Noahs inbred family tree [addendum]
Dave Gaban drrod@slip.net
Mon, 04 Jan 1999 00:16:56 -0800 (00915459416, 369078F8.2001@slip.net)
Joseph Crea wrote:
>
> Hello again, everyone!
>
> At 08:45 PM 1/3/99 +0000, Joseph Crea wrote:
> >Hello, Achilles!
>
> >>Achilles
> >>
> >>My genetics background is limited to some reading on my own and one >class, butI did get an A in it and will respond as well as I can. I >believe yourstatement is essentially accurate. 8 People would be far >too few to produce a viable gene pool, especially considering that they >were all supposedly close relatives. 250-300 is the number I remember >hearing, there have been smaller populations that worked but they also >relied on some exogamy (marrying outside their own people) so that the >effective population size was larger, including their neighbours.
> >
> >
> >CREA
> > Out of curiosity I dug through a couple of my evolutionary >biology textsfor pertinent data on this topic and discovered (in >Douglas Futuyma's__Evolutionary Biology__, 2nd edition, 1986) that the >population of elephantseals had been reduced by hunting to about 20 >individuals around 1890.Since then their numbers have grown to around >30,000 with no indicationsthat they suffer genetic anomalies in >disproportionate numbers.
>
> CREA
> Further digging produced the following:
>
> "Inbreeding, in itself, increases phenotypic variation, as measured by the variance of a trait, as long as the phenotype of a heterozygote does not lie outside the range spanned by the homozygotes. This is merely because homozygous genotypes at either end of the phenotypic spectrum increase in frequency, whereas heterozygotes decrease. Inbreeding causes a decline in the mean phenotype if individuals with the greatest value of the trait are dominant homozygotes or are heterozygotes. (If heterozygotes have the greatest value, the locus is said to be OVERDOMINANT). Such a decline, called INBREEDING DEPRESSION, is generally attributed to increased frequency of homozygotes for recessive alleles. Conversely, outbreeding, by increasing the frequency of heterozygotes and so concealing more of the recessive alleles, can increase the mean -- a phenomenon called HETEROSIS. When recessive alleles lower survival or reproduction (i.e., when they cause lowered fitness), they will b!
e eliminated more rapidly from an inbred than
from an outbred population because inbreeding exposes such alleles in
homozygous form. Thus the frequency of deleterious alleles should be
lower in inbred populations. This also means that if a randomly mating
population that harbors deleterious alleles becomes inbred, there will
initially be considerable inbreeding derpression as the alleles are
exposed, but the population will eventually regain high average fitness
as natural selection purges the population of the deleterious alleles.
Such a population will have less genetic variation than a randomly
mating population, not because of inbreeding in itself, but because of
the combined action of inbreeding and natural selection."
>
> (Futuyma, D. __Evolutionary Biology__, 2nd ed. 1986, page 124 )
>
> CREA
> Hope that this helps.
>
Dave G.
Interesting. I had always been under the impression "inbreeding" always
led to genetic defects, infertility or both. It makes sense the latent
defects would magnify upon repeated inbreeding, thus natural selection
would remove those defects from the pool.
The problem with the Noah model, is that the surviving pool would still
DNA code very closely, and would not account for the current diversity
of the Homo sapiens gene pool.