Re: How can the evolutionary progress from slime to humans be linear?

William Morse wrote to sci.bio.evolution on the 14/12/2005:

> "Francois Sabot" <sabotfran@xxxxxxxx> wrote in
> news:dnngdi$1hp3$1 @darwin.ediacara.org:
>
> > Uno Lapideus wrote to sci.bio.evolution on the
> > 01/12/2005:
>
> Again I would note the wrong date - I hadn't noticed this
> before but may pay more attention to it.
>
>
> (snip)
>
> >> This means that 3 000 000 000 base pairs has been added
> >> in 800 000 000 Years... By random mutations in the germ
> >> line... All of them favourable! All of them
> >> "saturating" the existing genome! Wow!
>
> (snip)
>
> >> Seeing this straight line of evolution from "simple" to
> >> "complex," from the Ediacaran to humans, how can anyone
> >> call this "random"? Shouldn't we at least expect
> >> something resembling a random walk pattern? Please
> help >> me understand this!
> >>
> >> Uno
> > au sein du forum de News sci.bio.evolution
> >
> > Ce à quoi je me propose de répondre:
> >
> > Just a last thing: human genome is far not the biggest
> > one (4000 Mb, and some plants from the Liliaceae family
> > have more than 1 000 000 Mb!), and not the most complex
> > (25-35 000 genes, more than 35 000 in wheat/barley even
> > in their diploid form). The only thing different with
> > human is the level of intelligence. And this is not a
> > genetic or genomic componant.
> >
> > The size of genome does not reflect the complexity of an
> > organism nor its evolution level!
> >
>
> Au contraire, mon ami.
> (Actually my French is extremely limited - and no real
> French speaker might use that phrase - but I figured it
> can't hurt to do my bit to keep the internet more
> cosmopolitan)
>
> The size of genome does reflect a complexity difference
> between prokaryotes and eukaryotes, although within each
> of these two groups your statement may be correct.
>
> I doubt that I could prove it, but I would bet that the
> size of the genome in eukaryotes would as a general rule
> reflect the length of time from the development of
> eukaryotes to the organism's birth. This argument
> follows Gould's drunkard's walk idea, that there is a
> steady increase in "complexity" simply because the low
> side of complexity is fixed at 0 while the high side is
> infinite. Ignoring true "complexity" - however we define
> that - and looking simply at size of the genome, it would
> seem that absent mechanisms that severely limit the size
> of the genome, there should be a steady increase over
> time in genome size. So if I could measure the size of
> the genome of Cambrian life forms and compare it to the
> size of the genome in current life forms, I think you
> would find a significant increase.
>
> But I do agree that there is no simple relationship
> between genome size and organism development, and that it
> is not valid to equate "Time Required to Evolve" with
> rate of addition of base pairs to the gene pool. This is
> especially true since rate of addition of base pairs is
> in any case not a function of mutation rate, as others
> have noted.
>
> Yours,
>
> Bill Morse
au sein du forum de News sci.bio.evolution

Ce à quoi je me propose de répondre:

Dear Bill,
Your french is not so bad, we can use this sentence indeed
^^.
For your statement about the increase of genome length over
the million years, I must correct this. It has been shown
that genomes can decrease their size simply by heterologous
recombination. Example is Arabidopsis thaliana, which is a
quite recent plant, so on your statement a big genome. But
its size is about 115 Mb. In the same way, if the genome
size always increase during time, why all the plants from
the same taxonomic family have extremely diverse genome
size (rice/oat, from 400 Mb to 8,000 Mb). Thus, the genome
size is not only correlated to the potential complexity,
but also to mechanistical and adaptative reasons. A bigger
genome could provide a bigger nucleus, thus a a more
concentrated cytosol, and then a higher resistance to
freezing for plants. And there are other examples.
The trouble is that we do not know exactly what is
responsible of genome size, but it seems that at least
there is a genome size ratio between "genes" and "garbage"
(TE,...), as supposed by Rabinowitz (2000) or Vitte &
Panaud (2002). If there are too much "garbage", the
reducive mechanism starts.
Best regard
Francois

--



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