within 18 geographical regions of Texas yielded an estimate of mean interfarm variance within
regions. The two loci are equally variable,
and for both the interbarn variance is approximately three-quarters as large as the
interfarm variance. It is apparent that
there is little genetic interchange between
populations occupying different barns on
the same farm, even when they are located
only a few yards apart. To account for the
origin of the observed interbarn heterogeneity, it is necessary to postulate that
either (1) there is marked variation in
selective factors among barns, which
seems unlikely, especially on farms where
the barns are identical in structure and
appearance, or that (2) barn populations
are established by a small number of founders, which rapidly increases to "saturate" the available space. Pertinent to
this problem is the observation that the
interbarn pattern of variation in frequency
of alleles for a given locus is largely random, as would be expected if the frequency in a barn population were largely a
consequence of the genetic composition of
a small number of founding individuals.
1 Expected proportions of phenotypes were calculated using Levene's (1949) unbiased formula for
small samples.
58
ROBERT K. SELANDER
TABLE 3. Frequencies of alleles for three loci in samples from Nieschwietz
1
Es-2
Barn
Es-3
d
Number
of mice
Es-2'
"Silent"
Es-2"
F
Es-2'
M
Es-2
S
49
86
101
25
50
106
90
31
47
31
616
0.00
0.00
0.06
0.00
0.13
0.00
0.09
0.00
0.00
0.00
0.77
0.84
0.78
0.68
0.63
0.78
0.76
0.73
0.89
0.86
0.778
0.03
0.01
0.01
0.10
0.08
0.04
0.03
0.11
0.01
0.03
0.035
0.20
0.15
0.15
0.22
0.16
0.18
0.12
0.16
0.10
0.11
0.153
1
2
3
4
5
7
8
9
10
11
Pooled
0.034
Farm, Ensinal,
2
Texas
Hbb'
Es-3"
M
Es-3'
S
Hbb"
Diffuse
Hbb'
Single
0.34
0.37
0.45
0.38
0.32
0.43
0.52
0.66
0.28
0.42
0A20
0.66
0.63
0.55
0.62
0.68
0.57
0.48
0.34
0.72
0.58
0.17
0.23
0.17
0.22
0.10
0.17
0.11
0.13
0.22
0.11
0.580
0.165
0.83
0.77
0.83
0.78
0.90
0.83
0.89
0.87
0.78
0.89
0835
1
Heterogeneity x'an — 119.17**, calculating numbers of alleles from maximal likelihood estimates of frequencies.
2
Heterogeneity x'm = 40.12**.
3
Heterogeneity x2m = 19.92*.
'
TABLE 4. Pooled interbarn and interfarm variances in frequency of alleles (arcsin
for two loci in samples from Texas
transformation)
Arcsin variance (in degrees)
Number of farms
or regions pooled
Source of variation
Interbarn within farms
Interfarm within regions
26
18
Moreover, this interpretation is supported
by the fact of nonconcordance of patterns
of interbarn variation among different loci
(see, for example, barns at Nieschwietz
Farm, Table 3).
Intrabarn Variation. As predicted from
the ecological and behavioral evidence of
tribe formation, subsampling within large
barns reveals significant spatial heterogeneity in gene frequencies. Representative data are presented in Table 5 for a
large chicken barn at Lay Egg Ranch, Austin, where spatial subsamples were taken
TABLE 5. Frequencies of alleles for two loci in samples from a barn at Lay Ranch, Austin
region,
Texas
Es-31
Hbb'
d
Sample
Number
of mice
Es-3"
M
Es-3'
S
Hbb
Diffuse
Hbb'
Single
West 1
West 2
West 3
West 4
96
75
70
59
0.542
0.520
0.314
0.119
0.458
0.480
0.686
0.881
0.255
0.120
0.250
0.424
0.745
0.880
0.750
0.576
1
2
Heterogeneity x°m
Heterogeneity x'm
= 68.26**.
— 32.22**.
—
Es-3"
Hbb'
52.39
71.38
43.81
72.91
in each of four quarters of the west half of
the barn; the area of each quarter was
approximately 108 square yards. At the
Es-2 locus the Es-2b allele is fixed in this
barn, but highly significant spatial heterogeneity is apparent for frequencies of alleles
at the Es-3 and Hbb loci. Data of this
type, which are available for a number of
barns, indicated clearly that populations
within large barns are not panmictic, and
suggested the possibility of directly demonstrating tribal structure by mass trapping
within a single barn.
For a detailed analysis of intrabarn variation, four large chicken barns at Hildreth
Farm, Dripping Springs, Texas, were selected (Table 6). These barns, which
house laying hens held in three longitudinal rows of cages suspended above the
earthen floor, are 192 feet long and 24 feet
wide and are set in a row, side-by-side, at
intervals of 80 feet. Mice occur throughout
the barns, where they burrow into the
ground and into mounds of dried chicken
feces accumulating beneath the cages.
59
BEHAVIOR AND GENETICS OF MUS
TABLE 6. Frequencies of alleles for three loci in samples from Hildrelh
Es-2
Sample
Barn 1
Whole Barn
AVhole Barn
West Side
East Side
AVhole Barn
West Side
East Side
East Side
Barn 2
Whole Barn
West Side
East Side
Whole Barn
West Side
East Side
AVhole Barn
West Side
East Side
Bum 3
Whole Barn
West Side
East Side
Whole Barn
Barn 4
Whole Barn
West Side
East Side
Whole Barn
Date of
collection
Number
of mice
Es-2'
"Silent"
Es-2
F
April 1967
July 1967
July 1967
July 1967
Dec. 1967
Dec. 1967
Dec. 1967
June 1968
54
108
54
54
186
48
27
583
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
Aug. 1967
Aug. 1967
Aug. 1967
Dec. 1967
Dec. 1967
Dec. 1967
May 1968
May 1968
May 1968
108
54
54
310
101
74
802
423
379
Aug. 1967
Aug. 1967
Aug. 1967
Dec. 1967
Aug. 1967
Aug. 1967
Aug. 1967
Dec. 1967
1
Farm, Austin
Es-3
C
region,
Texas
Hbb
U
Es-3"
M
Es-3°
S
Hbb*
Diffuse
HbbSingle
0.907
0.912
0.981
0.843
0.946
0.969
0.981
0.880
0.093
0.088
0.019
0.157
0.054
0.031
0.019
0.120
0.167
0.144
0.148
0.139
0.188
0.292
0.185
0.122
0.833
0.856
0.852
0.861
0.812
0.708
0.815
0.878
0.171
0.134
0.128
0.139
0.167
0.187
0.148
0.166
0.829
0.866
0.872
0.861
0.833
0.813
0.852
0.834
0.135
0.086
0.166
0.064
0.092
0.000
0.082
0.094
0.068
0.832
0.895
0.788
0.897
0.860
0.986
0.876
0.871
0.882
0.033
0.019
0.046
0.039
0.048
0.014
0.042
0.036
0.050
0.407
0.472
0.343
0.385
0.446
0.284
0.393
0.433
0.350
0.593
0.528
0.657
0.615
0.554
0.716
0.607
0.567
0.650
0.413
0.470
0.360
0.329
0.396
0.270
0.353
0.402
0.298
0.587
0.530
0.640
0.671
0.6O4
0.730
0.647
0.598
0.702
108
54
54
80
0.089
0.000
0.152
0.000
0.897
0.972
0.848
0.981
0.014
0.028
0.000
0.019
0.407
0.435
0.380
0.394
0.593
0.565
0.620
0.606
0.218
0.194
0.241
0.194
0.782
0.806
0.759
0.806
108
54
54
198
0.060
0.041
0.075
0.046
0.856
0.880
0.834
0.859
0.084
0.079
0.090
0.095
0.343
0.333
0.352
0.407
0.657
0.667
0.648
0.593
0.189
0.183
0.194
0.220
0.811
0.817
0.806
0.780
Mark-and-recapture studies in August,
1967, indicated an average population size
of 3,000 mice per barn, or a density of 5.9
mice per square yard. (This is one of the
highest densities recorded for the house
mouse.) Population size remains relatively
constant throughout the year, with reproduction occurring at a very low rate, and
seasonal variation in frequencies of alleles
has not been detected (Table 6). As noted
earlier, interbarn heterogeneity in allele
frequency is apparent for the three loci
studied.
To obtain a picture of spatial variation
in allele frequencies within a barn, mice
were live-trapped in a 1.6-foot grid-pattern
throughout Barn 2. A total of 870 traps
set in the grid-pattern on the west side of
Barn 2 the night of May 10, 1968, yielded
395 adult and 48 juvenile mice; and a
similar trapping effort on the east side of
the barn on the night of May 15, 1968,
Es-2
yielded 303 adult and 80 juvenile mice.
Grid-points at which mice were caught on
the east side of Barn 2, together with the
genotypes for the Es-3 locus are shown in
Figure 2.
Regional heterogeneity within Barn 2
involves a significant difference between
the west and east sides of the barn in frequency of alleles at both the Es-3 and Hbb
loci (Table 6). These differences were
apparent in all three sample periods, and
thus are known to have persisted for at
least eight months. The large samples from
Barn 2 taken in May, 1968, also suggest
east-west differences in population density,
sex ratio, age composition, and proportion of pregnant females (Table 7).
The grid samples show that, for the
three loci, variation in allele frequencies
within Barn 2 is not simply clinal from
east to west; rather, for each locus, there
is a complex mosaic pattern, with small
60
ROBERT K. SELANDER
©
© •
©
©
O
®
©
• 50-
©©
©
o • ©©
©o ©© ® ©s
©
©•
© •
© © • ,'
© «o«« © 8
© © O ©
D 50
©
r- °i
' Q? • • •
•
© • • • • ® ©O©«
©' D v*®_ • •©©•oo»« ! • •
• «/O . ' 4> • ©
J3O~7ft#)
© ©
•
©
©©• • • • • © « o ® •©: O
, - O''®©O«©©©
•
®O®®® • ®® ©©©©©«.
• s ® « o ©_ • '®OO',»»
• ©
• ®•®
© • • ®© x « *m ®\OOO® • © • ©
•
•
© (V— 80—
»tt»'OO0\ • ; ©
• © • © © • ®o» OO
a©, ©• 0 0 0 t / » •
• ®p®^'
•
• © © • ©©©•© • • • • /
©OS) / © O
i • ©•$/•• •
Q ® © • • » < H#®
#
O • • ( * ) • • © • © • © • © 'eoo«q»"'
®O
' © • •7®
©©•©1
•
© O
o ••©
®®o! ® ©(•••• •
•«
•©(•?•© • © • © • © ©/
(iig
®® © © • • ®©;o®o©®:» ®J O0®«o ; • •
• •
• • • • \ © mm * o ® •©©
1
® |W
0
i
B'O
• •
^—
1
80
•
© ©
ES-3
OMM ©MS
FIG. 2. Esteiase 3 genotypes of mice collected in a
grid-pattern on the east side of Barn 2 at Hildreth
regions of high and low frequencies of the
Es-3C and Hbb'1 alleles on either side of the
barn (Fig. 2).
On either side of Barn 2, for each locus
the spatial distribution of genotypes of
adult mice, as well as of adults and
juveniles combined, is non-random, with a
clustering of like genotypes. Nonrandomness of distribution was demonstrated by variance tests of homogeneity
of the binomial distribution, employing
counts of either genotypes or alleles from
mice collected on 22 non-overlapping
"quadrats" of 25 grid-points each on either
side of the barn (Table 8). Similar results
obtained from a sample of 583 mice collected in a 1.6-foot grid-pattern on the east
side of Barn 1 are also presented in Table
8.2
The spatial clustering of similar genotypes at several loci in adult mice from
Barns 1 and 2 provides direct evidence of
1
• SS BARN 2 EAST SIDE
Farm; 50% and 80% isofrequency lines are indicated (see text).
tribal subdivision. To visualize better the
pattern of regional variation in gene frequency within Barn 2, the genotypes
TABLE 8. Tests of homogeneity in distribution of
alleles among 22 "quadrats" in two barns at Hildreth Farm
X2m) values for indicated locus
Barn and side
Es-2
Es-1
Hbb
Adults and Juveniles
Barn 2
West Side
63.26**
East Side
48.33**
Barn 1
38.96**
East Side
60.65**
65.59**
33.93*
48.86**
50.37**
32.13*
Adults
Barn 2
West Side
East Side
53.54* •
66.70**
36.01*
41.78**
and
Es-3°/Es-3e,
61.44**
28.55
Es-3b/Es-B*>, Es-3b/Es-3°,
were scored 1, 2, and 3, respectively, and
a frequency of the Es-3e allele for each
TABLE 7. Composition of samples of house mice collected in May, 1968, at Hildreth Farm
Sample
Barn 1
East Side
Barn 2
West Side
East Side
1
Number
of mice
trapped
Per cent
adult
Per cent
juvenile
Per cent of
females in
adult class
Per cent of
adult females
pregnant
Number of mice
processed for
electrophoresis
5911
90.0
10.0
46.5
J4.9
583
443
.'183
89.2
79.1
10.8
20.9
57.1
6.2
19.7
423
379
Two nights of trapping (see text).
- Four hundred mice were caught in 870 traps set
on June 3; the unsuccessful traps were left in
position on the grid over night and yielded an
additional 191 mice.
BEHAVIOR AND GENETICS OF
Mits
61
TABLE 9. Frequency of alleles at two loci in relation to estimated size of population in samples
from the Austin region, Texas
Type of
population
Number
of populations
sampled
Median
estimated size
of population
Es-3"
Hbb'
Es-B1"-
Small (n < 50)
Large (n > 50)
29
13
10
200
0.418
0.372
0.849
0.843
0.0506
0.0125
Mean allele
frequency
Variance of allele
frequency
Hbb"
0.1883
0.0083
-F = 22.72**.
grid-point was estimated by computing a
two-dimensional running average of the
scores, in which the score of a mouse taken
at a grid-point was weighted 1.0 and the
scores of mice taken at grid-points in the
five adjacent rows and columns on all sides
of the grid-point were weighted by the
inverse square of their distance from the
grid-point. With frequency estimates for
the Es-3e allele available for each gridpoint, isofrequency contours can be
drawn, as shown in Figure 2. Comparable
contours were also obtained for the Hbbd
and Es-2C alleles. Presumably the major
"peaks" and "valleys" correspond to tribes
or to groups of genetically related tribes
occupying neighboring territories. The
tribal territories apparently are very small,
perhaps on the order of 20 square feet in
area.
Genetic Drift and the Variance of Allele
Frequencies. Evidence of genetic drift
may be obtained by comparing variances
of allele frequencies in samples from populations of various sizes, since with drift
the variance should increase with decreasing population size. In Table 9, I have
summarized the results of a study of allele
frequencies at two loci in 42 barn populations in the Austin region. For these populations, estimates of size were derived by
counting mice collected over several days
of "saturation" trapping (small barns)
or by mark-and-recapture studies (large
barns). Estimated size of populations
ranged from four to 3,000 mice. The smallest populations sampled presumably consisted of single tribes, whereas several hundred tribes may be represented in samples
from the largest populations. Because the
trapping effort in barns housing small pop-
ulations was sufficiently intense to obtain
most, if not all, adult individuals present, I
feel justified in concluding that sampling
error is not the major component of the
variance of frequencies for these samples.
At the Es-3 locus, the frequency of the
Es-3b allele does not vary significantly with
estimated population size, but the variance
of frequency does, being roughly four
times greater in samples of small populations than in those of large populations. In
two small populations, the Es-3" allele appears to be fixed, and, although fixation of
Es-3b was not observed, a frequency of
0.9 was recorded in one sample of seven
mice from a barn estimated to house a
population of 10. For the same 42 populations, the overall mean frequency of the
Hbbs allele is 0.847, and the variances of
samples from large and small populations
are 0.0083 and 0.1883, respectively. The
Hbb8 allele apparently was fixed in 52% of
the small populations but in only 8% of the
large populations.
The "Wahlund Effect": Hetcrozygole
Deficiency as an Index to Subdivision.
Several attempts have been made by other
workers to determine the extent of subdivision in rodent populations through use
of an inbreeding coefficient derived from
observed deficiencies of heterozygotes in
samples (the Wahlund effect), but the validity of this approach is questionable on
several grounds. For the loci which I have
examined, deviations in proportions of
heterozygotes clearly do not provide a basis
for investigating subdivision.
In a study of the Pm erythrocytic antigen locus in 227 deer mice (Peromyscus
maniculatus) collected in Michigan, Rasmussen (1964) found a 29.1% heterozygote
62
ROBERT K. SELANDER
deficiency.3 Perhaps much or all of this
deficiency reflects inbreeding resulting
from subdivision, but, because the possibility of occurrence of a "silent" (nonreactive) allele cannot be excluded, the
results of Rasmussen's study are, as he admits, equivocal. (According to my calculations, a non-reactive allele at a frequency
of 0.08 would account for all of the 29.1%
deficiency of heterozygotes).
Following Rasmussen's
(1964) approach, Petras (1967a) reported heterozygote deficiencies of 18% for the Es-2 locus
and 13% for the Hbb locus in a pooled
sample of 296 house mice collected over a
three-year period in numerous small
buildings on five adjacent farms near Ann
Arbor, Michigan. In this case, the period
over which collections were made is so
long that seasonal and secular changes in
allele frequencies may have contributed to
the observed heterozygote deficiency. Petras' (1967a) data do, in fact, suggest that
the frequencies of both the Es-2a and the
Hbbd alleles increased in the populations
from I960 to 1962.
A further difficulty encountered in using
proportions of heterozygotes as an index
to inbreeding arises from the fact that,
even when temporal variation is excluded,
the effect of subdivision on this variable
may be partially or completely masked by
heterotic effects, which may vary in intensity seasonally, secularly, and geographically. For the material collected in the
present study, it is apparent that an
analysis of subdivision based on the
Wahlund effect is impossible. In this investigation, the £5-2 locus is not considered
because of the presence of the "silent" allele Es-2a, which precludes direct translation of all phenotypes to genotypes. For the
Es-3 and Hbb loci, however, genotypes can
be determined directly, permitting comparison of observed and expected proportions of heterozygotes in samples.
3 The coefficient adopted by Rasmussen (1964)
is F = (Hr — Ho)/He, where H, and Ho are the expected and observed proportions of heterozygotes,
but the coefficient D — (Ho — He)/H, is preferable,
because observed deficiencies are indicated by negative values.
ES-3 N= 159 Samples
n= 2-130
I- •' .• •
SAMPLE SIZE In)
FIG. 3. Relationship of heterozygote deviation (D)
at the Es-3 locus and sample size (indexing population size) in samples from Texas.
For both loci, a preliminary examination of the data from samples collected in
Texas suggested that the direction of
heterozygote deviation (D) was not independent of the size of the barn from which
samples were obtained, being, on the average, negative in samples from large barns
and positive in those from small barns.
This observation was confirmed when D
was plotted against sample size for groups
of samples in which sample size is in some
degree positively correlated with barn size
and, hence, presumably, with population
size. For both the Es-3 and the Hbb loci,
D is inversely related to population size, as
this is indexed by sample size (Figs. 3 and
4).4 The mean value of D for the 159
samples used in the analysis of variation at
4 The observed variation is highly significant,
both by chi-square tests of the distribution of
samples with D < 0 or D > 0 in different size
classes and by regression analysis in which D values
are weighted by the reciprocals of their variances. For the Es-3 locus, D = 0.148 — 0.00208 N (F
— 42-79*», with 1 and 157 d.f.); and for the
Hbb locus. D = 0 09!) _ 0.00143 N (F — 19.77**,
with 1 and 108 d.f.).
It should be noted that Levene's (1949) small
sample formula, used in this study to calculate
expected genotypic proportions, eliminates the bias
toward heterozygotic excess that is inherent in
calculations based on a binomial expansion.
BEHAVIOR AND GENETICS OF MIIS
63
housing only 5 to 20 mice, it is likely that
food and cover are available only temporarily, so that populations do not persist long
enough to become closely inbred.
In concluding this section, I wish to note
that, even if my interpretation of the
causal basis for the observed variation in D
is in part or wholly erroneous, the results
of my investigations clearly point up the
difficulties inherent in any attempt to
deduce the extent of subdivision in natural
populations on the basis of heterozygotic
deviations in samples.
REGIONAL EQUILIBRIUM
In view of the great—indeed, almost bewildering—genetic heterogeneity within
barns, among barns on the same farm, and
FIG. 4. Relationship of heterozygote deviation (D)
among farms in the same area, it is of
at the Hbb locus and sample size (indexing populaspecial interest and importance to examine
tion size) in samples from Texas.
the pattern of variation at a higher level
of
populational structure, that of physiothe Es-3 locus is +0.0114; and, for the 110
samples for which D values for the Hbb graphic, faunistic, and climatological relocus have been calculated, mean D = gions. If a strong stochastic element were
apparent in the geographic patterns of
+0.0077.
variation
in allele frequencies, we would
In interpreting the observed relationship
be
tempted
to conclude that we are
between D and barn or population size, I
dealing
with
selectively
neutral or weakly
suggest that the negative values obtained
for large populations reflect inbreeding re- adaptive characters. But a regularity of
sulting from tribal subdivision and, per- pattern geographically, with clinal variahaps to a lesser degree, isolation by dis- tion between physiographic or other natutance within large barns, as previously ral regions, would be strongly indicative of
demonstrated by direct mapping of geno- the action of selection in maintaining
types. Apparently in large and temporally equilibrium frequencies.
For purposes of analyzing geographic
stable populations, the inbreeding effect on
proportion of heterozygotes is strong variation, the single barn or field was takenough to override any heterotic effect, en as the sampling unit. Sample localities
which would otherwise result in an excess in Texas were grouped into sample regions
of heterozygotes. The fact that D tends to (Fig. 1), and presumed equilibrium-frequenbe positive in small populations suggests cies for each locus were estimated by comheterosis at each of the two loci, although puting unweighted mean frequencies for
the variation could be accounted for by all barns and fields represented by samples
directional selection in populations that larger than nine mice. The results of this
have drifted away from equilibrium fre- analysis are reported elsewhere (Selander,
quencies. In any event, for the heterotic Yang, and Hunt, 1969). For present pureffect to be manifested, we must presume poses, it will suffice to note that for none of
that the small populations (consisting of the loci is there evidence that random
one or a few denies) are only weakly inbred, events play a significant role in determinif at all. This is plausible since, in small ing regional frequencies. Rather, geograbarns, sheds, or other structures capable of phic patterns of variation are characterized
64
ROBERT K. SELANDER
FIG. 5. Variation in regional mean frequencies o£
alleles at the Es-2 locus in Texas.
by uniformity of allele frequency in the
major regions of the state, with clinal variation between regions, as would be
expected if selection is determining the
frequencies. As an example, geographic
variation at the Es-2 locus is shown in
Figure 5. At the regional level there is no
evidence that chance factors have a major
or dominant effect in determining frequencies of alleles at the local level.
DISCUSSION AND CONCLUSIONS
The results of the investigations I have
reported provide clear support for
Wright's (1948) contention that random
drift due to sampling error in small populations is an important factor governing
gene frequencies and one which must be
taken into account, along with selection
and other determinant factors, in a comprehensive consideration of the evolutionary process. For any individual or small
population of the house mouse, it would
seem that stochastic processes play the
dominant role in determining genotype
(individual) or gene frequency (population).
With fine-scale subdivision and consequent genetic drift, regional equilibrium
gene frequencies become, as Lewontin
(19675, p. 47) puts it, "no longer a single
composition to which the population
tends, but a statistical ensemble of compositions with an equilibrium set of associated probabilities." The implications of
this situation for sampling procedures in
population genetics, ecology, systematics,
and other areas of populational biology
are apparent. For species in which populations are subdivided like those of the house
mouse, samples taken from one or a few
populations will not be sufficient for testing predictions. In any region allele frequencies at a given locus will be at selectively determined equilibrium values in
few, if any, populations sampled. Estimates
of regional equilibrium frequencies must,
therefore, be based on samples from, many
populations. There is an urgent need for
studies of subdivision in a variety of organisms in order that systematists and other
populational biologists can modify their
sampling procedures to minimize the potential sources of error stemming from
genetic heterogeneity within species populations.
We come, finally, to a consideration of
the significance of populational structure
in the evolution of the house mouse. Elsewhere (Selander and Yang, 1969a), I have
suggested that the polygamous, territorial,
tribal social system of the house mouse is
adaptive in terms of efficient exploitation
by individuals of environmental resources,
particularly food, and that the resulting
genetic heterogeneity is an inevitable consequence of subdivision, having no adaptive significance per se. But whatever the
cause of subdivision, it is apparent that the
populational structure of this species corresponds to that described by Wright
(1931, 1940) as potentially favorable for
evolutionary trial and error among local
populations. Although evolutionary biologists have in recent years tended to minimize the importance of random fluctuations
in the evolutionary process, a balanced assessment would seem to require much
additional information from empirical
studies. In the case of the house mouse, we
should like to know the average "life"
span of the tribal units under various envi-
BEHAVIOR AND GENETICS OF MUS
ronmental conditions in order to judge
how long groups may be withdrawn from
the common gene pool (see discussion by
Simpson, 1953, p. 123). For populations of
this species living in agricultural areas,
the duration of periods of withdrawal of
tribes may be very short. Populations of
mice may experience repeated cycles involving finding of a supply of food by a
few mice, establishing one or more founding tribes, rapid proliferating of tribes
from the founding stock, inbreeding and
drift within tribes, and eventual depletion
of the food supply, leading to dispersal of
tribal members and the initiation of a new
cycle. In the long run, drift in individual
tribes may have little significance in evolution, other than providing an opportunity
for variant alleles to increase in frequency
locally, if tribes do not remain intact and
isolated long enough for pervasive genetic
changes to occur. However, if, as the evidence seems to indicate, large populations
are founded by a few individuals invading
a barn (or field), the founder effect
(A'fayr, 1954) may play a role in evolution
of this species, with the total variance in
gene frequencies among populations significantly augmented by shifts in genome
structure arising from the fact of populations being founded by a non-random sample of the gene pool. The reality of this
phenomenon has been demonstrated experimentally for Drosophila pscudoobscura
by Dobzhansky and Pavlovsky (1957). At
present it would seem wise to withhold
final judgment as to the evolutionary significance of drift and other stochastic processes in evolution, for clearly this area of
evolutionary biology, and especially that
aspect concerning the relationships of social and genetic structures of species populations, has been largely unexplored.
REFERENCES
Anderson, P. K., and J. L. Hill. 1965. Mus musculus: experimental induction of territory formation. Science 148:1753-1755.
Croucroft, P., and F. P. Roue. 1963. Social organization and territorial behaviour in the wild
house mouse (Mus musculus J,.). l'roc. Zool.
.Soc. London 140:517-531.
65
Dobzhansky, T., and O. Pavlovsky. 1957. An experimental study of interaction between genetic drift
and natural selection. Evolution 11:311-319.
King, f. A. 1967. Behavioral modification of (he
gene pool, p. 22-43. In J. Hirsch, [ed.], Behaviorgenetic analysis. McGraw-Hill, New York.
I.evene, H. 1949. On a matching problem arising in
genetics. Ann. Math. Statist. 20:91-94.
Lewontin, R. C. 1967a. An estimate of average
heterozygosity in man. Amer. J. Hum. Genet.
19:681-685.
Lewontin, R. C. 19676. Population genetics. Annu.
Rev. Genet. 1:37-70.
Lewontin, R. C, and L. C. Dunn. 1960. The
evolutionary dynamics of a polymorphism in the
house mouse. Genetics 45:705-722.
Lewontin, R. C, and J. L. Hubby. 1966. A molecular approach to the study of genie heterozygosity
in natural populations. II. Amount of variation
and degree of heterozygosity in natural populations of Drosophila pseudoobscura. Genetics
54:595-609.
Mayr, E. 1954. Change of genetic environment and
evolution, p. 157-180. In J. Huxley, et al., [ed.],
Evolution as a process. Allen and Unwin, London.
Petras, M. L. 1967a. Studies of natural populations
of Mus. I. Biochemical polymorphisms and their
bearing on breeding structure. Evolution 21:259274.
Petras, M. L. 19676. Studies of natural populations
of Mus. II. Polymorphism at the T locus. Evolution 21:466-478.
Rasmussen, D. I. 1964. Blood group polymorphism
and inbreeding in natural populations of the
deer mouse Peromyscus maniculatus. Evolution
18:219-229.
Reimer, J. D., and M. L. Petras. 1967. Breeding
structure of the house mouse, Mus musculus, in
a population cage. J. Mammalogy 48:88-99.
Reimer, J. D., and M. L. Petras. 1968. Some aspects
of commensal populations of Mus musculus
in southwestern Ontario. Can. Field Xatur.
82:32-42.
Selander, R. K., W. G. Hunt, and S. Y. Yang. 1969.
Protein polymorphism and genie heterozygosity
in two European subspecies of the house mouse.
Evolution 23:379-390.
Selander, R. K., and S. V. Vang. 1969a. Biochemical
genetics and behavior in wild house mouse
populations. In C. Lind/ey and D. D. Thiessen,
[ed.], Contributions to behavior-genetic analysis:
the mouse as a prototype. Appleton-CenturyCrofts, New York. (In press)
Selander, R. K., and S. Y. Yang. 19696. Protein
polymorphism and genic heterozygosity in a wild
population of the house mouse (Mus musculus).
Genetics 63: (In press).
Selander, R. K., S. Y. Yang, and W. G. Hunt. 1969.
Polymorphism in esterases and hemoglobin in
wild populations of the house mouse (Mus musculus). Studies in genetics V. Univ. Texas Publ.
6918:271-338.
66
ROBERT K. SELANDER
Simpson, G. G. 1953. The major features of evolution. Columbia Univ. Press, New York.
Southern, H. N., and E. M. O. Laurie. 1946. The
house mouse (Mus musculus) in corn ricks. J.
Anim. Ecol. 7:134-149.
Wright, S. 1931. Evolution in Mendelian populations. Genetics 16:97-159.
Wright, S. 1940. The statistical consequences of
Mendelian heredity in relation to speciation, p.
161-183. In J. Huxley, [ed.], The new systematics.
Clarendon Press, Oxford.
Wright, S. 1948. On the roles of directed and
random changes in gene frequency in the genetics of populations. Evolution 2:279-295.
Yasuda, N., and N. E. Morton. 1967. Studies on
human population structure, p. 249-265. In J. F.
Crow and J. V. Neel, [ed.], Proceedings of the
Third International Congress of Human Genetics: Plenary sessions and symposia. Johns Hopkins
Univ. Press, Baltimore.
Young, H., R. L. Strecker, and J. T. Emlen, Jr.
1950. Localization of activity in two indoor populations of house mice, Mus musculus. J. Mammalogy 31:403-410.