The main
postulate in Darwin's theory of evolution was that a species evolves because
natural selection acts on small inheritable variations in the members of the
species (cf. Part 31).
But it was
argued by his opponents that, since a species is also characterized by
interbreeding, such small variations should get averaged away. Darwin had no
answer to counter this because the actual mechanism of inheritance was not known
at that time.
The answer in
fact had been provided in 1865 (i.e. during the lifetime of Darwin, but
apparently unknown to him) by the work of Gregor Mendel, the founder
of the subject of genetics. We
now know that the 'genotype' or the genome of an organism is its genetic
blueprint, and is present in the nucleus of every cell of the organism. The 'phenotype',
on the other hand, is the end-product (the organism) which emerges through
execution of the instructions carried by the genotype. It is the phenotype that
is subjected to the battle for survival and natural selection, but it is the
genotype which carries the accumulated evolutionary benefits to succeeding
generations. The phenotypes compete, and the fittest among them have a higher
chance of exchanging genes among themselves.
Mendel’s laws of genetics were rediscovered independently by quite a few
workers. One of them was the Dutch botanist Hugo de Vries, who not only
rediscovered Mendel’s laws for the inheritance of 'dominant' (or expressed)
and 'recessive'
(or suppressed) characteristics, but also discovered genetic mutations. These were sudden (unexplained) changes of form
which were inherited by the offspring.
The present, post-Darwinian,
picture is that the inherited characteristics of the progeny are carried by
genes. In sexually reproducing organisms, each parent provides one complete set
of genes to the offspring. Genes are portions of molecules of DNA, and their
specificity is governed by the sequences in which their four bases (adenine
(A), thymine (T), guanine (G), and cytosine (C)) are arranged. The double-helix
structure of DNA, together with the restriction on the pairing of bases comprising the DNA
molecule to only A-T and G-C, provides a mechanism for the exact replication of DNA molecules. And the DNA sequences on genes
determine the sequence of amino acids in the specific proteins created by the
live organism.
Genes
programme embryos to develop into adults with certain characteristics, and
these characteristics are not entirely identical among the individuals in the
population. Genes of individuals with characteristics that enable them to
reproduce successfully tend to survive in the gene pool, at the expense of genes that tend to fail. This feature
of natural selection at the gene level has consequences which become manifest
at the organism or phenotype level. Cumulative natural selection is NOT a
random process.
If like begets
like (through inheritance of characteristics), by what mechanism do slight
differences arise in the gene pool of successive generations so that the
species evolves towards evolutionary novelty? One mechanism is that of mutations.
Mutations, brought about by radiation or by chemicals in the environment, or by
any other agents causing replication errors, change the sequence of the
four bases in the DNA molecules comprising the genes. Most mutations are
deleterious and get weeded out by natural-selection processes, but those which
happen to be beneficial to the population have a selection advantage and get further
propagated in the population.
If all living
beings have the same or only a few ancestors, how have the various species
arisen? The Darwinistic answer lies in isolation
and branching, aided by evolution. Migrations of populations also play a
role in the evolutionary development of species. If there are barriers to
interbreeding, geographical or otherwise, single populations can branch and
evolve into distinct species over long enough periods of time. Each such
branching event is a 'speciation': A population accidentally
separates into two, and they evolve independently. When separate evolution has
reached a stage that no interbreeding is possible even when there is no longer
any geographical or other barrier, a new species is said to have originated.
The term neo-Darwinism essentially connotes a modification of the
original ideas of Darwin in the light of later knowledge about the mechanism of
transmittal of genetic information from one generation to the next. Margulis and Sagan (2002), who disagreed
with this neo-Darwinistic view of the origin of species (I shall describe their
work in the next post), summed up neo-Darwinism as follows:
‘All organisms
derive from common ancestors by natural selection. Random mutations (heritable
changes) appear in the genes, the DNA of organisms, and the best “mutants”
(individuals bearing the mutations) in competition with the others, are
naturally selected to survive and persist. The unsuited offspring die – they
tend to be called “unfit” – with fitness, a technical term, referring to the
relative numbers of offspring left by an individual to the next generation. The
most fit, by definition, produce the largest number of offspring. The mutant
variations then leave more offspring, and populations evolve; that is, they
change through time. When the number of changes in the offspring accumulates to
recognizable proportions, in geographically isolated populations, new species
gradually emerge. When sufficient numbers of changes in offspring populations
accumulate, higher (more inclusive) taxa gradually appear. Over geological
periods of time new species and higher taxa (genera, families, orders, classes,
phyla, and so on) are easily distinguished from their ancestors.’
As emphasized
by Stuart Kauffman, evolution of
biological complexity is determined by two factors: natural selection, and self-organization. Self-organization
creates order in any complex system. Darwinian natural selection acts on this
existing order and hones it further.
The phrase
'selfish gene' was
introduced by Richard Dawkins: 'The most inspiring way of teaching evolution is
to say that it's all about the genes. It's the genes that, for their own good,
are manipulating the bodies they ride about in. The individual organism is a
survival machine for its genes.'
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