Re: rates of change of mitochondrial DNA

Stephen Montgomery-Smith wrote:
> John Harshman wrote:
>
>
>>Are we now clear on what "fixation" means, by the way? It
>>just means "spreads throughout the population to the point where all
>>other alleles are eliminated".
>
>
> I now think that I'm not clear on this. I had assumed that fixation
> meant something "the mutation takes" that is, the mutation appears
> sufficiently often in the population that it doesn't disappear anytime
> soon. I would guess that most mutations simply disappear after one
> generation.

An allele is fixed when it is the only form of that locus in a population. If
it reaches a certain frequency and remains there we say it has reached
equilibrium (until the population structure or the selective pressures
change). If it randomly varies in frequency, we say it drifts. A fixed allele
can get there by drift, by the way.

Most mutations are more or less neutral, selectively, and if a genome has a
mutation then it will tend to be passed on Mendelianly.
>
> So, for example, I did spend quite a bit of time thinking about
> mitochondrial DNA (albeit from a state of relative ignorance), and it
> wasn't at all clear to me why there shouldn't be many variants
> co-existing in a single species (say human beings) at the same time.

Mitochondrial DNA is one of the population group markers used to work out
ancestry and geographical origin. Humans are polytypical for mtDNA alleles.
>
> Indeed the only reason I could think of why there wouldn't even be many
> variants in a single human being is because every few years or so there
> is a bottleneck effect when the next offspring happens in a single
> cell. Well that is if the egg cell only carries a single mitochondria.
> (I did ask some knowledgable friends how many mitochondria a typical
> cell contains, and the answer I mostly got was - its hard to know.)

Sperm have around 50-100 and eggs have at least a hundred thousand
<http://cellbio.utmb.edu/cellbio/mitoch2.htm>. Some paternal mtDNA is
inherited, but less than .01% of the zygotic mtDNA is paternal.
>
> I was also wondering about when the egg cells are actually produced -
> are they produced in the human being when they are very young, or when
> the emale is relatively mature? If the former, this gives a really
> rather short time for any kind of mutation to happen in the
> mitochondria if it is to get passed to the next generation.

Mitochondria are continually formed by division within the cells.
....

--
John S. Wilkins, Postdoctoral Research Fellow, Biohumanities Project
University of Queensland - Blog: evolvethought.blogspot.com
"Darwin's theory has no more to do with philosophy than any other
hypothesis in natural science." Tractatus 4.1122

.

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