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

Master Index Current Directory Index Go to SkepticTank Go to Human Rights activist Keith Henson Go to Scientology cult

Skeptic Tank!

Chris Colby's "eve" quiz Organization: UTCC Public Access From: (L.A. Moran) Message-ID: Newsgroups: 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.) Chris asks, "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)


E-Mail Fredric L. Rice / The Skeptic Tank