Out of Africa: geographyic parsimony errors

arlin at ac.dal.ca arlin at ac.dal.ca
Sun Nov 17 15:56:06 EST 1991


In article <robison.690333981 at ribo>, robison at ribo.harvard.edu (Keith Robison) writes:
> lamoran at gpu.utcs.utoronto.ca (L.A. Moran) writes:
> 
>>Keith, can you construct an Out-of-Africa senario that is so obvioulsy more
>>reasonable and more simple? Any senarios that I can think of still require
>>many migrations between continents. For example, an early population
>>would have had to migrate out of Africa but some descendents of this group 
>>returned. Or, several distinct populations left Africa at different times. 
> 
> Using the Vigilant et al (1991, Science 253:1503-7) data, and trying to
> minimize the number of migrations, I get the following table:
> 
> 
> Origin assumption		  Africa		Somewhere else
> ----------------------------------------------------------------------
> Migrations from origin	    5			      2  
> 
> Back-migrations to origin	    7			     26
> 				-------------------------------
> Total migrations		   12			     28
> 
> 
> Does this answer your question?
> 
> Keith Robison
>  
Keith Robison's analysis of "migration" events necessary to explain
the present distribution of mtDNA types is flawed in two major
respects.

First, in this unfortunate method of parsimony analysis on geographic
states, migrating populations are being treated as lineages rather
than populations.  That is, the parsimony analysis assumes that there
is a one-to-one correspondence between historical human migration
events and the translocation of single lineages of mtDNAs, so that
each migration transferred one and only one lineage from point A to
point B.  This is an unwarranted assumption that has recently been
falsified by mtDNA studies on native americans (Ward, et al. 1991,
Proc. Natl. Acad. Sci. 88: 8720).

Second, even if we could treat population migrations as
character-state changes in lineages, we would not come up with the
numbers that Keith Robison derives. A parsimony analysis is carried
out by using allowable character state transitions to explain a
distribution of observed character states within a pre-specified
phylogeny. In the analysis that Keith Robison is presenting, the
observed character states are geographic locations (Africa or
not-Africa) of the bearers of mtDNA types, and the phylogeny is the
supposed phylogeny of the mtDNA types.  The observed character states
for extant lineages are given on p. 1505 of the September 27 issue of
Science (Vigilant, et al., 1991), along with the tree.  The arguments
below will not make sense unless you are looking at the diagram on p.
1505.  The question that Keith is trying to answer is this: how many
character state changes (Africa-to-not-Africa or not-Africa-to-Africa)
does it take to explain the observed character states if A) the
ancestral character state was *Africa* or B) *not Africa*.  His
answers are A) 12 and B) 28.

The first answer is correct but the second is incorrect.  Looking at
the tree on p. 1505, it is easy to see that if the ancestral state was
*Africa*, then changes to *not-Africa* are needed for 1) type 23; 2)
type 28; 3) the ancestor of 49 & 50; 4) type 58; 5) the ancestor of
types 74 through 135.  In addition, many descendants of the ancestor
of types 74-135 went back to Africa, so we must postulate
back-migrations for 6) type 76; 7) the ancestor of types 77 & 78; 8)
type 83; 9) type 100; 10) type 103; 11) the ancestor of types 105-107;
and 12) type 127.  Keith seems to have correctly identified all of
these events (at least he came up with the right total of migrations
and back-migrations).

If the ancestral state was *not-Africa*, then changes to *Africa* must
be postulated for 1) the ancestor of types 1-9; and 2) the ancestor of
types 10-135.  Back migrations will then have to be postulated for 3)
type 23; 4) type 28; 5) the ancestor of 49 & 50; 6) type 58; 7) the
ancestor of types 74 through 135. In addition, many descendants of the
ancestor of types 74-135 went back to Africa, so we must postulate
migrations for 8) type 76; 9) the ancestor of types 77 & 78; 10) type
83; 11) type 100; 12) type 103; 13) the ancestor of types 105-107; and
14) type 127.  [these numbers should look familiar-- see previous
paragraph].  So the parsimony tally for this hypothesis should be 14,
not 28, as Keith Robison suggested (Keith seems not to have recognized
the necessity of event #7, and so missed finding the most 
parsimonious solution for the out-of-not-Africa hypothesis). 


So, the score is 12 for out-of-Africa and 14 for out-of-not-Africa, so
it looks like out-of-Africa is the more parsimonious by a small
margin.  But what does this result really mean?  I am just posting
this in order to correct the analysis given by Robison, NOT to support
his conclusion that "the molecular evidence for an African origin of
humans [sic] is rather certain."  In my opinion, although the result FAVORS
the hypothesis that *mtDNA types* now present in the population
originated in Africa, it in no way makes it CERTAIN or even makes it highly
likely, because of erroneous assumptions (treating populations like
lineages), methodological errors and statistical sleight-of-hand.  More
importantly, the question of the geographic origin of extant *mtDNA
types* is not the same as the question of the origin of the whole
extant human *gene pool*, and it is hardly equivalent to the question
of the origin of the human *species*.  Most people reading this
newsgroup should already know from years of discussions about "Eve"
that these questions should be kept conceptually distinct, though they
are in some ways related.

Arlin Stoltzfus

Arlin at ac.dal.ca



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