A question about evolution and chromosomes. {Deludedgod?? -HD}

JillSwift
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A question about evolution and chromosomes. {Deludedgod?? -HD}

If I recall correctly, somewhere in the evolutionary line leading to homo sapiens there was a chromosome "loss" (a pair merged - we have 46 and other apes 48).

Now, this mutation would mean the new individual(s) with the 46 chromosomes shouldn't be able to produce offspring themselves capable of reproduction - by mating with members of the population that still had 48 chromosomes.

Yet, obviously something worked out. Anyone have any idea how it worked out? My knowlege is obviously deficient. =0.o=

 

 

{MOD EDIT:  Appeal to authority  Smiling  -HD}

"Anyone can repress a woman, but you need 'dictated' scriptures to feel you're really right in repressing her. In the same way, homophobes thrive everywhere. But you must feel you've got scripture on your side to come up with the tedious 'Adam and Eve not Adam and Steve' style arguments instead of just recognising that some people are different." - Douglas Murray


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I've wondered that myself.

I've wondered that myself. Random speculation: Since the 48 became 46 by fusing two chromosomes, perhaps the 'damage' wouldn't be so severe, since all the genes are still there, just in a slightly different configuration.

Specifically, two chromosomes would fuse in one individual, resulting in a single fused chromosome 2, which means this individual would have 47 chromosomes. Some gametes would carry 23 chromosomes and some 24. Crossover might still be possible to some extent, in a similar manner to how the X and Y chromosomes can crossover, even though they are different lengths.

This individual would give rise to a number of individuals with 47 chromosomes. Eventually, two of these would mate, and some of their offspring would have 46 chromosomes, inheriting the fused version from both parents.

Either the fusing created a selective advantage, or the population was small enough that genetic drift just happened to wipe out the non-fused variety, giving rise to an entire sub-population that had only 46 chromosomes. This sub-population would likely diverge more quickly from the 48s due to the different configuration (again, I'm speculating here). Once they diverge enough that they no longer can mate with the 47s or 48s, then you get a new species.

One issue I can see with this idea is that if the fused chromosome came from one individual, and only participated in crossover with (presumably) the larger of the non-fused chromosomes, then a portion of the fused version would be pretty much identical when the two 47s mate to produce the first 46. If these two portions are mostly identical, that would lead to some pretty serious inbreeding depression, which usually leads to lower fitness, meaning that the 46s wouldn't be likely to take over.

On the other hand, perhaps the fused variety participated in crossover with both non-fused chromosomes alternately, sometimes getting a bit of variation from one, sometimes from the other. That would give rise to several different variants of the fused chromosome with less effect of inbreeding depression when two mostly-different fused variants join to make a 46.

On the third hand, it strikes me that maybe this whole thing arose from inbreeding in the first place! If the sub-population was already suffering from inbreeding depression, then the fusing of a chromosome wouldn't be as deleterious, relatively speaking.

Who knows? Again, I'm just speculating. There's probably a better explanation in google-space somewhere.

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I just came across a blog

I just came across a blog entry on this that suggests a possible pathway.

Apparently chromosome 2 has a region in the middle that looks very much like the sequences at the ends of a chromosome, suggesting that it is made up of what were at some stage two separate chromosomes. So it is suggested that two originally separate chromosomes became linked, end to end, in some individual, due to some variation in their code. Each section could probably still link up with the separate versions from other individuals, and so reproduce, and spread, if there was some slight advantage in either the particular sequence or this smaller number of genes itself.

Seems plausible to me.

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 Bob, that makes sense to

 Bob, that makes sense to me, but I hope Deludedgod weighs in on this.  I feel certain he has some knowledge of this.

 

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 Apropos of nothing, I just

 Apropos of nothing, I just read your sig, JillSwift.  I love Terry Pratchett, and think that's a great line.  What book was that from?  Small Gods?

 

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I had written something in

I had written something in response to a nearly identical question which I am trawling through old threads for. Back soon.

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

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 Holy shit.. speaking of

 Holy shit.. speaking of sigs... Deludedgod, my hat is off to you, sir.

 

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Apologies to Natural, I was

Apologies to Natural, I was distracted as I started to read your post, only just realized when I went back and read it that you were describing basically the same thing as I dug up...

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deludedgod
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Quote:Now, this mutation

Quote:

Now, this mutation would mean the new individual(s) with the 46 chromosomes shouldn't be able to produce offspring themselves capable of reproduction - by mating with members of the population that still had 48 chromosomes.

Two words:

Chromosomal polymorphism.

I wrote something about it in a previous post which I am sifting for. In general, the processes responsible for chromosomal variation among and within species (translocation, flipping, etc.) do not necessarily block interbreeding, and in general the process happens slowly enough for the mating of distinct polymorphs to be possible.

"Physical reality” isn’t some arbitrary demarcation. It is defined in terms of what we can systematically investigate, directly or not, by means of our senses. It is preposterous to assert that the process of systematic scientific reasoning arbitrarily excludes “non-physical explanations” because the very notion of “non-physical explanation” is contradictory.

-Me

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JillSwift
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deludedgod wrote:Two

deludedgod wrote:
Two words:

Chromosomal polymorphism.

I wrote something about it in a previous post which I am sifting for. In general, the processes responsible for chromosomal variation among and within species (translocation, flipping, etc.) do not necessarily block interbreeding, and in general the process happens slowly enough for the mating of distinct polymorphs to be possible.

Wow. Never heard of that before. The Wikipedia article has some nice links about it.

Thanks bunches everyone! Laughing out loud

 

"Anyone can repress a woman, but you need 'dictated' scriptures to feel you're really right in repressing her. In the same way, homophobes thrive everywhere. But you must feel you've got scripture on your side to come up with the tedious 'Adam and Eve not Adam and Steve' style arguments instead of just recognising that some people are different." - Douglas Murray


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Hambydammit wrote: Apropos

Hambydammit wrote:
Apropos of nothing, I just read your sig, JillSwift.  I love Terry Pratchett, and think that's a great line.  What book was that from?  Small Gods?
Honestly, I dunno. I can't read Pratchet, He's too nonlinear for me. I just get confused. I saw it on a QOTD list and liked it.


 

"Anyone can repress a woman, but you need 'dictated' scriptures to feel you're really right in repressing her. In the same way, homophobes thrive everywhere. But you must feel you've got scripture on your side to come up with the tedious 'Adam and Eve not Adam and Steve' style arguments instead of just recognising that some people are different." - Douglas Murray