Re: Can genomes sense their population size and adjust their amount of junk DNA accordingly?
- From: "Perplexed in Peoria" <jimmenegay@xxxxxxxxxxxxx>
- Date: Wed, 21 Dec 2005 20:13:51 -0500 (EST)
"Jim McGinn" <jimmcginn@xxxxxxxxxxxxx> wrote in message news:doacvi$88c$1@xxxxxxxxxxxxxxxxxxxxxx
> Scientists fish for origins of genome complexity
> Mon 19 Dec 2005 04:37 PM CST
> ATLANTA (myDNA News)
>
> I cut and pasted the following relevant quotes from this article:
>
> http://www.mydna.com/genes/genetics/news/news_20051219_genome_complexity.html
>
> *** Begin cut and paste ***
>
> the size and complexity of a species' genome is not an evolutionary
> adaptation per se, but can result as simply a consequence of a reduction in
> a species' effective population size.
> most of the mutations that arise in organisms are not advantageous and that
> the smaller a species effective population size is (the number of
> individuals who contribute genes to the next generation), the larger the
> genome will be.
>
> freshwater species have a smaller effective population size than their
> marine counterparts. If the hypothesis was correct, the genome size of these
> freshwater fish would be larger than that of the saltwater dwellers. It was.
>
> they found that species with a smaller effective population had larger
> genomes.
>
> "We see a very strong negative linear relationship between genome size and
> the effective population size,
>
> *** End cut and paste ***
Thx Jim.
The web page is essentially a Georgia Tech press release reporting on
an empirical study which is published in the Dec 2005 issue of TIG.
The hypothesis which this study supports was apparently published
in 2003 by researchers from other universities, but the Georgia Tech
press release doesn't say who they were or where they published. If
readers of TIG could provide a citation for the 2003 hypothesis, it would
be appreciated.
> My interpretation of what they are saying here is that, all other things
> being equal, a sublineage that has a smaller effective population size
> (fresh water ray-fin fish in the above example) will tend to have larger
> stores of "junk" DNA than sublineages that have a larger effective
> population size (salt water ray-fin fish in the above example). I think
> this observation is confirming of (or, in the least, not refuting of) the
> hypothesis being championed here by myself and others that "junk" DNA is a
> lineage level adaptation that enables a population/lineage to more rapidly
> achieve new adaptations to future changes in environment/habitat.
>
> It makes sense in that fresh water fish would be more susceptible to
> extinction as a result of relatively slight changes in environmental
> factors. Consequently they have greater likelihood of making use of 'junk'
> DNA.
>
> This suggests that Genomes are more sophisticated than the bean-bag genes
> notion that has been assumed by neoDarwinists. Having evolved over billions
> of years, I suspect that the genome has the ability to sense it's population
> size and adjust factors, such as the size of its 'junk' DNA, accordingly.
Your interpretation seems correct to me. To answer the question posed
in your title - Yes. A genome (in a single individual) can "sense" the
effective size of the population from which it was derived by a variety
of techniques. The most obvious is to simply count the number of homozygotic
loci in a set of loci known to be quite variable. Homozygosity correlates
with inbreeding which in turn negatively correlates with effective population
size.
However, it is also easy to construct models in which you get the same
pattern of junk vs population size which have nothing to do with selection
or adaptation. Simply imagine that the amount of junk appearing in a species
results from an equilibrium between the rate of junk generation and
junk removal. Also assume that the most effective method of junk removal
is to occasionally remove stuff on one chromosome which seems to be an insertion
as compared to the other chromosome. (A biased gene conversion mechanism.)
In a large population, this may be an effective strategy, but in a small
inbred population, the junk may become homozygous before the scrubber
gets around to removing it.
I would be curious whether this alternative explanation has been published.
Its hard to imagine that it would not have been.
I'm also curious whether publication of something like this in TIG
is considered more or less peer-reviewed and authoritative than something
in a straight research journal.
.
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