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Geographical
Variation
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Timothy A Mousseau University of South Carolina, Columbia, South Carolina, USA
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Alexander E Olvido University of Nebraska, Lincoln, Nebraska, USA
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Advanced
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doi:10.1038/npg.els.0001736
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Geographical
variation refers to differences among populations of organisms in
genetically based traits across the geographical range of that species.
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Introduction: Why
Study Geographical Variation?
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Elucidating the mechanisms that create and
maintain geographical variation can reveal important facets of evolution.
Since evolutionary response depends on genetic variation, factors affecting
the amount of genetic variation within and among populations are of
fundamental importance. The study of geographical variation can provide
insights to the adaptive mechanisms that operate in the wild, and
ultimately in the divergence of populations into genetically distinct
units, i.e. species. Thus geographical variation is the starting point for
understanding the generation of biodiversity. Further, an understanding of
the genetic basis to geographical variation that presently exists may
provide the basis for predicting how populations and species will adapt to
future environmental changes.
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The factors that give rise to genetic
variation in a population are also responsible for creating and maintaining
geographical variation. It is useful to divide these factors into two
groups: random effects that arise as a function of population size,
mutation rates, demography and proximity to other populations, and those
that result from the effects of natural selection.
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Creating and
Maintaining Geographical Variation
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In populations of finite size, random
genetic drift will promote the loss of genetic variation as well as changes
in allele frequencies (Figure 1). The effects of
drift are inversely proportional to population size, with small populations
showing the greatest effects. In the absence of mutation or immigration
small populations will eventually lose much of their genetic variation due
to drift. Also, populations that are initiated by few individuals will tend
to be different from other such populations as a consequence of founder
effects (Figure 2). Founder effects
occur because founders will have a small random selection of the genes
found in the parental population. These properties of small population sizes
are of great importance to conservation biology. However, most populations
experience some degree of genetic mutation that will tend to maintain
genetic variation. In addition, although gene flow among populations will
maintain local variation it will also act to homogenize variation among
populations (Figure 3). Without gene flow,
the original species range will become fragmented into localized gene
pools. Over time, as a consequence of genetic isolation, founder effects
and drift, populations are expected to diverge. See also: Drift: theoretical aspects
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Figure 1
The effects of population size on random genetic drift. This figure was
created using the Populus computer simulation
program available from Dr Donald Alstad at the University of Minnesota
(http://biosci.cbs.umn.edu/software/populus.html). The top panel
illustrates the rapid loss of genetic variation often observed in small
populations. The bottom panel demonstrates that large populations ar ...
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Figure 2
Founder effects and population differentiation. When a new population is
initiated by one or a few individuals dispersing from a larger,
genetically heterogeneous source population, these ‘satellite’
populations may have different gene frequencies from the source (and from
each other) as a result of founder effects. Founder effects are believed
to play a role in rapid divergence and speciati
...
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Figure 3
The effects of migration on genetic variation within and among
populations. In small populations any migration among populations will
tend to eliminate genetic differentiation and increase genetic variation within
populations (top). In the absence immigration, genetic drift will lead to
differentiation among populations and the loss of genetic variation
within populations (bottom). ...
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Environmental differences due to biotic or
abiotic factors between regions can also lead to
variation among populations that is a result of natural selection. For
example, the presence or absence of predators, parasites, hosts or
competitors are biotic factors that can select for geographical variation.
Variation in climate and nutrient availability are examples of abiotic factors that can result in geographic variation
among populations. The degree of divergence among populations will reflect
a balance between the effects of selection imposed on local populations, and
the amount of gene flow among divergent populations. In regions of high
gene flow (i.e. immigration among populations) selection must be very
strong in order to maintain genetically based differences among
populations. In the absence of gene flow local populations can respond to
local selection unimpeded by the deleterious effects of gene flow from neighbouring populations.
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Clinal Variation
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A particularly interesting form of
geographical variation is a cline, or ‘a gradient
in a measurable character’ (Endler, 1977, p. 6). Clines can be
organized into two general categories, graded and stepped, although the
distinction may only depend on the scale of investigation and not really on
the environmental differences that produce the cline. Regardless of
perspective, clines can signify adaptive responses of organisms to natural
selection. Clines are frequently observed in response to variation in
latitude, altitude, ocean or lake depth, or nutrient gradients (Figure 4). The observation of repeated, independently
evolved clines can be very strong evidence for the action of natural
selection on organismal variation. For example, a
number of cricket species from Japan and North America show similar
latitudinal clines in body size with body size smallest in northern
populations. The fact that this clinal pattern
has evolved independently in several different systems and geographical
locations strongly supports the hypothesis that natural selection favours small size in cold, northern environments.
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Figure 4
Clinal variation in body size.
When a trait shows gradual change along an environmental gradient (e.g.
latitude or altitude) this is called a cline. Many endotherms
(e.g. birds and mammals) show an increase in body size from south to
north, and a pattern of variation known as Bergmann’s rule. Many ectotherms (e.g. insects) show the ‘converse to
Bergmann’s rule’, whereby body size decreases w
...
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Genetic versus
Environmental Causes of Variation
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Patterns of variation in the wild can
result either from genetically based differences among populations, or from
the effects of environmental variation. Many organisms display some degree
of phenotypic plasticity that is caused by differences in the environment. For
example, humans tan in response to ultraviolet radiation, while trees grown
in shade tend to have larger leaves than the same trees grown in direct
sunlight. Given the widespread incidence of environmentally induced
plasticity, it is quite possible that many of the reported cases of
geographical variation in plants and animals are the result of development
in different environments and not due to genetically based differences
among populations. This distinction is critical since only genetically
based differences among populations are likely to result in local
adaptation and speciation. The simplest test for the genetic basis to
variation among populations is the common garden experiment. Here
representatives from each study population are raised in a greenhouse or in
the laboratory in a common environment. If phenotypic differences persist
following two or more generations in a common environment this provides
strong evidence for genetic differences among the study populations. It is
important that common garden tests be conducted for several generations
(minimum of two) since environmental effects are often transmitted to
offspring via maternal effects that can extend for
several generations. The common garden approach can be extended to multiple
environments to determine if similarities or differences among populations
are consistent under a variety of rearing conditions. For organisms that
cannot be raised in the laboratory, the reciprocal transplant technique can
be used. In this design individuals from two or more populations are
reciprocally transplanted into each of the environments. As with the common
garden technique, if phenotypic differences persist even when individuals
are reared side-by-side in the different environments, this provides strong
evidence that phenotypic differences are genetically based. As with the
simple common garden approach it is necessary to conduct the experiment
through two or more generations to remove potentially confounding maternal
effects.
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Studies of
Geographical Variation
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Sickle cell anaemia
and balanced polymorphism in human populations
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Human populations display a wide range of
genetically based variation. Perhaps the best known concerns the
distribution of alleles for sickle cell anaemia.
The sickle cell allele results from an amino acid substitution at the
structural-gene locus for the β chain of haemoglobin.
The abnormal haemoglobin crystallizes at low
oxygen tension, and the red blood cells deform and haemolyse.
Individuals with two sickle cell alleles (i.e. homozygotes)
have severe anaemia, and survivorship is low. Heterozygotes (i.e. one sickle cell allele and one
normal allele) have a mild anaemia and under
ordinary circumstances have the same or only slightly lower fitness than
normal homozygotes. However, in regions of Africa with a high
incidence of the malarial parasite, Plasmodium falciparum,
heterozygotes have a higher fitness than normal homozygotes because the presence of some sickling haemoglobin protects
them from malaria by selective removal of the parasite from the
bloodstream. Where malaria is absent, as in North America, the fitness advantage
of heterozygosity is lost. In this case,
geographic variation in the incidence of malaria (a biotic factor) selects
for genetic variation in human populations. See also: Fitness
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Warfarin resistance in
wild rats
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The brown rat, Rattus
norvegicus, has evolved resistance to the rodenticide, warfarin, in
regions where this chemical has been used to control population size. Warfarin is an anticoagulant that prevents blood
clotting via disruption of the vitamin K cycle. Studies have shown that a
single, dominant gene controls resistance to warfarin
and that different alleles have independently evolved in different
geographical locations (Greaves, 1985). As with sickle
cell anaemia in humans, the alleles bestowing
resistance to warfarin in rats are believed to be
maintained as a consequence of heterozygote advantage in regions where rats
are extensively poisoned. Individuals that possess two resistance alleles
suffer impairment of the vitamin K cycle and may have lower fitness in the
wild due to dramatically increased needs for dietary sources of vitamin K,
while homozygous susceptible genotypes are prone to death in regions where warfarin is used as a pesticide. Recently, it has been
found that heterozygous individuals from a population from southern England suffer no fitness
costs and may have an advantage over homozygous susceptible genotypes even
in the absence of poison (Smith et al., 1993). It
is very likely that this new resistance allele, which shows no cost, will
spread to other populations even in the absence of warfarin
use. See also: Vitamin K: structure and function
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Industrial melanism
in the peppered moth
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Industrial melanism
in the peppered moth, Biston betularia, is another well-documented case of a
trait that behaves as a single-locus polymorphism. For a thorough review of
this phenomenon see Majerus (1998). The peppered moth
possesses several genetically based colour
polymorphisms that result from different alleles. There are three basic
forms of B. betularia, with several grades
of melanism in between each: carbonaria
(fully melanic), insularia
(dark with white speckling or white fringes on wings), and typica (speckled).
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From the mid-1700s onward, coal production
and consumption changed the landscape of many northern European cities.
Particularly in and around industrialized areas, lichens that once covered
tree trunks disappeared, probably as a consequence of acid rains, and were
replaced with ash and soot from the accompanying smoke. Numerous studies
since then (e.g. Bleasdale, 1952; Went, 1955; Hawksworth | |
