Dienstag, Dezember 07, 2010


Unimportant sites evolve as predicted by the neutral theory, whereas
important sites are more influenced by natural selection, and the difference
in the patterns provides an opportunity to detect selection. Many scientists
have now recognized that the strictly neutral theory is not satisfactory, and
the issue has entered into a new phase

The selectionist position seems to be logical in a Darwinian sort of way.
The neutralist position, however, needs a bit more explanation.
How does an adaptively neutral or equivalent change (one that isn't 'seen' by natural selection) become fixed in the population?
The answer is genetic drift.


Here, the idea is that the ratio of one gene over another, say a mutated version versus the original, fluctuates over generations but eventually settles to 1 or 0. A thought experiment can be used to illustrate this, or, if you have an excessive amount of free time, you can actually try it. Take 10 coins, 5 heads and 5 tails, choose 5 of them at random and remove them, this step represents the coins
that came to an untimely death
We chose randomly because of the assumption that natural selection has no effect on whether heads are better than tails. Now make the
remaining 5 coins have children and boost the population back up to 10, but keep the
ratio of heads to tails the same. For example, if you had 2 heads and 3 tails then after
reproduction you should have 4 heads and 6 tails. Repeat this process a few times and
before you know it all your coins will be heads or tails. The mutation has become fixed in the population and natural selection contributed nothing.
Our job now is to determine what is really going on and so we turn our attention to the predictive aspects of each theory. It turns out that the predicted rate at which mutations accumulate is different for each hypothesis. If the selectionists are correct then mutations cannot accumulate as fast because Natural Selection keeps on removing deleterious mutations. The rate at which mutations accumulate for the neutral theory will be much higher since they aren't constantly being removed. One example of supposed evidence in favour of the neutral theory came with the analysis of mutations at different positions on a codon. A codon is a sequence of 3 nucleotides that encodes for a specific amino acid. In general, if either of the first 2 nucleotides of a codon changes, the resultant amino acid
is different. The 3rd site, however, is usually silent; a change here does not change the amino acid. A mutation at this site is called a synonymous substitution. It was found, by comparing two species of sea urchin, that there were 5 mutations at such sites in the gene that coded for the protein histone IV. These 5 mutations were located within a stretch of only 11 codons, apparent confirmation of the neutral theory. Other data, such as an analysis of RNA viruses, also seems to confirm the neutral hypothesis.

RNA has an intrinsically higher rate of mutation.
The neutralists argue that since the rate of accumulation of mutations is high as well, that it is in keeping with the Neutral hypothesis. But wouldn't a higher intrinsic rate of mutation also lead to a higher number of favourable mutations that would be kept under the selectionist scheme also? According to Roger Lewin "The question is, therefore, whether the observed maximum rate of change better fits the predicted effects of selection or the random accumulation of neutral alleles. The answer, unequivocally, has been the latter, and represents strong support for the neutral theory

Always beware of words like "unequivocally". Remember the synonymous substitution example? Well, it doesn't seem too amazing that it obeys the predicted results of the neutral theory because the sites where mutations were looked at were silent, they were already known not to make any difference in the protein! It seems a little obvious that such a change would be invisible to the selective pressures of the environment. In a paper by Tomoko Ohta the situation is described more accurately.
DNA sequence data have rapidly increased in the nineties, enabling
comparison of the patterns of substitution at selectively important (such as
nonsynonymous) and unimportant (such as synonymous) sites.

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