Download Evolutionary consequences of restricted gene flow among natural

BULLETIN OF MARINE SCIENCE, 39(2): 526-535, 1986
LARVAL INVERTEBRATE WORKSHOP
EVOLUTIONARY CONSEQUENCES OF RESTRICTED GENE
FLOW AMONG NATURAL POPULATIONS OF THE
COPEPOD, TIGRlOPUS CALIFORNICUS
Ronald S. Burton
ABSTRACT
Extensive differentiation at electrophoretically-detected
gene loci is commonly observed
among natural populations of marine invertebrates. This differentiation reflects restricted
gene flow among con specific populations or the action of natural selection. While differentiation indicates the potential for adaptive evolution, few studies have demonstrated the
adaptive nature of genetic differentiation among marine invertebrate populations. Two lines
of evidence are presented that suggest that the extensive differentiation observed among T.
califarnicus populations has resulted in differential adaptation ofIocal populations. (I) Based
on the environmental conditions that might favor the GptF allele (predicted by previous
physiological genetic investigations) and knowledge of the population structure ofthis species,
an adaptive relationship between salinity variation and allele frequency was predicted. Data
suggesting that such a relationship may hold among natural populations is presented. (2) The
genetic structure of T. califarnicus populations (isolated populations differentiated at many
loci) appears to be conducive to the establishment of polygenic interaction systems postulated
by Wright's shifting balance theory of evolution. Inter-population hybridization experiments
show that the F2 larvae of between-population matings show significantly higher mortality
in response to hyperosmotic stress than those of within-population matings. This work suggests that each population possesses a gene pool consisting of an integrated complex of alleles;
hybridization between populations breaks up these complexes and results in individuals
lacking the physiological capacities present among individuals from each parental population.
Substantial progress has recently been made toward understanding the genetic
structure of natural populations of marine invertebrates (reviews by Burton, 1983;
Gooch, 1975; Levinton, 1980). Primarily through the use of protein electrophoresis, these efforts have demonstrated that genetic differentiation of conspecific
populations is frequently observed among marine species (Berger, 1973; Berglund
and Lagercrantz, 1983; Bulnheim and Scholl, 1981; Burton and Feldman, 1981;
Janson and Ward, 1984; Schopf and Gooch, 1971; and many others) and is not
uncommon even among species with long-lived planktonic larval stages (Johnson
and Black, 1982; Koehn et al., 1980; Marcus, 1977; Theisen, 1978; Tracey et al.,
1975). Differentiation in species with high dispersal potential could be the result
of restriction in effective dispersal imposed by either the environment or behavior
of dispersal stages (Burton and Feldman, 1982a), or may result from natural
selection favoring resident over immigrant recruits to a population (Koehn et al.,
1980). While in the latter case the adaptive significance of population differentiation is evident, relatively few cases of differentiation (especially at enzymecoding gene loci) have been shown to be the direct result of natural selection.
Hence, while allozyme studies have shown ubiquitous genetic differentiation among
marine invertebrate populations, we know little ofthe evolutionary consequences
of the genetically sub-divided population structures found among these species.
In previous papers, we have investigated the genetic structure of natural populations of the harpacticoid copepod Tigriopus calif amicus, a common, freeswimming inhabitant of high intertidal and supralittoral rock pools along the
California coast. Despite its apparently high dispersal capacity, T. califomicus
526
BURTON: CONSEQUENCES OF RESTRICTED GENE FLOW
527
populations exhibit extensive differentiation (Burton et aI., 1979; Burton and
Feldman, 1981); populations inhabiting neighboring rock outcrops often possess
unique (i.e., found in only one or a few local populations) alleles in high frequency
at one or more loci. Field transplantation experiments (Burton and Swisher, 1984)
have further refined our understanding of population structure in this species by
demonstrating that gene flow is extensive among T. californicus populations inhabiting pools located on single rock outcrops. Gene flow between outcrops separated by sandy beach, however, is highly restricted (Burton and Feldman, 1981).
In another line of investigation, Burton and Feldman (1983) presented evidence
for physiological differences among genotypes at one of the loci that had been
used as a genetic marker: Gpt. This locus codes for the enzyme glutamate-pyruvate
transaminase, which catalyzes the final step of alanine biosynthesis. Since rapid
accumulation of alanine appears to play an important role in response to hyperosmotic stress in this species (Burton and Feldman, 1982b), polymorphism at the
Gpt locus could result in differences in physiological response among genotypes.
Data presented in Burton and Feldman (1983) suggest that such differences are
observed not only at the physiological level (i.e., rates of alanine accumulation),
but also in differential larval survivorship among Gpt genotypes subjected to
hyperosmotic stress.
Here I will attempt to integrate our population structure analyses with our
physiological genetic investigations in order to address the evolutionary consequences of restricted gene flow among T. californicus populations. While our
previous work on the genetic structure of natural populations of this species has
focused on documenting levels of population genetic differentiation and gene flow,
the investigations presented here provide our first indications that restriction of
gene flow has resulted in adaptive differentiation of T. californicus populations.
MATERIALS
AND METHODS
Sampling of natural populations of T. californicus in the vicinity of Santa Cruz, California, was
carried out on an irregular basis from January 1980, to August 1984; some of the data used in the
analysis here were previously reported in Burton and Feldman (1981). Pool salinities for each population sampled were measured to the nearest part per thousand using a hand-held refractometer. In
March 1984, 10 coastal populations from Moss Beach (San Mateo County) to La Jolla '(San Diego
County) were sampled and returned alive to Philadelphia where laboratory populations were established and maintained in continuous culture using artificial seawater (Instant Ocean). Field-collected
animals were used for electrophoretic analysis, while animals used for the inter-population crosses
reported below were FI and F, individuals from the laboratory populations. Electrophoretic analyses
were carried out as described in Burton and Feldman (1981; 1982).
Intra- and inter-population matings were obtained by isolating a group of adult males (approximately
20 individuals) from one population in a 100 x 15-mm petri dish and adding approximately 20 virgin
females from the second population, Virgin females were obtained by collecting clasped pairs of T.
californicus from the required population and dissecting the adult male from his immature "mate";
immature individuals obtained in this way are invariably virgin females (Burton et aI., 1981; Burton,
1985). Inter-population hybrids (FI individuals) were then reared en masse, or as clasped pairs were
formed, single-pair crosses were isolated. The F, larvae used for the salinity-shock treatments were
the progeny ofFI x FI matings.
Salinity-shock treatments consisted of taking Stage I-II nauplii hatched and reared for at least 24
h in 50% seawater (salinity = 170/00)
and pipetting known numbers (25-35 per trial) into 100% seawater
(340/00).Survivorship was counted after 6 days. It should be noted that this stress simulates that which
occurs when a low salinity tidepool (common habitat for T. californicus) is inundated by seawater
during a period of high wave action; adult T. californicus show no mortality in response to this shock.
RESULTS
Salinity Versus Gpt Allele Frequencies. -Over the period of 1980-1984, allele
frequencies and salinities were determined for 53 samples representing pools
528
BULLETIN OF MARINE SCIENCE, YOLo 39, NO.2.
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20
30
40
SALINITY
50
60
70
80
90
(Ppt)
Figure 1. Frequency of the Gpt allele in 53 T. californicus population samples plotted against the
salinity of the pool from which the sample was taken. Different symbols represent samples taken from
pools on different rock outcrops.
F
located on 5 outcrops located between Natural Bridges State Park and Capitola
in Santa Cruz County, California. Sample sizes for allele frequency determinations
ranged from 30 to 160 individuals (average = 76). The relationship between pool
salinity and GptF allele frequency is plotted in Figure 1; no significant relationship
can be discerned (linear regression of arc-sine transformed GptF allele frequency
on pool salinity was not significant at 0.05 level). This plot did, however, suggest
that the five outcrops differed in average allele frequency and the range of salinity
variation observed. These outcrop parameters are tabulated in Table 1. Allele
frequencies are highly heterogeneous among outcrops (Kruskal-Wallace test, H =
31.0, P < 0.001). While mean salinities do not differ significantly (ANOV A, F =
0.99, P > 0.25), they are heteroscedastic (Bartlett's test, P < 0.01) and clearly
differ in range and coefficient of variation. Without continuous monitoring of
numerous pools on each outcrop it is difficult to assess the actual salinity regimes
each presents to its resident T. califomicus population. However, characterization
of each site (based on observations of fresh water input, range of pool elevations,
and wave exposure) suggests that the differences in levels of salinity variation
among outcrops tabulated in Table 1 accurately reflect both short- and long-term
differences in the salinity regimes of the outcrops. For example, the SCN and
PLA outcrops show the greatest range in salinity among pools at anyone time
and are also more variable through time than the other three outcrops. With the
exception of the typically low salinity CAP site, mean salinities on each of the
other outcrops are near 100% seawater. There is a suggestive tendency for outcrops
with extensive salinity variation to have higher GptF frequencies; all samples with
GptF frequency above the median (0.167) were taken from the two outcrops with
BURTON: CONSEQUENCES
OF RESTRICTED
529
GENE FLOW
Table I. Salinity variation and Gpt'" allele frequencies on five Santa Cruz outcrops sampled on 5 to
II dates in 1980-1984
Outcrop
Dates
SeN
PLA
NB
WCAP
CAP
II
6
9
5
7
No.
samples
24
8
9
5
7
Salinity:
Mean,
range (0/00)
34, 3-96
28, 4-80
36,34-52
34,31-40
20, 5-34
Salinity
CV(%o)
65.2
85.7
15.4
11.1
42.8
GptF x (±SEM)
0.219
0.150
0.134
0.086
0.056
(0.008)
(0.013)
(0.009)
(0.023)
(0.018)
the greatest salinity variation (SCN and PLA). Also notable is the perfect rankorder correlation between maximum outcrop salinity and allele frequency.
Geographic Distribution of Allozyme Variants at Four Enzyme-encoding Gene
Loci.-Allele
frequencies for Got-I, Got-2. Gpt, and Me at 10 sites along the
central to southern California coast are presented in Table 2. As we have previously
observed with other loci and populations, many alleles have highly restricted,
disjunct distributions and reach high frequency where they occur (Burton and
Feldman, 1981; Burton, 1983).
Inter-population Crosses and Larval Survivorship During Hyperosmotic Stress.Inter-population crosses were carried out using four source populations: two from
central California, Santa Cruz (SCN) and Pescadero (PES) and two from southern
California, Palos Verdes (AB, Abalone Cove Park) and La Jolla (LJ). Three classes
of crosses were obtained: (1) within-population (control) crosses, where both male
and female were taken from the same population; (2) within-region crosses, where
the male was taken from one and the female taken from the other population
within either the central or the southern California region; (3) between-region
crosses, where one parent was taken from a central California population and the
other from a southern California population. Survivorships ofF2 larvae subjected
to hyperosmotic stress are presented in Table 3. Most of the potential cross types,
including reciprocals, are represented in the data. Progeny of within-population
crosses show significantly higher survivorship following hyperosmotic stress than
those produced by interpopulation hybridization. Furthermore, there is a significant effect of region such that the apparent break-down of adaptations to hyperosmotic stress was more pronounced for between-region hybridizations than for
within-region hybridizations.
DISCUSSION
Inference of Gene Flow from Genetic Data. - The level of gene flow among geographically separated con specific populations may have important implications
for the evolutionary response of a species to differences in selective forces among
local environments (Crisp, 1978, for pertinent discussion focused on marine invertebrates). The extent to which populations can adapt to environmental heterogeneity depends both on environmental "grain" and gene flow. At the extremes,
panmixia over broad geographic ranges will prevent evolutionary response to a
spatially patchy environment, while total restriction of gene flow can result in the
evolution of extensive local adaptation and, in the limit, speciation.
The inference of levels of gene flow from either morphological or biochemical
genetic data is often difficult. In the former case, the genetic basis ofthe trait must
first be established and then the relative roles of natural selection and gene flow
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BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2,
1986
Table 2. Frequencies of common alleles (rounded to 0.1) for Gpt. Got-I, Got-2, and Me at study
sites along the California coast (All frequencies are based on electrophoretic analysis of at least 50
individuals collected in March, 1984)
Gpr
Site
F
M
GOI-I
F
S
Central California study sites
Moss Beach
Pescadero
Santa Cruz
0.3
Capitola
0.1
Monterey
Carmel
1.0
1.0
0.7
0.9
1.0
1.0
0.1
0.9
1.0
1.0
1.0
0.5
0.9
0.1
Southern California study sites
Abalone Cove
Laguna Beach
Aliso Beach
La Jolla
0.1
1.0
1.0
1.0
0.9
0.7
0.1
0.5
ES
VF
F
S
0.1
0.9
1.0
1.0
1.0
1.0
1.0
1.0
0.2
1.0
0.3
0.7
Me
Gor·2
VS
S
VF
F
0.2
0.1
0.4
1.0
0.3
0.7
1.0
1.0
M
S
1.0
0.8
0.9
0.6
1.0
1.0
1.0
1.0
0.9
0.1
1.0
must be somehow disentangled. Struhsaker's (1968) study of shell sculpture polymorphism in Littorina pieta suggests that this trait is under some level of genetic
control and that selection influences the frequencies of morph types at different
sites (although the mechanism by which selection acts is unclear since, as Struhsaker points out, the distribution of morphs in L. pieta is exactly opposite that
in another polymorphic species, L. saxatilis rudis). More pertinent to this discussion, however, is the fact that the role of gene flow in this system remains
unclear; while morph distributions of new recruits differ from those of older
individuals at the same site (strong evidence for selection), the higher diversity
ofmorph types among juveniles could be due to Mendelian segregation of resident
multilocus genotypes affecting sculpture as well as gene flow from neighboring
differentiated populations. Because this species has planktonic larvae, the latter
explanation has been rather uncritically excepted (Gooch, 1975). However, Struhsaker (1968) reported that plankton tows suggest that most larvae are found close
to shorelines where large adult populations occur, indicating that levels of gene
flow might be low.
An obvious advantage of protein polymorphisms as genetic markers for studies
of population genetic structure is their single-locus co-dominant mode of inheritance, which eliminates the potential problem posed by complex segregation of
morphological characters. A second and equally important advantage is the (frequent) availability of numerous polymorph isms independently segregating in the
same natural populations, Still, inference of gene flow patterns among natural
populations based on protein polymorphisms is not always straightforward and
is usually based on ad hoc arguments specific to individual study systems (Christiansen and Frydenberg, 1974). Slatkin (1981) has proposed a semi-quantitative
approach for inferring levels of gene flow in natural populations based on the
frequencies of alleles present in varying proportions of the study populations.
While this method is simple, objective, and gaining in popularity (Buroker, 1984),
the extent to which it is an improvement over ad hoc methods is unclear for at
least two important reasons: (1) it does not take the actual geographic distribution
of alleles into account (an allele's presence in 2 of 10 populations can mean quite
different things depending on whether the two populations are adjacent or dis-
BURTON: CONSEQUENCES
Table 3.
531
OF RESTRICTED GENE FLOW
Survivorship ofF2 larvae of within- and between-population
crosses to hyperosmotic stress
Survivorship
Cross type
Within populations
(PES x PES, AB x AB, etc.)
Within regions
(PES x SC, AB x U)
Between regions
(PES x U, U x SC)
N
(%)
626
77.3
716
68.6
406
53.2
R x C Test of Independence: G = 65.1. df = 2. P « 0.005.
junct), and (2) in averaging the frequencies of alleles with the same occupancy
number (proportion of populations where the allele occurs), much valuable information may be lost. For example, a unique allele fixed in one population tells
much about restriction of gene flow; yet when its frequency (1.0) is averaged with
many rare unique alleles, its information content can be effectively lost. More
recently, Slatkin (1985) has presented simulation results that suggest that the joint
parameter Nm (effective population size multiplied by the proportion of immigrant individuals per generation) can be estimated from the average frequency of
"private" (occurring in only one population) alleles. While overcoming problem
(1) above, the estimate shows strong sample size dependence (Slatkin provides a
sort of correction factor). More importantly, since we lack any information on
effective population sizes among marine invertebrate species, it remains difficult
to resolve actual migration (gene flow) rates. Given these reservations, Slatkin's
methods should be combined with simple analyses of the raw data in order to
provide a reasonable first approach for the analysis of gene flow based on allozyme
data.
Consequences of Restricted Gene Flow among T. californicus Populations: Microadaptation Among Neighboring Populations. - The widespread occurrence of
unique alleles in high frequency among Tigriopus califarnicus populations indicates that gene flow in this species is highly restricted (Burton and Feldman, 1981;
Burton, 1983). Both high predation pressure in the lower intertidal zone and the
behavior of T. californicus probably contribute to this restriction of gene flow
(Burton and Feldman, 1981). Consequently, the population structure of this species
is conducive to the evolution of adaptations to local environmental conditions.
Such adaptations could potentially involve the specific gene loci at which we have
documented population differentiation or they may involve a diversity of structural and regulatory loci which we have not yet studied. The data presented above
attempt to address both possibilities. In the former case, we know of only one
locus for which an appropriate gene/environment adaptation argument may be
constructed, i.e., Gpt (Burton and Feldman, 1983). Any number of adaptations
might be used to address the latter; here we have focused on larval survivorship
following hyperosmotic stress.
The Gpt locus might participate in local adaptation in the following way: Burton
and Feldman (1982b) demonstrated the rapid accumulation of intracellular free
amino acids (FAA) in T. californicus subjected to hyperosmotic stress, with alanine
playing an important role in the early stages of the response. Burton and Feldman
(1983) showed that Gpt genotypes differ significantly in rates of alanine accumulation during stress response (Gpt catalyzes the final step of alanine synthesis)
and larval survivorship following stress. While we lack information concerning
532
BULLETIN OF MARINE SCIENCE, VOL. 39, NO.2,
1986
other forces acting on the Gpt polymorphism, I propose that the higher activity
GptF allele will be favored in environments where T. calijomicus experience more
episodes of hyper osmotic stress. Under laboratory conditions of constant salinity
(54, 34, or 170/00), I have observed no change in Gpt allele frequencies for periods
of over 6 months (unpubl. data), suggesting that in the absence of salinity stress,
directional selection on this polymorphism may be weak or nonexistent. Unfortunately, I have not yet been able to maintain large population sizes in a laboratory
fluctuating salinity experiment to test the above hypothesis.
As demonstrated in Figure I, there is no direct relationship between Gpt allele
frequencies and pool salinities. Given that I now propose that the action of this
locus is as described above, the salinity history of the pool and not its current
salinity would clearly be a more appropriate environmental parameter. The relationship is further complicated by the population structure of this species because
there is extensive gene flow among pools on a given rock outcrop (Burton and
Swisher, 1984). Restriction of gene flow among these outcrops, stretching over
approximately II km of coastline, is evident from the differentiation observed at
the pgm locus (Burton and Feldman, 1981). Hence, the relevant parameters are
those quantifying salinity variation on an outcrop (in time and space) and the
average allele frequency on that outcrop. While we do not yet have an extensive
set of such data, Table 1 suggests that Gpt allele frequencies respond to the differing
levels of salinity variation experienced by local populations of T. califomicus.
Consequences of Restricted Gene Flow Among T. californicus Populations: Differentiation of the Genetic Basis of Physiological Response. -On a broad geographic scale, T. calijomicus populations are genetically isolated to the point where
they are currently undergoing independent evolution. A predicted consequence
of such independence is that the genetic basis of common physiological responses
could differentiate over time since each population will experience different mutational input, genetic drift, and selective regimes. Hence, while all T, califomicus
populations along the California coast will experience some degree of osmotic
stress and have the ability to respond to that stress, the biochemical genetic
processes underlying the physiological response may not be the same in each
population. Such predictions arise from the work of King (1955) on integration
of the gene pool in lines of Drosophila melanogaster independently selected for
DDT resistance and other related investigations (Wallace, 1981). These studies
indicate that physiological response (in this case DDT resistance) was built up
"by the consolidation of polygenic systems which are not identical in independently developed lines and in which the constituent factors are not simply additive
(King, 1955)." Such consolidation of polygenic systems would correspond to the
establishment of new "harmonious interaction systems," the adaptive peaks of
Wright's shifting balance theory of evolution (reviewed in Wright, 1977). When
populations with different interaction systems are hybridized, adaptations will
break down in the F2 generation as segregation breaks up the pleiotropic systems
found intact in each parental population. This breakdown of adaptations is also
thought to underlie the phenomenon of "optimal" outcrossing distances in plants
(refs. in Willson, 1984); a reduction in female reproductive success is observed
when pollen donors are from a relatively distant population ("outbreeding depression").
Extensive differentiation among T. calijomicus populations is apparent from
the data presented in Table 2. Based on the four loci studied here as genetic
markers, we can safely conclude that central and southern California populations
are differentiated at many loci and that the population structure of this species is
BURTON: CONSEQUENCES
OF RESTRICTED GENE FLOW
533
highly subdivided. These conditions are clearly conducive for the establishment
of different polygenic interaction systems (Wright, 1977). The data presented in
Table 3 indicate an apparent "outbreeding depression" exists both within and
among regional T. califomicus populations; I conclude that the reduced larval
survivorship among F2 interpopulation hybrids is the result of breakdown of the
successful interaction systems characterizing each of the parental lines resulting
from meiotic segregation. It should be noted that this conclusion makes no assumptions concerning the involvement of specific loci in osmotic response or
whether the differentiation observed at the four loci studied here is the result of
selective forces or random drift.
Evolutionary Consequences of Restricted Gene Flow. -My laboratory, like many
others, has put considerable effort into studying the spatial distribution of allelic
variants at enzyme coding gene loci among natural populations of marine organisms. The goal of this work is to understand the evolution of adaptations to local
environmental conditions. There are two general ways in which electrophoretic
data might contribute to this goal: (1) By indicating the extent of population
differentiation, allozyme data can aid in determining the extent of gene flow among
populations. Combined with other genetic data, such as heritable morphological
variation (Struhsaker, 1968) and mitochondrial DNA analyses (Avise and Lansman, 1983), allozyme studies can suggest the appropriate spatial scale at which
one should focus studies of adaptation. With respect to T. californicus electrophoretic surveys of natural populations and field transplantation experiments
indicate that while gene flow is too extensive to expect adaptive differentiation
among pools on a single outcrop, restriction of gene flow among neighboring
outcrops appears to be sufficient to allow such differentiation. (2) Since some
enzymes are known to function in specific, physiologically-relevant biochemical
pathways, allelic frequencies in those enzyme systems might directly reflect population adaptations. Unfortunately, while suggestive patterns of differentiation at
single gene loci have frequently been noted (i.e., allele frequencies are correlated
with some environmental parameter), a causal relationship has seldom been established (Koehn et aI., 1983).
In conclusion, surveys of allozyme variation have made invaluable contributions to our understanding of gene flow among marine invertebrate populations.
In fact, the detailed studies needed to resolve the genetic structure of natural
populations of marine invertebrates are still relatively few in number. However,
such studies alone can only aid in demarcating the boundaries of natural populations. While this may be a valuable goal from a fisheries management point of
view, it is rarely the ultimate goal of evolutionary ecology; here I have suggested
some ways in which this work can serve as a first step in elucidating the evolution
of physiological adaptations.
ACKNOWLEDGMENTS
This work was supported by a grant from the National Science Foundation (DEB-8207000). I thank
P. Noga for laboratory assistance.
LITERATURE CITED
Avise, J. C. and R. A. Landsman. 1983. Polymorphism in mitochondrial DNA in populations of
higher animals. Pages 147-164 in M. Nei and R. K. Koehn, eds. Evolution of genes and proteins.
Sinauer, Sunderland, Massachusetts.
Berger, E. 1973. Gene-enzyme variation in three sympatric species of Littorina. BioI. Bull. 145:
83-90.
534
BULLETIN OF MARINE SCIENCE. VOL. 39. NO.2.
1986
Berglund, A. and U. Lagercrantz. 1983. Genetic differentiation in populations of two Palaemon
prawn species at the Atlantic east coast: does gene flow prevent local adaptation? Mar. BioI. 77:
49-57.
Bulnhein, H.-P. and A. Scholl. 1981. Genetic variation between geographic populations of the
amphipods Gammarus zaddaehi and G. sa/inus. Mar. BioI. 64: 105-115.
Buroker, N. E. 1984. Gene flow in mainland and insular populations of Crassostrea (Mollusca). BioI.
Bull. 166: 550-557.
Burton, R. S. 1983. Protein polymorphisms and genetic differentiation of marine invertebrate populations. Mar. BioI. Letters 4: 193-206.
--.
1985. Mating system of the intertidal copepod Tigriopus ealifornieus. Mar. BioI. 86: 247252.
--and M. W. Feldman. 1981. Population genetics of Tigriopus ealifornieus: II. Differentiation
among neighboring populations. Evolution 35: 1192-1205.
--and ---.
1982a. Population genetics of coastal and estuarine invertebrates: does larval
behavior influence population structure? Pages 537-551 in Y. S. Kennedy, ed. Estuarine comparisons. Academic Press, New York.
--and ---.
1982b. Changes in free amino acid concentrations during osmotic response in
the intertidal copepod Tigriopus ealifornieus. Compo Biochem. Physiol. 73A: 441-445.
-and --.
1983. Physiological effects of an allozyme polymorphism: glutamate-pyruvate
transaminase and the response to hyperosmotic stress in the copepod Tigriopus ealifornieus.
Biochem. Genet. 21: 239-251.
--and S. G. Swisher. 1984. Population structure of the intertidal copepod Tigriopus ealifornieus
as revealed by field manipulation of allele frequencies. Oecologia 65: 108-111.
---,
M. W. Feldman and J. W. Curtsinger. 1979. Population genetics of Tigriopus ealifornieus
(Copepoda: Harpacticoida): I. Population structure along the central California coast. Mar. Ecol.
Prog. Ser. I: 29-39.
---,
--and S. G. Swisher. 1981. Linkage relationships among five enzyme-coding gene loci
in the copepod Tigriopus ealifornieus: a genetic confirmation of achiasmatic meiosis. Biochem.
Genet. 19: 1237-1245.
Christiansen, F. B. and O. Frydenberg. 1974. Geographic patterns offour pOlymorphisms in Zoarees
viviparous as evidence for selection. Genetics 77: 765-770.
Crisp, D. J. 1978. Genetic consequences of different reproductive strategies in marine invertebrates.
Pages 257-273 in B. Battaglia and J. A. Beardmore, eds. Marine organisms: genetics, ecology,
and evolution. Plenum Press, New York.
Gooch, J. L. 1975. Mechanisms ofevo]ution and population genetics. Pages 349-409 in O. Kinne,
ed. Marine ecology, Yol. II, Part 1. Wiley, London.
Janson, K. and R. D. Ward. 1984. Microgeographic variation in allozyme and shell characters in
Littorina saxatilis Olivi (Prosobranchia: Littorinidae). BioI. J. Linn. Soc. 22: 289-307.
Johnson, M. S. and R. Black. 1982. Chaotic genetic patchiness in an intertidal limpet, Siphonaria
sp. Mar. BioI. 70: 157-164.
King, J. C. 1955. Evidence for the integration of the gene pool from studies of DDT resistance in
Drosophila. Cold Spring Harbor Symp. Quant. BioI. 20: 311-317.
Koehn, R. K., R. I. E. Newell and F. Immermann.
1980. Maintenance of an aminopeptidase allele
frequency cline by natural selection. Proc. Natl. Acad. Sci. U.S.A. 77: 5385-5389.
--,
A. J. Zera and J. G. Hall. 1983. Enzyme polymorphism and natural selection. Pages 115136 in M. Nei and R. K. Koehn, eds. Evolution of genes and proteins. Sinauer, Sunderland,
Massachusetts.
Levinton, J. S. 1980. Genetic divergence in estuaries. Pages 509-520 in Y. S. Kennedy, ed. Estuarine
perspectives. Academic Press, New York.
Marcus, N. H. 1977. Genetic variation within and between geographically separated populations of
the sea urchin, Arbacia punetulata. BioI. Bull. 153: 560-576.
Schopf, T. 1. M. and J. L. Gooch. 1971. Gene frequencies in a marine ectoproct: a cline in natura]
populations related to sea temperature. Evolution 25: 286-289.
Slatkin, M. ] 981. Estimating levels of gene flow in natural populations. Genetics 99:323-335.
--.
]985. Rare alleles as indicators of gene flow. Evolution 39: 53-65.
Struhsaker, J. W. 1968. Selection mechanisms associated with intraspecific shell variation in Littorina
pieta (prosobranchia: Mesogastropoda). Evolution 22: 459-480.
Theisen, B. F. 1978. Allozyme clines and evidence of strong selection at three loci in Mytilus edu/is
(Bivalvia) from Danish waters. Ophelia 17: 135-142.
Tracey, M. L., K. Nelson, D. Hedgecock, R. A. Shleser and M. L. Pressick. 1975. Biochemical
genetics oflobsters: genetic variation and the structure of American lobster (Homarus amerieanus)
populations. 1. Fish. Res. Board Can. 32: 2091-2101.
BURTON:CONSEQUENCES
OF RESTRICTED
GENEFLOW
535
Wallace, B. 1981. Basic population genetics. Columbia University Press, New York. 688 pp.
Willson, M. F. 1984. Mating patterns in plants. Pages 261-276 in R. Dirzo and J. Sarukhan, eds.
Perspectives on plant population ecology. Sinauer, Sunderland, Massachusetts.
Wright, S. 1977. Evolution and the genetics of populations, Yol. 3: Experimental results and evolutionary deductions. University of Chicago Press, Chicago. 611 pp.
DATEACCEPTED: February II, 1986.
ADDRESS: Department of Biology, University of Pennsylvania. Philadelphia, Pennsylvania 19104.