146
4 Molecular Evolution
other Drosophila species show less alcohol tolerance [166]. Very similar ADH electromorphs are
found in the flour moth, Ephestia kuhniella and
in D. melanogaster, although the former never
encounters alcohols in its natural habitat [231]. In
experiments with the kelp fly, Coelopa frigida,
the Adh allele frequency was not affected by
0.2 % alcohol in the medium, perhaps because
here, unlike in Drosophila, ADH has no detoxifying function [86].
Correlation of the allele frequency with the
geographical site or ecological factors is often
given as evidence for the effect of selection. The
directed alteration of a gene frequency over a
geographical distance is known as a cline, according to the concept derived by Huxley in 1939.
Many environmental parameters also show directed site dependency such that, for example, a
north/south cline can be looked upon as an
adaptation to temperature. However, which of
the many site-dependent environmental parameters is really effective in the selection of a given
locus can, at best, be presumed. Neutral alleles
also form frequency gradients as they disperse
from the site of the mutation. There are many
examples of clines, and particularly of north/
south clines. In the sea anemone Metridium
senile, found on the east coast of North America,
the frequency of the alleloenzymes PGI-F of glucose phosphate isomerase declines from almost
100 % in the south to only 50 % in the north. The
explanation offered for this is that, based on the
temperature dependency of its kinetic properties,
PGI-F is better adapted than PGI-S to higher
temperatures in the surroundings. As in many
such conclusions, the question remains open as to
exactly which selective advantage maintains the
relatively high frequency of the PGI-S allele in
the northern population [178]. In the mussel Guekensia demissa, the frequency of a cryptic phosphoglucomutase allele, which was identified by its
higher temperature sensitivity, is correlated with
the average water temperature [145].
There are many particularly appropriate examples in the insects. The cline of the alcohol dehydrogenase of Drosophila melanogaster in the eastern USA has already been mentioned (p. 145); a
similar cline also exists in Australia but is correlated more with rainfall than with temperature.
The 6-phosphogluconate dehydrogenase of the
mosquito Culex pipiens is a dimeric enzyme for
which two subunit alleles, F and S, are known.
The frequency of S declines from 30 % in the
north to below 10 % in the south. This may be
pardy explained by the fact that the temperature
stability increases in the order SS < FS < FF;
however, the exact selective advantage of S is
again not clear. Further enzymes of this species
also show north/south clines [337, 338]. In various fish species, north/south clines have been
found for lactate dehydrogenase and other enzymes [330, 351]. Selection is indicated as the
cause of polymorphism when parallel clines
appear in closely related species existing in close
proximity (sympatric), or the same cline is seen on
different continents for widely dispersed species.
Thus, the sympatric cricket species Gryllus veletis
and G. pennsylvanicus, which are phenotypically
very similar but sexually isolated, show the same
type of site dependency in the frequency of glucose phosphate isomerase alleles [159]. A similar
dependency on geographical range is found for a
whole series of polymorphic enzymes in populations of Drosophila melanogaster from all continents [7]. If a protein polymorphism allows
adaptation to climatic factors, it should show
regular seasonal changes; however, no annual
periodicity was detected, for example, in 12
enzyme loci in D. melanogaster [61].
In several cases, a correlation of gene frequency with climatic or ecological factors has
been directly demonstrated. Two populations of
the barnacle Balanus amphitrite live in the inflowing and outflowing cooling-water canals of a
power station in Haifa, with a temperature difference of 9-12 °e. The animals in the cooler canal
are three times larger and have a four-fold higher
population density than those in the warmer
canal. Differences in allele frequency were found
at 8 out of 12 loci, and these may be looked upon
as adaptation to the different conditions [299]. In
Drosophila simulans, sequential electrophoresis
distinguishes 27 alleles of esterase 6; the pattern
relates more to the available fruit than to the
location and thus represents ecological races [3].
The electrophoregrams of various enzymes of
aphid species reveal host-plant-specific differences [393]. Amongst different North American
Drosophila species, whose larvae develop on
cacti, D.longicornis, which is restricted to Opuntia, shows only half as much polymorphism at
four enzyme loci as the species D. mojavensis and
D. arizonensis, which live on various cactus genera [345]. The mosquito species Anopheles aquasalis, whose larvae require salt water for their
development, have a significantly lower polymorphism (H = 0.081) than five related species
with H = 0.17-0.27 [284].
Coincidental electrophoretic patterns in sexually isolated populations are commonly observed
4 Molecular Evolution
other Drosophila species show less alcohol tolerance [166]. Very similar ADH electromorphs are
found in the flour moth, Ephestia kuhniella and
in D. melanogaster, although the former never
encounters alcohols in its natural habitat [231]. In
experiments with the kelp fly, Coelopa frigida,
the Adh allele frequency was not affected by
0.2 % alcohol in the medium, perhaps because
here, unlike in Drosophila, ADH has no detoxifying function [86].
Correlation of the allele frequency with the
geographical site or ecological factors is often
given as evidence for the effect of selection. The
directed alteration of a gene frequency over a
geographical distance is known as a cline, according to the concept derived by Huxley in 1939.
Many environmental parameters also show directed site dependency such that, for example, a
north/south cline can be looked upon as an
adaptation to temperature. However, which of
the many site-dependent environmental parameters is really effective in the selection of a given
locus can, at best, be presumed. Neutral alleles
also form frequency gradients as they disperse
from the site of the mutation. There are many
examples of clines, and particularly of north/
south clines. In the sea anemone Metridium
senile, found on the east coast of North America,
the frequency of the alleloenzymes PGI-F of glucose phosphate isomerase declines from almost
100 % in the south to only 50 % in the north. The
explanation offered for this is that, based on the
temperature dependency of its kinetic properties,
PGI-F is better adapted than PGI-S to higher
temperatures in the surroundings. As in many
such conclusions, the question remains open as to
exactly which selective advantage maintains the
relatively high frequency of the PGI-S allele in
the northern population [178]. In the mussel Guekensia demissa, the frequency of a cryptic phosphoglucomutase allele, which was identified by its
higher temperature sensitivity, is correlated with
the average water temperature [145].
There are many particularly appropriate examples in the insects. The cline of the alcohol dehydrogenase of Drosophila melanogaster in the eastern USA has already been mentioned (p. 145); a
similar cline also exists in Australia but is correlated more with rainfall than with temperature.
The 6-phosphogluconate dehydrogenase of the
mosquito Culex pipiens is a dimeric enzyme for
which two subunit alleles, F and S, are known.
The frequency of S declines from 30 % in the
north to below 10 % in the south. This may be
pardy explained by the fact that the temperature
stability increases in the order SS < FS < FF;
however, the exact selective advantage of S is
again not clear. Further enzymes of this species
also show north/south clines [337, 338]. In various fish species, north/south clines have been
found for lactate dehydrogenase and other enzymes [330, 351]. Selection is indicated as the
cause of polymorphism when parallel clines
appear in closely related species existing in close
proximity (sympatric), or the same cline is seen on
different continents for widely dispersed species.
Thus, the sympatric cricket species Gryllus veletis
and G. pennsylvanicus, which are phenotypically
very similar but sexually isolated, show the same
type of site dependency in the frequency of glucose phosphate isomerase alleles [159]. A similar
dependency on geographical range is found for a
whole series of polymorphic enzymes in populations of Drosophila melanogaster from all continents [7]. If a protein polymorphism allows
adaptation to climatic factors, it should show
regular seasonal changes; however, no annual
periodicity was detected, for example, in 12
enzyme loci in D. melanogaster [61].
In several cases, a correlation of gene frequency with climatic or ecological factors has
been directly demonstrated. Two populations of
the barnacle Balanus amphitrite live in the inflowing and outflowing cooling-water canals of a
power station in Haifa, with a temperature difference of 9-12 °e. The animals in the cooler canal
are three times larger and have a four-fold higher
population density than those in the warmer
canal. Differences in allele frequency were found
at 8 out of 12 loci, and these may be looked upon
as adaptation to the different conditions [299]. In
Drosophila simulans, sequential electrophoresis
distinguishes 27 alleles of esterase 6; the pattern
relates more to the available fruit than to the
location and thus represents ecological races [3].
The electrophoregrams of various enzymes of
aphid species reveal host-plant-specific differences [393]. Amongst different North American
Drosophila species, whose larvae develop on
cacti, D.longicornis, which is restricted to Opuntia, shows only half as much polymorphism at
four enzyme loci as the species D. mojavensis and
D. arizonensis, which live on various cactus genera [345]. The mosquito species Anopheles aquasalis, whose larvae require salt water for their
development, have a significantly lower polymorphism (H = 0.081) than five related species
with H = 0.17-0.27 [284].
Coincidental electrophoretic patterns in sexually isolated populations are commonly observed