Re: A Specification for a
Paul Crowley (Paul@crowleyp.demon.co.uk)
Sun, 05 Nov 95 15:52:08 GMT
In article <4783na$i40@geraldo.cc.utexas.edu>
aduncan@mail.utexas.edu "Alex Duncan" writes:
> Paul@crowleyp.demon.co.uk writes:
>
> > . . it is parsimonious to assume that all the features
> >that make a species distinctive appeared at the point of speciation,
> >if they were not present in the CA.
> Our current understanding of evolution is that it is ALWAYS happening.
> It has been demonstrated that there are some genetic loci in which no
> change occurs for short periods of time (or even very long periods of
> time, e.g., gene for histone). However, the norm is for evolutionary
> change. In other words, your "parsimonious" assumption is incompatable
> with the way the world is observed to work.
Evolution is always happening, but the vast bulk of changes that
persist are neutral as regards selective advantage; they appear
to have no, or negligible, effects on morphology or behaviour.
We only know about them from looking at the DNA. Taking chimps as
an example: we are fairly sure that they have occupied the same,
or closely similar, environment for the last 7 Myr. We have to
ask the question: "Were they significantly different then?"
If we say "Yes, they probably were" we are implying that they
solved their survival problems in a different way - and in effect
that they were a different species (in other than a purely DNA
sense).
We can't know the answer. But, given that we need to have a
working hypothesis, the answer "No, they were probably quite
similar" is IMO better - i.e. more parsimonious.
> Assume that we had no fossil evidence for human evolution. Following
> your assumption, we would posit that at the point of speciation from the
> CA, fully modern humans appeared, and then didn't change for 5 - 6 Myr.
We do have the evidence of our own bodies. Our brain is much
larger than that of apes. Its growth was either gradual or
sudden. Gradual makes more sense. So, *substantially*, I am
saying that the human species appeared 5-6 mya and the only major
change in 5 Myr was in brain size. (I'm not denying lots of
less important changes.)
> >Nakedness is a most distinctive feature; so is bipedalism. Both
> >would seem to represent very particular adaptions to a new form of
> >life. Our job is to identify that form of life. We know large
> >brains came later. Maybe, if we could link nakedness with large
> >brains we could escape the problem; but until we have such an
> >explanation, a link with bipedalism can be (weakly) assumed.
>
> Frankly, I don't think this can even be "weakly" assumed. When we
> examine the fossil record, we see a gradual accumulation of the features
> that make us "human." It is true that we are only examining skeletal
> evidence, but your "weak assumption" would force an unparsimonious model
> in which all of the soft tissue features of humans appeared at the time
> of speciation from the CA, and skeletal evolution proceeded at a more
> leisurely pace. It seems unlikely, to put it mildly.
A weak assumption about nakedness being associated with speciation
has no particular implications for "all of the soft tissue changes".
Many of the latter (e.g descent of the laranyx) could be linked to
brain development and use of language and could be later. May main
point is that nakedness is a most peculiar adaption - as also is
bipedalism - that to separate them is to require two explanations
and imply that there were two radical changes in habitat and/or
behaviour.
Paul.
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