Reflectorites
On Wed, 2 Aug 2000 19:29:28 -0700, billwald@juno.com wrote:
[...]
>SJ>We divided into groups of four and each group used 60 green and yellow
>>beads in containers to represent alleles in individuals in a population.
>We drew the breads out singly at random to simulate genetic drift while
>>reducing the population by 5 pairs each generation, i.e. 55, 50, 45
>pairs, and recorded the gene frequency and phenotypic effect.
BW>Don't understand the experiment. If you removed beads randomly the ratio
>of green to yellow should remain constant if enough trials were made.
Sorry, but my explanation was necessarily abbreviated. We followed the
genotype-phenotype/dominant-recessive rules of Mendelian genetics.
Besides, it does not follow that random removal of beads should remain
constant. Over a very long period with a large enough population it would.
But with a small population it wouldn't necessarily.
BW>maybe 120 is to small a starting population.
The population was 60. The alleles were 120.
It wouldn't matter what it *started* as. The question was what happens when
a population got smaller. And populations do get down to 60 and in fact
down to 1, when they go extinct.
In fact, small populations are thought now to be how evolutionary change
happens, because favourable genes get diluted in large populations:
"All major theories of speciation maintain that splitting takes place
rapidly in very small populations. The theory of geographic, or
allopatric, speciation is preferred by most evolutionists for most
situations (allopatric means "in another place"). A new species can
arise when a small segment of the ancestral population is isolated at
the periphery of the ancestral range. Large, stable central
populations exert a strong homogenizing influence. New and
favorable mutations are diluted by the sheer bulk of the population
through which they must spread. They may build slowly in
frequency, but changing environments usually cancel their selective
value long before they reach fixation. Thus, phyletic transformation
in large populations should be very rare as the fossil record
proclaims. But small, peripherally isolated groups are cut off from
their parental stock. They live as tiny populations in geographic
corners of the ancestral range. Selective pressures are usually
intense because peripheries mark the edge of ecological tolerance
for ancestral forms. Favorable variations spread quickly. Small,
peripheral isolates are a laboratory of evolutionary change." (Gould
S.J., "The Episodic Nature of Evolutionary Change," in "The
Panda's Thumb," 1990, pp.152-153)
But the take-home point is that when populations get smaller, random
genetic drift becomes the dominant factor. Natural selection is not as
strong a force as we intuitively think.
When you think of it. Even with the strong selection of artificial selection
(i.e. selective breeding), it is hard to obtain the desired results. That is why
they are spending so much effort on animal cloning. It is only by cloning a
known genotype that they can guarantee the phonotype will be exactly
what they want.
[...]
Steve
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"It was a Russian biochemist, A. I. Oparin, who in 1936 first suggested
how inert chemicals might link together into an organic chain. Although it
was impossible to create life from non-life in our present oxygen- heavy
environment, he said (oxygen literally eats up any primitive organic
chemical such as an amino acid), this might not have been the case in
conditions billions of years ago. He suggested that there was a 'reducing'
atmosphere - free of oxygen, and consisting of such gases as methane,
ammonia, water and hydrogen. All experiments, including Stanley Miller's,
have been based on this hypothesis. Without oxygen, there is no ozone
canopy to protect Earth from the sun's ultraviolet rays. Nowadays, as
established by NASA's early space probes, this canopy blankets us between
fifteen and thirty miles above Earth's surface, effectively shielding us from
certain death. So with oxygen in the air, the first amino acid would never
have got started; without oxygen, it would have been wiped out by cosmic
rays. Imaginative and elaborate solutions have been written to this
conundrum. Perhaps the amino acid was formed at the edge of a volcano,
and then sank into a lake where it dropped the few metres below the
surface necessary to protect it from radiation; perhaps the Earth's waters
were covered by a layer of tar-like chemicals which stopped ultraviolet
light; perhaps the amino acid was protectively dehydrated or 'frozen' in
some way on dry rock or clay, waiting for an improvement in the
atmosphere. For every suggestion, there is a seemingly insuperable
objection: beneath the surface of the water there would not be enough
energy to activate further chemical reactions; water in any case inhibits the
growth of more complex molecules; unlike conditions in laboratory
experiments, the amino acids and their constituents could not be kept pure
and isolated. In other words, the theoretical chances of getting through
even this first and relatively easy stage in the evolution of life are
forbidding." (Hitching F., "The Neck of the Giraffe: Or Where Darwin
Went Wrong", Pan: London, 1982, p.64) .
Stephen E. Jones | sejones@iinet.net.au | http://www.iinet.net.au/~sejones
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