The Selfish Gene Chapter 3 by Richard
Dawkins. Immortal Coils. In this chapter,
Dawkins
elaborates more on the definition of
genes and why it is a practical unit for
studying natural selection. Through his
explanation, he makes clear why the title
of the book is called The Selfish Gene.
Different survival machines appear very
different inside and outside. For example,
an octopus is nothing like a mouse and
an oak tree. However, the fundamental
chemistries of their genes are the same. We are all survival machines for the same
kind of replicators, molecules called DNA.
Monkeys machine preserves genes up trees
and fish is a machine that preserves
genes in the water. In his discussion of
DNAs, Dawkins issues that DNAs are much
the same as the first replicators in the
primeval soup. Of course they may be
related or totally different, or their
survival machines were taken at a later
stage by DNA. Alexander Graham Cairns-Smith
hypothesized that the first
replicators may have been inorganic
crystals. So, what is DNA?
Deoxyribonucleic acid or DNA consists of
a long chain of building blocks called
nucleotides and pair nucleus are chains
that twist together in a double helix or what
Dawkins calls the immortal coil. There are
only four different kinds of building
blocks within all animals and plants and
the difference is in this sequence of
the building blocks. The amazing thing is
that your DNA is distributed across all
cells in the body, and almost every cell
contains a complete copy of the body's
DNA. They are housed in the nucleus of
the cell. DNA molecules perform two
important functions. Firstly, they
replicate and make copies of themselves
and have become extremely adept at this
function. The most obvious example is how all human beings can start off as just a
single cell and are faithfully
replicated until each human becomes
billions of cells.
Secondly, DNA indirectly supervises the
manufacture of different kinds of
molecules known as proteins. DNA, through
RNA gives instructions to produce amino
acids, which make proteins. At the most
basic level, the DNA sequence
contains start and stop signals that
provide instructions for proteins.
Proteins make up the body and regulate
its chemical processes. This means that
genes that excel in the control of
embryonic development and passes genes
on would be naturally selected and
favoured. Dawkins once again reminds us
that replicators operate blindly and have
no conscious foresight or purpose. Modern
replicators are highly gregarious and
work together with other genes to create
its machine. This interdependence is
further promoted in sexual reproduction
where genes are mixed and shuffled. An
individual body is just a temporary
vehicle for a short-lived combination of
genes. The combination is short-lived but
the genes are potentially very
long-lived and immortal genes can cross
and re-cross and meet with each other
down the generations.
For humans, we house
46 volumes of DNA called chromosomes or
23 pairs of chromosomes. For each pair,
we inherit half of
chromosomes from the father through the
sperm and half from the chromosomes from
the mother through the egg. If there is a
contradiction in the instructions, one
reading of the genes will prevail over
another. For example, in the instance of
eye colour where two instructions are
available, blue eyes will be ignored and
the body would manufacture brown eyes.
Genes that are ignored are called
recessive. The genes for brown eyes are
dominant genes.
Therefore, blue eyes will only exist if
both copies are passed on. Genes that
compete for the same slot on a
chromosome are called alleles of each
other. For genes, Dawkins argues that
alleles are synonymous with rival. Our
genes are given during conception and
when we refer to the gene pool were
referring to the genes of the population.
When cells divide normal cell division
is called mitosis.
Another kind of cell division called
meiosis occurs in the production of sex
cells, the sperm or egg, which are unique
because they only contain 23 sets of
chromosomes, not 46 like another somatic
or non-sex cells. This is because they
fuse in sexual fertilization to make
a new cell that has 46 sets of chromosomes.
In meiosis, bits of chromosomes usually
swap and reshuffle in a process called
crossing over. This creates patchwork
arrangements of maternal and paternal
genes of chromosomes. We come back to the
question of how to define a gene. If we
like, we can define genes as a sequence
of nucleotides that lie between the start
and end signals that code for proteins
which are called cistrons, a term that is
sometimes used interchangeably by the
term genes. Dawkins uses George C
Williams definition of a gene as any
portion of the chromosomal material that
potentially last long enough generations
to serve as a unit of natural selection.
We can imagine a sequence of code
letters a length of chromosome called
genetic unit the shorter the genetic
unit the longer in generations.
This genetic unit is likely to live
because there is a smaller probability
it will be split up during crossing over.
For a very large genetic unit, there is a
50% chance that the unit will be split
up at each meiosis. If a genetic unit is
1% of the length the chromosome, it makes
sense to assume it only has 1% chance of
being split during mitotic division. By
tracing the ancestry of a small genetic
unit back enough, we can also work out
its original creator and common ancestor.
Other than crossing over, a new genetic
unit can also be formed through
mutations. Point mutations are errors,
changing one code of the sequence.
Mutations can also be inversions where
pieces of chromosomes detach and
re-attaches itself in the inverted
position. Although usually devastating
sometimes inversions can be beneficial
by bringing a closer linkage of genetic
materials that complement and reinforce
each other.
Natural selection would then favor new
genetic units that create beneficial
effects through considerable
rearrangement and editing. Dawkins
explains the phenomenon of mimicry and
gives an example of how disgusting-tasting
butterflies are brightly and
distinctively coloured as warning marks
for birds. Other butterflies may try to
mimic them by displaying the same color
markings and fool birds into not eating
them. In this way, natural selection
favours genes for mimicry and this is how
it evolves. Mimics, however, cannot try to
mimic all species. It must commit to one
species. Any intermediates or those that
try to mimic both would be eaten and
therefore negatively selected out. If say
one butterfly mimic species A and
another mimic species B, Dawkins argues
that one gene probably cannot control
all aspects of mimicry.
For example, colour shape, spot patterns,
flight rhythms. However, through the
editing by inversions and other
rearrangements, a large complex of
formerly separate genes come together in
a tight linkage and behaves like a
single gene. The clusters are rarely
split up by crossing over, so an
intermediate butterfly is never seen in
nature but can turn up if large numbers
are bred in laboratories. Dawkins
clarifies his use of the term gene and
says that lengths of chromosome likely
to be split up by crossing over or
mutations makes it less qualified to be
called a gene. A cistron may qualify as
a gene but so may a dozen cistrons
closely linked together that they
constitute a single long-lived genetic
unit and travel together during
meiosis on their journey down to the
next generation. Gregory Mendel stated
that hereditary units can be treated in
practice as indivisible and independent.
Although cistrons are sometimes
divisible and two genes on the same
chromosome are not wholly independent,
Dawkins definition of a gene as a units
that is seldom divided, present or absent
in an individual, and passes from
grandparent to grandchild is close to
the ideals of Mendelian inheritance.
This means that genes do not
continuously blend with each other,
otherwise, natural selection would be
impossible. In this sense, genes are
potentially immortal and do not grow old. senile as they can live in forms of copies
of itself, leaping from survival machine
to survival machine down the generations,
potentially for hundreds of millions of
years. Dawkins argues that individuals,
groups or species are too large and
temporary to qualify as a significant
unit of natural selection. A practical
unit of natural selection should be the
largest entity which possesses the
properties of longevity, fecundity, and
copying fidelity, which will be a gene.
As genes compete directly with their
alleles for survival, any gene that
behaves to increase their chances of
survival by acting selfishly instead of
altruistically tend to survive. Hence the
gene is the basic unit of selfishness. It
is useful to remember that genes are not
free and independent agents in
controlling embryonic development, as
they interact in complex ways with other
genes in the external environment (such
as temperature, food, predators companions). Like there is no single factor that
makes a plant grow, there are infinite
numbers of antecedent causes that makes
a baby. Nonetheless, differences between
babies, for example, the difference in the
lengths of the legs may be traced to one
or a few differences in environments or
genes. Differences matter in the
competitive struggle to survive.
Furthermore, if a herbivore acquires
genes that confer sharp meat-eating
teeth but without the right sort of
intestine for eating meat, they would be
bad genes for a herbivore gene pool.
Another quality of successful genes is a
tendency to postpone the death of their
survival machines until after
reproduction. Genes that make its
possessors die are called lethal genes and
those that have debilitating effects are
called semi-lethal genes. Based on Peter
Medawar theory of aging, lethal genes
will tend to be removed from the gene pool,
but some late acting lethal genes maybe
more stable and successful in the gene
pool than early acting one, provided the
lethal effects show after the body has
at least reproduced. This may explain genes that cause cancer in old age that may be
passed to numerous offsprings, whereas
genes that give young adult cancer will
not be passed to many offsprings.
According to this theory, senile decay is
the accumulation in the gene pool of late
acting lethal and semi-lethal genes, as
there is less selection pressure against
these genes. Dawkins also addresses the
question of what sexual reproduction and
crossing over are good for. Crossing over
does not happen in all species. Male fruit
flies do not do it and female fruit
flies have genes that can suppress it.
Also, there are alternatives to sex.
Female green flies can bear fatherless
female offsprings containing all the
genes of its mother. Plants can propagate
by sending our suckers and become
detached from the parent. Sex appears
paradoxical because is an inefficient
way for individuals to propagate their
genes. But if we consider individuals as
short-lived survival machines, efficiency
is irrelevant.
Genes for sexuality, crossing over, or
those that induce mutations manipulates
other genes for its own selfish ends and
this is sufficient explanation for its
own existence, which Dawkins concedes is
dangerously close to being a circular
argument. Dawkins states that the immediate manifestation of natural selection is
almost always at the individual level
but changes in gene frequencies in the
gene pool are the long-term consequences
of non-random individual death and
reproductive success -  the process of
evolution. Sex and crossing over keeps
the gene pool well-stirred and partially
shuffles the genes. When considering the
evolution of a certain characteristic,
Dawkins reminds us to get into the habit
of thinking about the effects on the
frequencies of the genes in the gene
pool.
 
