Text 560, 136 rader
Skriven 2004-10-28 06:18:00 av William Morse (1:278/230)
Ärende: Re: Is bipedalism neutral
=================================


"Anon." <bob.ohara@NOSPAM.helsinki.fi> wrote in
news:cllqtv$1ttp$1@darwin.ediacara.org: 

> William Morse wrote:
> 
>> "Anon." <bob.ohara@NOSPAM.helsinki.fi> wrote in
>> news:clh816$j0o$1@darwin.ediacara.org: 
>> 
>> 
>>>William Morse wrote:
>>>
>>>>"Anon." <bob.ohara@NOSPAM.helsinki.fi> wrote in news:cl65sn$ave$1
>>>>@darwin.ediacara.org:
>>>
>>><snip>
>>>>  bipedalism in humans is
>>>>simply a result of drift - our ancestors could have as easily
>>>>switched to knuckle-walking. 

(snip)

>>>For drift to be the explanation, we have to posit that all of the
>>>genes involved in the shift to bipedalism have drifted to fixation. 
>>>So, we'll need to know how many genes are involved, the probabilities
>>>of fixation of neutral alleles (which depend on the population size
>>>and structure), and the mutation rate of these alleles.  We'll also
>>>need to know the time since the shift to bipedalism started.

 
>> In my drift scenario  I am counting heavily on sampling error as
>> opposed to neutral drift to fix the alleles - so I need to have a
>> small effective population size. I am also counting on our ancestors
>> as having been primarily arboreal with brachiating locomotion, and
>> using limited bipedalism when walking on the ground.  
 
> I've no idea what the effective population size was then, so I'll
> accept it was sufficiently small for the sake of this argument.
 
> I'm a bit surprised that you're assuming that bipedalism evolved when 
> our ancestors were still in the trees - baboons are primarily ground 
> dwelling, but are still largely quadrapedal.  This implies that 
> bipedalism could have evolved after the descent from the trees (David 
> Attenborough pointed this out in Life On Earth, so it must be right!).

The idea about bipedalism evolving in the trees is based on gibbons, who 
are primarily arboreal but are bipedal when walking on the ground. But I 
am incorrect in characterizing the arboreal locomotion as brachiating - 
after doing some more research it appears more likely that arboreal 
locomotion of the larger apes is better characterized as vertical 
climbing and orthograde clambering - and I think this is typical of the 
climbing behavior that is common among human youngsters. 


  
>>>The probability of bipedalism evolving over that time scale is then
>>>the product of the probabilities for each allele.  The probability
>>>for each allele is approximately the probability of fixation divided
>>>by the mutation rate.
 
>> Again, my supposition is that bipedalism actually evolved over a 
>> relatively short time scale, when a small group of primates was faced
>> with a choice of bipedalism vs. knuckle-walking in adapting to a
>> primarly ground-based existence. 
 
> I think the short time scale you need is a big problem for your
> theory: you need a lot of related changes to have occured at the same
> time. 

You are right about that - as I did a bit more reading it became apparent 
that even with an arboreal existence with a bipedal gait during 
occasional forays on the ground, switching to a terrestrial bipedal gait 
that is at all efficient (i.e. will let you  gather food and avoid 
predators while spending a lot of time on the ground) still takes a lot 
of changes. 

 
> Incidentally, when you write about "...a choice of bipedalism vs. 
> knuckle-walking in adapting to ...", doesn't that sound like selection
> is involved?

You're going to make Larry Moran very upset :-)

 
>>>This is, of course, not what we want: we know that bipedalism has 
>>>evolved, and want to know whether it was by drift or something else. 
>>>To estimate the probability that it was drift wot done it, we have to
>>>calculate the probability that it some other factor than drift caused
>>>bipedalism, and then use Bayes formula to invert the probabilities. 
>>>At this point, I will only point out that probabilities of fixation
>>>of alleles can be increased by giving them a positive selection
>>>coefficient - we would need to know what the selection coefficients
>>>were to do the calculations.
 
>> Aye, there's the rub. I am assuming fairly steep selection against
>> the switch from quadripedalism to bipedalism, i.e. a steep adaptive
>> valley to cross, even though each peak is relatively stable. 
 
> Ah, that's where you're coming from!
 
(snip)

> I think the best approach to this problem is through comparative 
> anatomy.  We have to assume that the species we see today aren't in an
> adaptive trough, and then we identify some of the intermediate forms 
> that humans could have passed through: a bit like stepping stones 
> between the peaks, to really mix metaphores.  Given the amount of 
> speculation we need anyway, I think the morphologies would not need to
> be exactly the same (and we can refine the theories later).

The following link to an article in the Journal of Anatomy discusses a 
number of the possibilities for intermediate forms - and suggests that 
even fairly late ancestors of homo sapiens may have been partly arboreal. 
Of course that would tend to shoot my drift theory.


http://www.blackwell-synergy.com/links/doi/10.1111/j.0021-
8782.2004.00296.x/full/
 

But I still sort of like the thought that one of the crucial changes that 
led to the development of humans - bipedalism - may have been largely a 
matter of luck. Rather Gouldian of me, but it does teach humility.


Yours,

Bill Morse
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