4.2.9 Molecular Mechanisms in the Evolution of Complex Characters
129
rinase of insects. The evolution of new enzymes
can be observed in bacterial experiments; for
example, during the culture of ~-galactosidase
(lacZ)-deficient E. coli mutants on lactosecontaining medium, a new enzyme, known as
"evolved ~-galactosidase" (Ebg) appears. All that
is needed here are a few mutations in the gene
ebg 0, a weak catalytic ~-galactosidase found in
the wild type, and in the gene egb R of the associated repressor [239].
The de novo synthesis of a longer DNA
sequence coding for a functional protein would
appear to be excluded on the grounds of probability. It is also difficult to imagine the appearance of
a completely novel protein by the diversifying
evolution of a duplicated gene. Gradual alteration of a DNA sequence could never really produce a functional structural gene. Frame-shift
mutations and other lethal changes would accumulate in a gene that has become inactive such
that as a whole it could hardly become active
again. Thus, at the molecular level the evolution
of novelty may come mainly by a process
described by Jacob as "tinkering" [98]. By this it
is meant that evolution, unlike technology, does
not build new structure from scratch but achieves
novelty by the modification and recombination of
what is already available, in the same way that a
father constructs a toy car for his son from the
wheels of an old pushchair, an orange box and
other bits and pieces. The molecular mechanisms
of tinkering include, in particular, the processes
of gene rearrangement, i.e. the insertion or deletion of longer sequences, duplication, transposition and fusion of gene segments (exon shuffling) , and also perhaps gene transfer across species boundaries.
4.2.9 Molecular Mechanisms in the Evolution
of Complex Characters
It has been shown in many investigations that
there is no strong correlation between the rate
and extent of molecular evolution and the evolution of morphological and other complex characters of the phenotype. The use of molecular data
in the construction of species' ancestral trees
gives, in many cases, branching schemes similar
to those based on morphological characters,
although the lengths of the branches, which indicate the extent of the changes, are usually quite
different. Evolutionary changes occur, on the one
hand, within a line of evolution (anagenesis) and,
on the other hand, at the separation of the lines
(cladogenesis or speciation). It is controversial
whether the evolutionary alteration of morphological characters predominantly occurs anagenetically (gradualism) or during speciation (punctuationalism) [72, 107,301]. The evolution of proteins is, in any case, not accelerated by the formation of species; the genetic distances determined
by enzyme electrophoresis are not larger in
species-rich groups of animals than in those with
few species [14, 16].
The rate of phenotypic evolution differs quite
markedly between and within different lines of
evolution; one extreme is represented by species
referred to as "living fossils", whose morphological characters have remained constant over
many millions of years [106]. Such species include,
for example, the brachiopod genus Lingula,
which looked exactly the same in the Silurian
400 million years ago as it does today; the horseshoe crab Limulus (Xiphosura); or, amongst the
vertebrates, the lizard Sphenodon, the crocodile
and the opossum, all of which have hardly
changed since the Jurassic. In contrast to their
morphological evolution, the molecular evolution
of the living fossils has in no way been retarded;
e.g. the genetic variability of natural populations
of Limulus is not less than that in other animals
(see Table 4.8). So-called sibling species are
hardly distinguishable morphologically, but at a
molecular level they are no less different than
other closely related species (see Fig. 4.18).
There are, however, also groups of species
which show an especially rapid morphological
evolution and whose molecular similarity is much
greater than the morphological similarity. This is
true, for example, of the Hawaiian Drosophila
species D. silvestris and D. heteroneura, which are
easily distinguishable morphologically but have
only a small genetic distance (D) of 0.063 [370]. A
similarly low molecular distance is shown by the
approximately 300 species of the chequered perch
(Cichlidae), which are found only in the African
Malawi lake and are apparently the result of very
rapid speciation [219], and the five North American species of toothed carp, belonging to the
genus Cyprinodon [415]. In Israel, the blind
mouse, Spalax ehrenbergi, has several forms with
different chromosome numbers (2n = 52, 54 or
60) and these forms are sexually isolated due to
behavioural characteristics, but they have only a
very low genetic distance of 0.035 [366].
Two particularly convincing examples for the
low correlation between molecular and morphological differences are the comparison
between the anurans and the placental mammals,
129
rinase of insects. The evolution of new enzymes
can be observed in bacterial experiments; for
example, during the culture of ~-galactosidase
(lacZ)-deficient E. coli mutants on lactosecontaining medium, a new enzyme, known as
"evolved ~-galactosidase" (Ebg) appears. All that
is needed here are a few mutations in the gene
ebg 0, a weak catalytic ~-galactosidase found in
the wild type, and in the gene egb R of the associated repressor [239].
The de novo synthesis of a longer DNA
sequence coding for a functional protein would
appear to be excluded on the grounds of probability. It is also difficult to imagine the appearance of
a completely novel protein by the diversifying
evolution of a duplicated gene. Gradual alteration of a DNA sequence could never really produce a functional structural gene. Frame-shift
mutations and other lethal changes would accumulate in a gene that has become inactive such
that as a whole it could hardly become active
again. Thus, at the molecular level the evolution
of novelty may come mainly by a process
described by Jacob as "tinkering" [98]. By this it
is meant that evolution, unlike technology, does
not build new structure from scratch but achieves
novelty by the modification and recombination of
what is already available, in the same way that a
father constructs a toy car for his son from the
wheels of an old pushchair, an orange box and
other bits and pieces. The molecular mechanisms
of tinkering include, in particular, the processes
of gene rearrangement, i.e. the insertion or deletion of longer sequences, duplication, transposition and fusion of gene segments (exon shuffling) , and also perhaps gene transfer across species boundaries.
4.2.9 Molecular Mechanisms in the Evolution
of Complex Characters
It has been shown in many investigations that
there is no strong correlation between the rate
and extent of molecular evolution and the evolution of morphological and other complex characters of the phenotype. The use of molecular data
in the construction of species' ancestral trees
gives, in many cases, branching schemes similar
to those based on morphological characters,
although the lengths of the branches, which indicate the extent of the changes, are usually quite
different. Evolutionary changes occur, on the one
hand, within a line of evolution (anagenesis) and,
on the other hand, at the separation of the lines
(cladogenesis or speciation). It is controversial
whether the evolutionary alteration of morphological characters predominantly occurs anagenetically (gradualism) or during speciation (punctuationalism) [72, 107,301]. The evolution of proteins is, in any case, not accelerated by the formation of species; the genetic distances determined
by enzyme electrophoresis are not larger in
species-rich groups of animals than in those with
few species [14, 16].
The rate of phenotypic evolution differs quite
markedly between and within different lines of
evolution; one extreme is represented by species
referred to as "living fossils", whose morphological characters have remained constant over
many millions of years [106]. Such species include,
for example, the brachiopod genus Lingula,
which looked exactly the same in the Silurian
400 million years ago as it does today; the horseshoe crab Limulus (Xiphosura); or, amongst the
vertebrates, the lizard Sphenodon, the crocodile
and the opossum, all of which have hardly
changed since the Jurassic. In contrast to their
morphological evolution, the molecular evolution
of the living fossils has in no way been retarded;
e.g. the genetic variability of natural populations
of Limulus is not less than that in other animals
(see Table 4.8). So-called sibling species are
hardly distinguishable morphologically, but at a
molecular level they are no less different than
other closely related species (see Fig. 4.18).
There are, however, also groups of species
which show an especially rapid morphological
evolution and whose molecular similarity is much
greater than the morphological similarity. This is
true, for example, of the Hawaiian Drosophila
species D. silvestris and D. heteroneura, which are
easily distinguishable morphologically but have
only a small genetic distance (D) of 0.063 [370]. A
similarly low molecular distance is shown by the
approximately 300 species of the chequered perch
(Cichlidae), which are found only in the African
Malawi lake and are apparently the result of very
rapid speciation [219], and the five North American species of toothed carp, belonging to the
genus Cyprinodon [415]. In Israel, the blind
mouse, Spalax ehrenbergi, has several forms with
different chromosome numbers (2n = 52, 54 or
60) and these forms are sexually isolated due to
behavioural characteristics, but they have only a
very low genetic distance of 0.035 [366].
Two particularly convincing examples for the
low correlation between molecular and morphological differences are the comparison
between the anurans and the placental mammals,