Biology Reference
In-Depth Information
Wright, 1931; Haldane, 1932; Dobzhansky, 1937, 1951). These scientists had been ardent supporters
of Darwin's theory and later this came to be known as neo-Darwinism. Zuckerkandl and Pauling
(1965) were the fi rst who put forward the idea that DNA and protein sequences can be used to draw
evolutionary relationships and it is possible to construct a TOL. The studies on molecular evolution
gave importance to mutations since it is due to the mutations the change in the sequence of DNA or
proteins is affected. But this idea was also not acceptable to neo-Darwinians as according to them
mutations have nothing to do with the phenotypic evolution. Although it was recognized that the
basic underlying force for evolution is mutation and selection but subtle distinction has been made
between phenotypic evolution and molecular evolution. It was noted that selection plays greater role
in phenotypic evolution rather than in molecular evolution. The neutral theory of molecular evolution
put forward by Kimura (1980) emphasizes 'phyletic gradualism' as the basis of evolution where the
evolution of molecular characters is gradual and clock-like and that of the morphological characters
can also evolve in a gradual fashion. Another aspect that lends support to this theory is mutations
that occur in neutrally-evolving genes are either indifferent or negative (Kimura, 1983). Proteins
encoded by neutrally-evolving genes may even show more than 70% sequence divergence but still
retain the conserved function and the three-dimensional structure. It means due to a strong selection
pressure the structure and function of such proteins is maintained. Thus in case of macroevolutionary
characters as well there is a possibility for the retention of the same protein structure and function.
However, Kimura's neutral theory suffered due to two drawbacks. The fi rst one is that he took into
account nucleotide substitutions in the whole genome instead of taking into account gene as a unit
because nucleotide substitutions in the non-coding regions have no relevance in either mutation or
selection. Secondly, the defi nition of selective neutrality took into account the probability of fi xation
or the time until fi xation of mutation. Many molecular biologists consider a neutral or nearly neutral
mutation that does not have appreciable effect on the gene function. The views expressed by King and
Jukes (1969) on neutral mutations seem to be more convincing. They argued that though mutation
is the primary force for evolution, the role of natural selection is to eliminate harmful mutations.
According to them proteins that have rigid functional and structural constraint show lower amino
acid substitutions than the proteins requiring weak functional constraints. Additional molecular data
on the sequences of functionally important proteins lends support to this theory than to Kimura's
computation of natural selection. Thus the concept of neutral mutations conceived by Kimura
and convincingly put forward by King and Jukes is contrary to the Darwin's positive selection of
adaptations but by stabilizing or purifying selection eliminates deleterious mutations allowing
the fi xation of neutral mutations by genetic drift. It is through the evolution of multigene families
involving gene duplications and gene loss that an effect at phenotypic level would be refl ected (Nei
and Rooney, 2005). Taking into account these facts and other supportive molecular data Nei (1983,
1984) proposed the term neomutationism which was later called by him as neoclassical theory of
evolution (Nei, 1987). Nei (2005) discussed at length the evolution of multigene families and their
likely impact in shaping the evolution of new genetic systems.
The term 'phylogeny' was for the fi rst time coined by E. Haeckel who followed Darwin's
concepts and constructed phylogenetic trees for various groups of organisms. For many scientists
who supported Charles Darwin the TOL is a biological fact while someothers believe TOL to be
a hypothesis that is yet to be proved. In view of the progress achieved in the molecular biology
of prokaryotes the Darwinian concept needed a re-examination. Some of the relevant issues that
require attention are as follows: (i) would it be possible to test the Darwin's theory in the context of
molecular phylogenetic data obtained during recent years on prokaryotes? (ii) the phylogenetic trees
built on the basis of molecular sequences would be congruent with the phylogenetic trees built on
