130
4 Molecular Evolution
and the comparison between man and the chimpanzee. The anurans, i.e. the frogs and toads, are
a morphologically uniform group of 3000 species
which arose about 159 million years ago and
which can all be placed in the same order. The
"placental mammals" with an age of about
75 million years, include about 4600 species,
amongst which are such diverse forms as cats,
humans, bats and whales; they are, therefore,
distributed over about 16-20 orders. Although,
as can be seen from the above, the organismic
evolution of the two groups has progressed at
very different rates, the rate of molecular evolution, as measured by, for example, the immunological distances of the serum albumins or the
mtDNA sequences, appears not to be significantly different. Frog species which are similar
enough to be placed in the same genus may differ
at the molecular level to the same degree as the
bat and the whale [123,205, 440]. Man and the
chimpanzee differ in molecular characteristics
more than sibling species of other animal groups
(Table 4.7); however, on the basis of their morphological differences they are placed in different
families (Hominidae and Pongidae) [115, 214,440].
Table 4.7. A molecular comparison of man and the chimpanzee [176, 214, 361]
Amino acid sequences:
Fibrinopeptide A and B; cytochrome c; and u-, ~- and yglobin chains are identical
A single difference in myoglobin and the b-globin chain
Immunological distance:
Lysozyme = 0, carboanhydrase = 3, serum albumin = 6,
serum transferrin = 8
Electrophoretic data:
For 31 intracellular and 13 extracellular proteins there
are, on average, 2.4 amino acid exchanges per protein
with an average length of 293 amino acids, i.e. a mean
sequence difference of 0.8 %
DNA data:
12S rRNA genes in mtDNA show 3.7% sequence
difference (compared with an average of 0.36 % between
the mtDNA of different human individuals)
The ~-globin gene differs in the region 1-1396 in 14-19
positions (compared with differences at 9 positions
between human individuals)
According to different authors, unique DNA has ~Tm
values of 0.7-1.5 °C, corresponding to an average
sequence difference of 1.1 % (in comparison Xenopus
laevis and X. borealis show ~Tm = 12°C, i.e. 12 %
sequence difference)
Genetic distance:
For 44 loci, D = 0.62 (for a comparison, see Fig. 4.18;
similar D values are found between sibling species of
Drosophila; D = 1.76 for the species Rana pipiens and
R. corrugata)
The evolution of morphological characters,
right up to the appearance of completely new
forms of organisms, cannot be explained simply
on the basis of evolutionary changes in the amino
acid sequences of proteins. Morphological characters appear to be dependent not so much upon
the coding sequences of structural genes as upon
the regulatory processes that determine the
expression of the structural genes at a particular
time during development and at a particular site
in the embryo. That organic evolution mainly
concerns changes in regulatory mechanisms for
gene expression is shown by the formation of species hybrids. The success of hybridization
between various species of the genus Mus correlates to the evolutionary distance as estimated
from molecular data [34]. However, crosses
between anuran species are possible over a 10times larger immunological distance (of the albumins) than in the mammals [440]. Furthermore,
in vitro fertilization experiments with eggs of the
teleost Micropterus salmoides floridanus and
sperms of other species showed that the incidence
of non-hatching or deformed embryos was not
always proportional to the genetic distance
between the species [316]. The molecular basis of
morphological evolution will only become accessible to research when it is known by what
mechanism the molecules of the body substances
are arranged into different species-specific, supramolecular structures during morphogenesis.
Molecular embryology has recently made important progress, e.g. with the identification and analysis of homeotic genes, and will undoubtedly
have further success. When the DNA sequences
responsible for guiding morphogenesis are known
in more detail, it should also become possible, by
comparing different animal species, to obtain an
insight into the molecular mechanisms of the
evolution of morphological characters.
4.3 Protein and Nucleic Acid
Polymorphism
All evolutionary processes are manifested by
changes in the gene stocks of populations. Until
the middle of the 1960s, the genetic structure of a
population could be assessed only indirectly by
examining phenotypes. As phenotypically recognizable spontaneous mutations are rare events, it
was generally held that the individuals of a
population are homozygous at almost all loci for
4 Molecular Evolution
and the comparison between man and the chimpanzee. The anurans, i.e. the frogs and toads, are
a morphologically uniform group of 3000 species
which arose about 159 million years ago and
which can all be placed in the same order. The
"placental mammals" with an age of about
75 million years, include about 4600 species,
amongst which are such diverse forms as cats,
humans, bats and whales; they are, therefore,
distributed over about 16-20 orders. Although,
as can be seen from the above, the organismic
evolution of the two groups has progressed at
very different rates, the rate of molecular evolution, as measured by, for example, the immunological distances of the serum albumins or the
mtDNA sequences, appears not to be significantly different. Frog species which are similar
enough to be placed in the same genus may differ
at the molecular level to the same degree as the
bat and the whale [123,205, 440]. Man and the
chimpanzee differ in molecular characteristics
more than sibling species of other animal groups
(Table 4.7); however, on the basis of their morphological differences they are placed in different
families (Hominidae and Pongidae) [115, 214,440].
Table 4.7. A molecular comparison of man and the chimpanzee [176, 214, 361]
Amino acid sequences:
Fibrinopeptide A and B; cytochrome c; and u-, ~- and yglobin chains are identical
A single difference in myoglobin and the b-globin chain
Immunological distance:
Lysozyme = 0, carboanhydrase = 3, serum albumin = 6,
serum transferrin = 8
Electrophoretic data:
For 31 intracellular and 13 extracellular proteins there
are, on average, 2.4 amino acid exchanges per protein
with an average length of 293 amino acids, i.e. a mean
sequence difference of 0.8 %
DNA data:
12S rRNA genes in mtDNA show 3.7% sequence
difference (compared with an average of 0.36 % between
the mtDNA of different human individuals)
The ~-globin gene differs in the region 1-1396 in 14-19
positions (compared with differences at 9 positions
between human individuals)
According to different authors, unique DNA has ~Tm
values of 0.7-1.5 °C, corresponding to an average
sequence difference of 1.1 % (in comparison Xenopus
laevis and X. borealis show ~Tm = 12°C, i.e. 12 %
sequence difference)
Genetic distance:
For 44 loci, D = 0.62 (for a comparison, see Fig. 4.18;
similar D values are found between sibling species of
Drosophila; D = 1.76 for the species Rana pipiens and
R. corrugata)
The evolution of morphological characters,
right up to the appearance of completely new
forms of organisms, cannot be explained simply
on the basis of evolutionary changes in the amino
acid sequences of proteins. Morphological characters appear to be dependent not so much upon
the coding sequences of structural genes as upon
the regulatory processes that determine the
expression of the structural genes at a particular
time during development and at a particular site
in the embryo. That organic evolution mainly
concerns changes in regulatory mechanisms for
gene expression is shown by the formation of species hybrids. The success of hybridization
between various species of the genus Mus correlates to the evolutionary distance as estimated
from molecular data [34]. However, crosses
between anuran species are possible over a 10times larger immunological distance (of the albumins) than in the mammals [440]. Furthermore,
in vitro fertilization experiments with eggs of the
teleost Micropterus salmoides floridanus and
sperms of other species showed that the incidence
of non-hatching or deformed embryos was not
always proportional to the genetic distance
between the species [316]. The molecular basis of
morphological evolution will only become accessible to research when it is known by what
mechanism the molecules of the body substances
are arranged into different species-specific, supramolecular structures during morphogenesis.
Molecular embryology has recently made important progress, e.g. with the identification and analysis of homeotic genes, and will undoubtedly
have further success. When the DNA sequences
responsible for guiding morphogenesis are known
in more detail, it should also become possible, by
comparing different animal species, to obtain an
insight into the molecular mechanisms of the
evolution of morphological characters.
4.3 Protein and Nucleic Acid
Polymorphism
All evolutionary processes are manifested by
changes in the gene stocks of populations. Until
the middle of the 1960s, the genetic structure of a
population could be assessed only indirectly by
examining phenotypes. As phenotypically recognizable spontaneous mutations are rare events, it
was generally held that the individuals of a
population are homozygous at almost all loci for