# Chris Colby's +quot;eve+quot; quiz Chris Colby writes that under some circumstance we can

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Chris Colby's "eve" quiz

Organization: UTCC Public Access
From: lamoran@gpu.utcc.utoronto.ca (L.A. Moran)
Message-ID:
Newsgroups: talk.origins

Chris Colby writes that under some circumstance we can determine the
geographical origin of a population from mitochondrial DNA;

"Allow me to explain using a simple model. Let's say there are 5
islands lying directly in a row. Lets say "Mom" (a much better
name than "eve", in my extremely biased opinion) lives on island
1. In the course of time, she has a female descendant migrate to
island 2. She (the colonizer of 2) eventually has a female descendant
migrate to 3 and so on out to five.

Now, where did "Mom" originate from? Simple, sequence the
maternal marker (mtDNA) and draw a cladogram. You'd get:
(numbers represent mtDNA from island #)

1    2   3   4   5
*    *   *   *   *
*    *   *   *
*    *   *
*    *
* *
*
*

"Mom" is always going to be from the outgroup. Note that it doesn't
matter if all islands are initially inhabited or not or which
island "Mom" originates on (proof is left as an excercise for the
reader -- try starting "Mom" on 3 for a start.)

Chris, your example looks so simple that I'll bet you can't understand how
anyone who questions the logic could be so stupid. The problem is that you
have "rigged" your example in such a way that it can't fail to give you
the result you want.

Here's another way of looking at it.

Imagine that our hypothetical population lives on island 3. Over a period of
time the population grows and shrinks until there are only five females
whose mitochondrial DNA is related in the following manner:

|------ A
|---|
|   |------ B
-----|
|    |----- C
|    |
|----|  |-- D
|--|
|-- E

Let's assume that female A migrates to island 2 and that lines B and D die
out. After a few more generations we have:

|--------- F
|
|-------A--|   |----- G
|---|          |---|
|   |------ b      |----- H
-----|
|                 |------ I
|    |----- C-----|
|    |            |------ J
|----|  |-- d
|--|      |--------- K
|-- E--|
|       |- L
|------ |
|- M

Suppose that female F migrates from island 2 to island 1 and that females I
and K move to island 4. Lines H, J and L die out. After a short time on island
4 a descendant of female I moves on to island 5. A descendant of female M
moves to island 2 in a "second wave" of migration".  After a few generations
we have:

|------- 1      ISLAND 1
|--|
|  |------- 2      ISLAND 1
|----------F-|
|            |---------- 3      ISLAND 1
|
|-------A--|   |------G------------ 4      ISLAND 2
|---|          |---|
|   |------ b      |----- h
-----|                              |------- 5      ISLAND 5
|                 |-------I----|
|    |----- C-----|            |------- 6      ISLAND 4
|    |            |------ j
|----|  |-- d
|--|      |--------- K ----------- 7      ISLAND 4
|-- E--|
|       |- l         |-- 8      ISLAND 2
|------ |      |-----|
|--M---|     |-- 9      ISLAND 3
|
|-------- 10     ISLAND 3

By looking at the mitochondrial DNA of females 1-10 how would you prove that
their ancestors lived on island 3? Don't you think that this is a more
realistc example than the one you used?

Chris, the problem with your logic, and that of many workers in the field, is
that you are making three unnecessary assumptions. You are assuming that the
populations are genetically simple and small. (I made a similar assumption in
my example but at least I had populations of more than one or two.) You are
assuming that the migrants are genetically homogeneous rather than a mixture
of different lineages. And, you are assuming that the migration is linear;
that is, each successive migration moves further away from the point of origin.
(This last assumption begs the question.)

"BONUS QUESTION: Could the geographic origin of "Mom" be found
using mtDNA sequences in the absense of genetic drift?
Explain. (Hint: what if each female had two daughters, and
each always reproduced (having two daughters)?)

I'll answer this if nobody else does. Just to annoy Onar, let's
say this is worth +25 in the t.o. home game 8-) This wouldn't
be a half-way bad question for a population genetics test."

Current differences in mitochondrial DNA sequences are presumed to be due
to random mutations that have become fixed in local populations (demes) due
to genetic drift. There is no evidence of natural selection. I assume that
in the absence of genetic drift you mean that mutations could not become
fixed in a population due to chance alone. Thus the answer to your question
is "no"; the geographic origin of "Mom" could not be found because everyone
would have the same mitochondrial DNA sequence. (Do I get +25?)

Laurence A. Moran (Larry)

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