ness), i.e. the ability to pass on genes to as many
progeny as possible; selection doesn't always
leave behind just corpses. Two types of selection
can be conceptually distinguished. Negative or
stabilizing selection eliminates those variants that
display reduced fitness because of marked phenotypic changes. Positive selection, i.e. the fixation
of fitness-increasing mutations, appears to be of
rarer occurrence in nature.
Adaptive evolution is often understood to
mean the "choice", made by the process of selection, of one of the many "solutions" available to
the organism because of genetic variation to a
"problem" posed by a change in the environment.
However, the relationships between genes, organisms and the environment are much more complicated, as the following considerations show:
1. The quantitative variability of almost all characteristics of an organism arises during the
unique development of that organism and
results from non-trivial interactions with the
environment; single genes or alleles cannot be
assigned a particular selective value independent of the current environment.
2. Due to the imperfect reproducibility of the
developmental
process
(developmental
noise), various phenotypes may result from
the same genotype in a given environment.
3. Internal and external factors have mutual
influences; for example, in many cases, the
selective pressure of the environment on a particular allele is dependent upon the genetic
milieu or the gene frequency.
4. As each organism is constantly changing, the
phenotype is determined not only by the genotype and the environment but also by the organism's own previous condition.
s. Organisms can actively alter the character of
their environments.
6. The "environment" of most life processes is
the internal milieu of the organism, and this is
largely determined and maintained by regulatory processes, independent of changes in the
outside world [21].
It was originally assumed that the direction and
rate of evolution was determined entirely by the
selection of advantageous variants. Impressed by
the unexpectedly high molecular polymorphism
in natural populations and the relatively constant
rate of protein evolution (the molecular clock),
Kimura, Ohta, King and Jukes formulated in
1968-1969 the "neutral theory" of molecular
evolution [208, 213]. According to this theory,
many perhaps most DNA alterations have such
4 Molecular Evolution
113
little importance for fitness that they are not subjected to any selection pressure and are only fixed
by chance in the population. One could say that
the organism itself does not "notice" such mutations, but they are, nevertheless, detectable by
the precise tools of the molecular biologist. The
lower the importance of the altered character for
fitness, the higher is the probability, and thus the
relative frequency, of selectively neutral alterations. Therefore, the proportion of selectively
neutral alterations decreases in the gene-function
cascade (Fig. 4.1) from the DNA to the complex
morphological and physiological characters; the
neutral theory is applicable most of all to the
evolution of nucleic acids and proteins. Mutations that are neutral at a given point in evolutionary time may have a significant selective
advantage or disadvantage at a later time and
under different internal or external conditions.
Such conditionally neutral variation represents a
reservoir of genetic variability in the population
for later evolutionary processes.
All in all, it may be said that research into
evolution has benefited from the evaluation of
molecular data by a whole series of fundamental
new findings: (1) innumerable new mechanisms
that give rise to genetic variation (mutation
mechanisms) have been discovered; (2) the
selective value of many evolutionary changes in
DNA and their products is apparently so little
that they are effectively "neutral" and are only
spread and fixed in populations by chance (neutral theory of evolution); (3) in accordance with
point 2 is the fact that the rate of evolution of
molecular characters is relatively constant
("molecular clock"); (4) innovative evolution,
e.g. the formation of proteins with a new function, never occurs via de novo synthesis of new
DNA sequences but always through the alteration
or recombination of existing sequences; and (5)
the evolution of morphological characters is correlated in neither rate nor extent with the evolution of DNA and proteins.
Since the formulation of the neutral theory of
evolution, the question of whether Darwinism
has been undermined, or even completely disproved, by the new results of molecular biology
has been discussed with great ideological enthusiasm. That King and Jukes chose Non-Darwinian
Evolution as the rather unfortunate title of their
first publication has undoubtedly contributed to
the argument [213]. If, however, one defines Darwinian evolution as "the gradual transformation
of a species over time by the forces of selection
operating upon genetic variability existing in the
progeny as possible; selection doesn't always
leave behind just corpses. Two types of selection
can be conceptually distinguished. Negative or
stabilizing selection eliminates those variants that
display reduced fitness because of marked phenotypic changes. Positive selection, i.e. the fixation
of fitness-increasing mutations, appears to be of
rarer occurrence in nature.
Adaptive evolution is often understood to
mean the "choice", made by the process of selection, of one of the many "solutions" available to
the organism because of genetic variation to a
"problem" posed by a change in the environment.
However, the relationships between genes, organisms and the environment are much more complicated, as the following considerations show:
1. The quantitative variability of almost all characteristics of an organism arises during the
unique development of that organism and
results from non-trivial interactions with the
environment; single genes or alleles cannot be
assigned a particular selective value independent of the current environment.
2. Due to the imperfect reproducibility of the
developmental
process
(developmental
noise), various phenotypes may result from
the same genotype in a given environment.
3. Internal and external factors have mutual
influences; for example, in many cases, the
selective pressure of the environment on a particular allele is dependent upon the genetic
milieu or the gene frequency.
4. As each organism is constantly changing, the
phenotype is determined not only by the genotype and the environment but also by the organism's own previous condition.
s. Organisms can actively alter the character of
their environments.
6. The "environment" of most life processes is
the internal milieu of the organism, and this is
largely determined and maintained by regulatory processes, independent of changes in the
outside world [21].
It was originally assumed that the direction and
rate of evolution was determined entirely by the
selection of advantageous variants. Impressed by
the unexpectedly high molecular polymorphism
in natural populations and the relatively constant
rate of protein evolution (the molecular clock),
Kimura, Ohta, King and Jukes formulated in
1968-1969 the "neutral theory" of molecular
evolution [208, 213]. According to this theory,
many perhaps most DNA alterations have such
4 Molecular Evolution
113
little importance for fitness that they are not subjected to any selection pressure and are only fixed
by chance in the population. One could say that
the organism itself does not "notice" such mutations, but they are, nevertheless, detectable by
the precise tools of the molecular biologist. The
lower the importance of the altered character for
fitness, the higher is the probability, and thus the
relative frequency, of selectively neutral alterations. Therefore, the proportion of selectively
neutral alterations decreases in the gene-function
cascade (Fig. 4.1) from the DNA to the complex
morphological and physiological characters; the
neutral theory is applicable most of all to the
evolution of nucleic acids and proteins. Mutations that are neutral at a given point in evolutionary time may have a significant selective
advantage or disadvantage at a later time and
under different internal or external conditions.
Such conditionally neutral variation represents a
reservoir of genetic variability in the population
for later evolutionary processes.
All in all, it may be said that research into
evolution has benefited from the evaluation of
molecular data by a whole series of fundamental
new findings: (1) innumerable new mechanisms
that give rise to genetic variation (mutation
mechanisms) have been discovered; (2) the
selective value of many evolutionary changes in
DNA and their products is apparently so little
that they are effectively "neutral" and are only
spread and fixed in populations by chance (neutral theory of evolution); (3) in accordance with
point 2 is the fact that the rate of evolution of
molecular characters is relatively constant
("molecular clock"); (4) innovative evolution,
e.g. the formation of proteins with a new function, never occurs via de novo synthesis of new
DNA sequences but always through the alteration
or recombination of existing sequences; and (5)
the evolution of morphological characters is correlated in neither rate nor extent with the evolution of DNA and proteins.
Since the formulation of the neutral theory of
evolution, the question of whether Darwinism
has been undermined, or even completely disproved, by the new results of molecular biology
has been discussed with great ideological enthusiasm. That King and Jukes chose Non-Darwinian
Evolution as the rather unfortunate title of their
first publication has undoubtedly contributed to
the argument [213]. If, however, one defines Darwinian evolution as "the gradual transformation
of a species over time by the forces of selection
operating upon genetic variability existing in the