166
4 Molecular Evolution
urchins Echinostreptus aciculatus and E. molaris
[265], the cephalopods Eledone massyae and
E. gaucha [235], the New Zealand species of
mackerel Trachurus declivis and T. novaezelandiae
[128], and the field-mouse species Microtus arvalis and M. subarvalis [453]. The ciliates of the
genus Tetrahymena can be divided into a complex
of more than 30 sibling species on the basis of
their rRNA sequences [336]. Species diagnosis in
the disease-transmitting mosquitoes of the genus
Anopheles, which are morphologically difficult to
distinguish, has made use of electrophoretic
methods as well as gas chromatographic analysis
of the cuticular hydrocarbons [56].
Of particular interest are cases in which apparently true species were first recognized after electrophoretic analysis as being pairs or groups of
sibling species. For example, the marine polychaete Capitella capitata, which as an indicator of
water pollution is of practical importance, turned
out to be really a complex of at least six sibling
species [147]. The Hawaiian teleost Albula virgata
could, by electrophoretic analysis, be separated
into two completely isolated populations, for
which only later diagnostic morphological differences were found [371]. The leopard frog Rana
pipiens, used in numerous experimental investigations, is apparently a complex of at least
eight sibling species which mostly live in separate
habitats (allopatric) and, in addition to possessing
electrophoretic differences, are also distinguishable by their mating calls [174]. Enzyme electrophoresis can also help to clarify the taxonomic
situation when morphologically quite different
individuals are mistakenly assigned to different
species. An example here is provided by the ant
species Formica rufa, R polyctena and R pratensis, which all have the same pattern of 68 protein
bands and 18 esterase bands. The morphological
variability in this case is so great that even animals from the same nest appear to belong to different species [408].
A further interesting biological-taxonomic
application of biochemical characters is the identification of hybrid species. Because, in contrast
to most morphological characters, molecular features are inherited co-dominantly, i.e. both alleles
are always recognizable, hybrids can be clearly
recognized by electrophoretic investigation. Crosses between true species are rarely found in the
field, even when they are easy to achieve in the
laboratory. Thus, at least 159 different hybrid species of Drosophila have so far been bred in the
laboratory, although only eight cases are known
in nature [152]. In most cases hybrid species are
sterile or, at least, show reduced fertility. In spite
of this, naturally occurring hybrids have been
detected for a whole series of species pairs using
electrophoretic methods. It turns out that on
average 0.4 %, and locally up to 5.9 %, of the
"salmon" caught in Irish rivers and lakes are
hybrids between the salmon species Salmo salar
and the trout S. trutta [82]. Hybrids occur especially in the overlap zone between neighbouring
habitats of two species as, for example, in the
case of the toads Bufo boreas and B. punctatus in
California, and Bombina bombina and B. variegata in Poland, and the marine turtles Eretmochelys imbricata and Caretta caretta on the coast of
Brazil [n, 110]. Hybrid species are more frequent
in fish and amphibians than in the reptiles, birds
and mammals. This is at least in part related to
the different rates of morphological and molecular evolution (p.129). Naturally occurring hybrid
species are also known in the invertebrates, e.g.
between the ants Solenopsis geminata and S. xyloni in Texas, or between the mussels Mytilus edulis and M. galloprovincialis on the English coast
[184, 380]. Molecular methods may also allow the
correction of false conclusions, which are based
on mistakes in the interpretation of morphological data, relating to hybrid species. For example, it was previously claimed that up to 29 % of
the populations of the darter (perch) Etheostoma
spectabile and E. caeruleum, living in the same
habitat (sympatric), were hybrid species. Tests
with enzyme electrophoresis, however, gave no
indication of hybrids but showed instead that
single individuals of E. caeruleum could be easily
confused visually with E. spectabile [271].
By far the most interesting example of naturally occurring hybrid species is that of the European aquatic or green frogs. In the course of the
evolution of these species, special genetic mechanisms arose by which the normal problems of
chromosome pairing during meiosis in hybrids
were overcome. The central European aquatic
frog Rana esculenta, which was originally
described by Linnaeus and has since been used in
innumerable physiological investigations, is really
a hybrid between the species R. ridibunda and
R.lessonae; this has been shown in various
investigations since 1968, above all by the electrophoretic examination of serum albumins and various enzymes and also by restriction analysis
of mtDNA. In southern France, R. ridibunda
hybridized with R. perezi, and in Italy and the
Balkans it hybridized with lessonae-like species;
hybrids between R. nigromaculata and R. brevipoda have been found in Japan [392]. The central
4 Molecular Evolution
urchins Echinostreptus aciculatus and E. molaris
[265], the cephalopods Eledone massyae and
E. gaucha [235], the New Zealand species of
mackerel Trachurus declivis and T. novaezelandiae
[128], and the field-mouse species Microtus arvalis and M. subarvalis [453]. The ciliates of the
genus Tetrahymena can be divided into a complex
of more than 30 sibling species on the basis of
their rRNA sequences [336]. Species diagnosis in
the disease-transmitting mosquitoes of the genus
Anopheles, which are morphologically difficult to
distinguish, has made use of electrophoretic
methods as well as gas chromatographic analysis
of the cuticular hydrocarbons [56].
Of particular interest are cases in which apparently true species were first recognized after electrophoretic analysis as being pairs or groups of
sibling species. For example, the marine polychaete Capitella capitata, which as an indicator of
water pollution is of practical importance, turned
out to be really a complex of at least six sibling
species [147]. The Hawaiian teleost Albula virgata
could, by electrophoretic analysis, be separated
into two completely isolated populations, for
which only later diagnostic morphological differences were found [371]. The leopard frog Rana
pipiens, used in numerous experimental investigations, is apparently a complex of at least
eight sibling species which mostly live in separate
habitats (allopatric) and, in addition to possessing
electrophoretic differences, are also distinguishable by their mating calls [174]. Enzyme electrophoresis can also help to clarify the taxonomic
situation when morphologically quite different
individuals are mistakenly assigned to different
species. An example here is provided by the ant
species Formica rufa, R polyctena and R pratensis, which all have the same pattern of 68 protein
bands and 18 esterase bands. The morphological
variability in this case is so great that even animals from the same nest appear to belong to different species [408].
A further interesting biological-taxonomic
application of biochemical characters is the identification of hybrid species. Because, in contrast
to most morphological characters, molecular features are inherited co-dominantly, i.e. both alleles
are always recognizable, hybrids can be clearly
recognized by electrophoretic investigation. Crosses between true species are rarely found in the
field, even when they are easy to achieve in the
laboratory. Thus, at least 159 different hybrid species of Drosophila have so far been bred in the
laboratory, although only eight cases are known
in nature [152]. In most cases hybrid species are
sterile or, at least, show reduced fertility. In spite
of this, naturally occurring hybrids have been
detected for a whole series of species pairs using
electrophoretic methods. It turns out that on
average 0.4 %, and locally up to 5.9 %, of the
"salmon" caught in Irish rivers and lakes are
hybrids between the salmon species Salmo salar
and the trout S. trutta [82]. Hybrids occur especially in the overlap zone between neighbouring
habitats of two species as, for example, in the
case of the toads Bufo boreas and B. punctatus in
California, and Bombina bombina and B. variegata in Poland, and the marine turtles Eretmochelys imbricata and Caretta caretta on the coast of
Brazil [n, 110]. Hybrid species are more frequent
in fish and amphibians than in the reptiles, birds
and mammals. This is at least in part related to
the different rates of morphological and molecular evolution (p.129). Naturally occurring hybrid
species are also known in the invertebrates, e.g.
between the ants Solenopsis geminata and S. xyloni in Texas, or between the mussels Mytilus edulis and M. galloprovincialis on the English coast
[184, 380]. Molecular methods may also allow the
correction of false conclusions, which are based
on mistakes in the interpretation of morphological data, relating to hybrid species. For example, it was previously claimed that up to 29 % of
the populations of the darter (perch) Etheostoma
spectabile and E. caeruleum, living in the same
habitat (sympatric), were hybrid species. Tests
with enzyme electrophoresis, however, gave no
indication of hybrids but showed instead that
single individuals of E. caeruleum could be easily
confused visually with E. spectabile [271].
By far the most interesting example of naturally occurring hybrid species is that of the European aquatic or green frogs. In the course of the
evolution of these species, special genetic mechanisms arose by which the normal problems of
chromosome pairing during meiosis in hybrids
were overcome. The central European aquatic
frog Rana esculenta, which was originally
described by Linnaeus and has since been used in
innumerable physiological investigations, is really
a hybrid between the species R. ridibunda and
R.lessonae; this has been shown in various
investigations since 1968, above all by the electrophoretic examination of serum albumins and various enzymes and also by restriction analysis
of mtDNA. In southern France, R. ridibunda
hybridized with R. perezi, and in Italy and the
Balkans it hybridized with lessonae-like species;
hybrids between R. nigromaculata and R. brevipoda have been found in Japan [392]. The central