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Rule -> RE: Believer(s) of god are plague to this world. (12/3/2009 9:07:01 AM)
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quote:
ORIGINAL: LadyEllen
Genes change over the generations. Naturally occurring faulty copies arise that are passed down the generations. With a larger gene pool it is less frequently the case that two people will come together, each of whom has a faulty copy in relation to a particular gene, so the frequency of offspring that have two faulty copies is low.
That is correct.
Now supposing that neither natural nor sexual selection occurs, the frequency of such deleterious alleles (and also of advantageous alleles) will be subject only to genetic drift. That is, purely by chance they will increase or decrease in frequency in the gene pool. Thus in extreme form we may expect that half of them purely by chance are shuffled out of the gene pool and that the other half of all deleterious mutations become dominant in the gene pool; the former are not important, but the latter cause a very serious problem to the afflicted population. (I know that it is mathematically not as simple as I pretend here, but the example will do.)
quote:
ORIGINAL: LadyEllen
If you have a small gene pool that constantly intermarries within itself, the naturally occurring faulty copies will still arise at the same rate as in a larger gene pool, but in the smaller pool which is intermarrying there is a higher frequency of two people coming together each of whom carry the faulty copy and so pass it to their offspring. Rinse and repeat over several generations and the frequency of faulty genes in the small gene pool population will be higher than it would be in the larger gene pool population. This applies regardless of environment and behavioral traits of the population, it is simply the effect of repeated intermarriage of the same smaller pool of genes.
This is not correct. Deleterious mutations (usually all mutations initially) are most often recessive. In heterozygous individuals not much of their effect is noticeable in the phenotype (and they might in such heterozygous individuals even have beneficial effect, as for example in sickle cell anemia). Because they are recessive and then scarcely manifest in the phenotype, therefore there will be no natural selection against them. If there also is no sexual selection against them, then again their frequency in the population will be subject only to the chance shuffling of genetic drift. As it happens such deleterious alleles are much more rapidly drifting either into or out of the gene pool of small populations than of larger populations. So that is the reason that congenital diseases are high in such populations.
In the absence of natural selection and of sexual selection, the only way to remove such deleterious alleles from the population is to cause the homozygous presence of these alleles in individuals, because then they become visible in the phenotype. Such homozygous alleles conferring congenital birth defects are often lethal, or cause visible defects. If lethal, then the two deleterious alleles are at once removed from the gene pool, lowering their frequency in the population (three quarters of afflicted individuals however are not homozygous for the deleterious alleles, but heterozygous, so the majority of deleterious alleles will remain present in the population). If not immediately lethal, but resulting in visible defects, such babies may be exposed to the elements (as was done in ancient Greece, in particular by the Spartans (essentially nearly an island population) and also in Italy, if I recall correctly. In Sparta it also was the customs for boys and girls to exercise naked in each others presence - thus enabling any physical congenital defects to be visible to their future prospective partners.
Thus intermarriage of blood relatives does not increase the frequency of deleterious alleles, but to the contrary decreases their frequency in the population somewhat.
quote:
ORIGINAL: LadyEllen
Yet the environmental and behavioural traits are important to this mechanism where they are the drivers of the repeated intermarriage with a smaller pool of genes. An environmental driver might be geographical isolation as we might find in "lost tribes" in the Amazon for instance.
Quite. Such isolated populations are in fact island populations, where due to the effects of genetic drift the genomes are homogenized, that is, because alleles are thrown out of the gene pool, there is a paucity of variant alleles and there eventually is little genetic variation except for the new mutations. And as the environment is often unchanging, there will arise an optimum allele configuration and as further evolution is impossible, mutation rates may conceivably even drop sharply.
quote:
ORIGINAL: LadyEllen
A behavioural driver could be a religious or cultural prohibition on marrying someone outside the religion or culture, such as we might find with some Jewish populations.
Such behaviour is also subject to natural selection. Any change in behaviour that adds to the benefit already conferred by a phenotypical change (in the case of Jews and Muslims circumcision which to some degree protects against various sexually transmitted diseases), will be selected for by natural selection. The benefit in the case of circumcision being protection against sexually transmitted diseases, resulting for the males in higher reproductive success, any change in behavior that further reduces the risk of being infected by such diseases, will be selected for. This is inevitable. Since adulterous females and homosexual males with their frequent sexual contacts with men that also have frequent sexual contacts with other men serve as vectors for such diseases, any behavior that severely represses their lewd behavior will be selected for. Hence why homosexuals are persecuted in the Old Testament and why girls that have sexual relations before marriage and adulterous women are stoned to death, have their throat cut, or end up drowned at the bottom of water wells in so-called 'honor' killings.
Eventually, conceivably, mutations will arise and be selected for that make such behavior instinctive, baking it into their genetic material. So such behaviour will no longer be simply a matter of culture, but of inherited urges.
quote:
ORIGINAL: LadyEllen
Regardless of the driver, the mechanism will operate - with repeated intermarriage within a smaller gene pool giving rise to higher frequency of faulty gene copies than in a larger gene pool.
As I explained before, this is incorrect. On the contrary: such marriages serve to eliminate progeny of those marriages that are homozygous for such deleterious alleles from the breeding population.
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