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Genetic
Variation - The Fuel for Evolution and the Cambrian Explosion
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Simply
put genetics is a science of difference, that is, variation
in genomes. Genetic or genomic variation among individuals
and in the gene pool of a population of individuals can be
considered the fuel required for
evolutionary
adaption to proceed. Without fuel, the machine stops, at least
until replenished by the accumulation of new mutations.
Our
Darwinian Machine
has little to produce in the absence of selective pressure
for an organism to adapt in order
to survive. A period of pseudo stasis ensues, where fuel stores
(mutations) increase as to stores of raw materials build up.
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The
Origin of Species - Charles Darwin - Chapter 14 - Recapitulation
and Conclusion
Darwin
on
Variation
That many and grave objections may be advanced against the
theory of descent with modification through natural selection,
I do not deny. I have endeavoured to give to them their full
force. Nothing at first can appear more difficult to believe
than that the more complex organs and instincts should have
been perfected not by means superior to, though analogous with,
human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless,
this difficulty, though appearing to our imagination insuperably
great, cannot be considered real if we admit the following
propositions, namely, -- that gradations in the perfection
of any organ or instinct, which we may consider, either do
now exist or could have existed, each good of its kind, --
that all organs and instincts are, in ever so slight a degree,
variable, -- and, lastly, that there is a struggle for existence
leading to the preservation of each profitable deviation of
structure or instinct. The truth of these propositions cannot,
I think, be disputed.
It is, no doubt, extremely difficult even to conjecture by
what gradations many structures have been perfected, more especially
amongst broken and failing groups of organic beings; but we
see so many strange gradations in nature, as is proclaimed
by the canon, `Natura non facit saltum,' that we ought to be
extremely cautious in saying that any organ or instinct, or
any whole being, could not have arrived at its present state
by many graduated steps. There are, it must be admitted, cases
of special difficulty on the theory of natural selection; and
one of the most curious of these is the existence of two or
three defined castes of workers or sterile females in the same
community of ants but I have attempted to show how this difficulty
can be mastered. With respect to the almost universal sterility
of species when first crossed, which forms so remarkable a
contrast with the almost universal fertility of varieties when
crossed, I must refer the reader to the recapitulation of the
facts given at the end of the eighth chapter, which seem to
me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted
together, but that it is incidental on constitutional differences
in the reproductive systems of the intercrossed species. We
see the truth of this conclusion in the vast difference in
the result, when the same two species are crossed reciprocally;
that is, when one species is first used as the father and then
as the mother
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There are three primary sources of genetic
variation:
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Mutations are changes in the DNA. A single
mutation can have a large effect, but in many cases, evolutionary
change
is based on the accumulation of
many mutations.
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Gene
flow is any movement of genes from one
population to another and is an important source of genetic
variation.
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Sex can introduce new gene combinations into
a population. This genetic shuffling is another important source
of genetic variation.
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Mutations and variation:
Variety
is required for evolution to proceed. Variety is used up in
the evolutionary process, but is then renewed by chemical
errors that become part of copied DNA - the mutations. A
mutation is a change in the nucleotide sequence of the genome
of an organism,
or a nonchromosomal genetic element that becomes a permanent
change in the chemical structure of a gene, or actually in
one of the two alleles of each gene in the genome of eukaryotic
organisms. Mutations result from unrepaired damage to DNA or
to RNA genomes
that are nomally
caused by radiation or chemical interactions),
errors in
replication of DNA, or from either the insertion or deletion
of segments of DNA. A mutation in a parent,
has a chance to passed to offspring
upon mating. A mutated offspring can pass the error on in
kind to its offspring.
Mutations
are rare events, but once they occur they can have a range
of effects on an organism, from essentally no effect, to
most deleterious, to new mutated alleles past
to offspring
that can be favored by the darwinian
machine, whereby the mutation selectively expands within
a population's gene pool, enabling adaptation under selective
pressure
from
environmental factors.
Darwin
called
the recipiants
of
favored
alleles favored
individuals.
Sex
and variation:
There
are many modern theories that explain the advantage affords
sex incorporate an idea originally proposed by Weismann more
than 100 years ago, but they boil down to the idea that sex
allows natural selection to proceed more effectively because
it increases genetic variation.Genetic
variation can also be caused by differing recombination of
chromosomes in
sexual reproduction, a process called independent assortment
that occurs in eukaryotics and produces a gamete with
a mixture of the organism's chromosomes; the chromosomes that
result
are randomly sorted from all the potential combinations
of
parental chromosomes. [1-3]
Gene
flow and variation:
Gene
flow is the transfer of alleles or genes from one population
to another. Include genetic drift here.
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Burt, A. Perspective: sex, recombination, and the efficacy of
selection--was Weismann right? Evolution. 2000 Apr;54(2):337-51.
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Goddard MR, Godfray HC, Burt A. Sex increases the efficacy of
natural selection in experimental yeast populations. Nature.
2005 Mar 31;434(7033):636-40.
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Hoekstra RF. Evolutionary biology: why sex is good. Nature.
2005 Mar 31;434(7033):571-3.
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