Re: [asa] Two questions... (biological bottlenecking with Adam and Eve)

> I think we may agree on something. Can we both agree that, biologically speaking, according to DNA, there is no way to distinguish a line between human and ape because it is a ring-species type of deal?<

Roughly. A couple caveats:
Ring species strictly applies to contemporaries across a geographic
region. Evolutionary continuity is the issue of interest.
DNA gives us no a priori basis for drawing the line within the
evolutionary lineage to humans from apes. However, it is possible to
make a definition of the dividing line for which molecular data could
be of relevance. For example, to take a standard biological
definition, having a certain level of genetic difference (especially
in genes that are involved in egg-sperm recognition or other aspects
of determining capability to interbreed) could be indictive of when
the chimp and human lineages (or Homo versus Australopithecus or
whatever pair you like) have diverged into separate species. However,
this tells us nothing about whether the human lineage has other traits
of interest, just about when it becomes reproductively isolated from
something else. Likewise, if you define human as having a certain
level of mental capacity, for example, you could look at genes that
relate to intelligence. Of course, if you had decided where to draw
the line on other grounds and could get all the relevant DNA samples,
you could probably find something that correlated.

> That is to mean, the DNA gradient change between ape and human is grayscale (on a continuum) with no radical DNA differentiation in between adjacent species- the only major differentiation comes into play is when we are missing the DNA (sample) from an intermediate species and then assume that the intermediate doesn't exist.<

Yes; the only difference I know of that might be considered "radical"
is the chromosome rearrangement, but we don't have any way to pin down
exactly when it occurred and it would have little effect besides
possibly interfering with interbreeding-other genes would continue
their relatively gradual changes.

The cummulative differences can be impressive or not, depending on how
you look at them. Morphologically, we're quite distinctive-we're
built for upright walking rather than knuckle walking and climbing,
but we can see those changes gradually coming in the fossil record.
Genetically, the amount of diference in DNA sequence isn't all that
remarkable (as a percentage-the effects can be impressive), but
activity is remarkably different in many ways (i.e., what gene is
turned on when and where). Except for anything recent enough for
comparison with late Pleistocene DNA (e.g., Neandertal), pinning down
just when the genetic changes occurred is not easy. Perhaps a lot of
the changes relating to mental capacity took place in a short
interval, but perhaps not.

Basically, I am being much stricter than you in my interpretation of "no way".

As far as I know, there are no abrupt physical or genetic markers to
say "fully human" versus "not quite". It is possible to pick out one
feature and use that as a divider, but nothing jumps out from
biological data as the clear choice.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
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