mes". The analysis of alleloenzymes has become
one of the most important tools in population
genetics, with the help of which it is possible, for
example, to decide whether two groups of individuals are exchanging genes, i.e. they constitute a
population, or are sexually isolated and form two
species.
By the use of molecular characters, it has been
possible to recognize very similar animals as
members of different species (sibling species),
and to demonstrate the hybrid nature of apparently bona fide species. Unfortunately, it has proven more difficult than expected to use molecular
data to solve taxonomic problems above the species level; in particular, the old problem of the
relationships between the animal phyla has not
been solved, despite the initial optimism. It is still
problematic to derive a quantitative measure of
evolutionary distance from nucleic acid- or
protein-sequence differences. Even if this should
be achieved, the application of a distance matrix
for constructing a family tree of sequences is still
fraught with difficulties. Such a tree may describe
the evolution of molecules but does not necessarily reflect relationships between the species
from which the molecules came. This is because
gene duplication and subsequent divergent development (diversification) can result in different
genes existing side by side in the same individual.
In spite of such difficulties, there are still convincing arguments in favour of molecular characters
as superior to all others for phylogenetic reconstruction.
1.4 Chance and Necessity
in Molecular Evolution
1.4.1 Non-Adaptive ("Neutral") Differences
Research in comparative biochemistry attempts
to describe structural or functional differences in
biochemical organization: the presence or
absence of particular compounds, differences in
structural characteristics of low or high molecular
weight substances, differences in enzyme activity
or in the concentration of metabolites, or differences in the direction or rate of metabolic processes. Normally, structural and functional differences are considered to be the result of adaptive
events, but one can actually also use the reverse
argument: that adaptation exploits already available differences. Here it is usually quietly
assumed that structural difference always means
1.4.1 Non-Adaptive ("Neutral") Differences
5
functional difference. Such assumptions arise
from the idealistic view that, as a result of selection, all the characters of an organism are optimal
for a particular environment. This may generally
be true for complex anatomical, physiological
and behavioral characters, even when it is rather
awkward to imagine, for example, that the different arrangements of bristles on the bodies of
closely related insects have adaptive significance.
For molecular evolution, the assumption that
each structural difference represents a selective
functional difference can be taken as categorically
disproven; in DNA and subsequent amino acid
substitutions in the resulting polypeptides there
are quite clearly genetic differences that have no
selective significance. Such genetically determined differences can be retained for long periods because they do not disturb, and can be
spread by chance through a population. This is
the central premise of the neutral theory, which
will be dealt with extensively in Chapter 4. Of
course, the concept "neutral" only applies under
particular selection conditions; changes in the
environment can confer on characters a positive
or a negative value. The existence of a "reserve"
of neutral variation in a population is a direct prerequisite for molecular evolution. Today, there
can no longer be any doubt of the existence of
neutral variation; the only discussion can be
about the extent of the genetically determined
differences which lack significant adaptive importance. However, as with all negative statements,
the opinion that a particular molecular difference
has no selection value is basically not provable.
The existence of neutral mutations is important
not only in the comparison of different individuals of one evolutionary line, but also in the comparison of distantly related species. When two
evolutionary lines separate, a range of neutral
variation can accumulate in both such that very
large differences without selective significance
can arise between species. Thus, for example,
proteins with different amino acid sequences can
be functionally equivalent. Epigenetically based
differences between products of the same gene,
e.g. the results of variable mRNA processing or
post-translational changes in proteins, do not
necessarily have adaptive significance, but are
more likely to be the result of chance differences
in enzyme activity. Such indications of the role of
chance in molecular evolution should not deny
the significance of molecular adaptation; this will
in fact be one of the main themes in the following
chapters. It should only be emphasized that it is
not obligatory to enquire each time about the
one of the most important tools in population
genetics, with the help of which it is possible, for
example, to decide whether two groups of individuals are exchanging genes, i.e. they constitute a
population, or are sexually isolated and form two
species.
By the use of molecular characters, it has been
possible to recognize very similar animals as
members of different species (sibling species),
and to demonstrate the hybrid nature of apparently bona fide species. Unfortunately, it has proven more difficult than expected to use molecular
data to solve taxonomic problems above the species level; in particular, the old problem of the
relationships between the animal phyla has not
been solved, despite the initial optimism. It is still
problematic to derive a quantitative measure of
evolutionary distance from nucleic acid- or
protein-sequence differences. Even if this should
be achieved, the application of a distance matrix
for constructing a family tree of sequences is still
fraught with difficulties. Such a tree may describe
the evolution of molecules but does not necessarily reflect relationships between the species
from which the molecules came. This is because
gene duplication and subsequent divergent development (diversification) can result in different
genes existing side by side in the same individual.
In spite of such difficulties, there are still convincing arguments in favour of molecular characters
as superior to all others for phylogenetic reconstruction.
1.4 Chance and Necessity
in Molecular Evolution
1.4.1 Non-Adaptive ("Neutral") Differences
Research in comparative biochemistry attempts
to describe structural or functional differences in
biochemical organization: the presence or
absence of particular compounds, differences in
structural characteristics of low or high molecular
weight substances, differences in enzyme activity
or in the concentration of metabolites, or differences in the direction or rate of metabolic processes. Normally, structural and functional differences are considered to be the result of adaptive
events, but one can actually also use the reverse
argument: that adaptation exploits already available differences. Here it is usually quietly
assumed that structural difference always means
1.4.1 Non-Adaptive ("Neutral") Differences
5
functional difference. Such assumptions arise
from the idealistic view that, as a result of selection, all the characters of an organism are optimal
for a particular environment. This may generally
be true for complex anatomical, physiological
and behavioral characters, even when it is rather
awkward to imagine, for example, that the different arrangements of bristles on the bodies of
closely related insects have adaptive significance.
For molecular evolution, the assumption that
each structural difference represents a selective
functional difference can be taken as categorically
disproven; in DNA and subsequent amino acid
substitutions in the resulting polypeptides there
are quite clearly genetic differences that have no
selective significance. Such genetically determined differences can be retained for long periods because they do not disturb, and can be
spread by chance through a population. This is
the central premise of the neutral theory, which
will be dealt with extensively in Chapter 4. Of
course, the concept "neutral" only applies under
particular selection conditions; changes in the
environment can confer on characters a positive
or a negative value. The existence of a "reserve"
of neutral variation in a population is a direct prerequisite for molecular evolution. Today, there
can no longer be any doubt of the existence of
neutral variation; the only discussion can be
about the extent of the genetically determined
differences which lack significant adaptive importance. However, as with all negative statements,
the opinion that a particular molecular difference
has no selection value is basically not provable.
The existence of neutral mutations is important
not only in the comparison of different individuals of one evolutionary line, but also in the comparison of distantly related species. When two
evolutionary lines separate, a range of neutral
variation can accumulate in both such that very
large differences without selective significance
can arise between species. Thus, for example,
proteins with different amino acid sequences can
be functionally equivalent. Epigenetically based
differences between products of the same gene,
e.g. the results of variable mRNA processing or
post-translational changes in proteins, do not
necessarily have adaptive significance, but are
more likely to be the result of chance differences
in enzyme activity. Such indications of the role of
chance in molecular evolution should not deny
the significance of molecular adaptation; this will
in fact be one of the main themes in the following
chapters. It should only be emphasized that it is
not obligatory to enquire each time about the