Help re: evolution

[Wiccan_Child]

To all those bringing up meiosis and different chromosome arrangements - still missing the point. Recombination is fine, but ONE of your parents still MUST have the gene. To have a new mutation, you MUST be born from someone who has it. Bacteria can absorb DNA from their environment or transfect each other with it. Eukaryotes can't. If a new mutation "spreads" to fill a whole population, it must EVERYONE in the population has come from the ONE individual (albeit combined with many others). My problem with that is that it seems very unlikely, and creates genetic bottlenecks... (Alternatively, I just can't get my head around it )

Nope: it's just a consequence of long-term population dynamics. If the population is stable (that is, it remains at a more or less constant value over many generations), then inbreeding is inevitable.

If everyone's ancestry was unique (that is, no one bred with a relative), then there should be trillions of humans living just a few generations ago (~40 generations, since 2[sup]40[/sup]=10[sup]12[/sup]). Clearly this isn't realistic. Over time, families interbreed. Family trees are more like elongated hedges, intertwining with one another.

That said, mutations occur at any point between the formation of your constituent gametes (i.e., when your sperm and egg first form), to the moment of conception itself (chiasmata formation, etc). Your parents mutations are passed on to you, but the act of conception itself adds new mutations into the mix.

(Also Wiccan_Child said something similar.)
Doesn't that make the situation even more unlikely? If hairlessness occured due to a single mutation (let's call it A) that would create one bottleneck, i.e. everyone would have to be descended from the person who first got A. But if hairlessness was due to many gradual mutations (A, B, C, D, E ...), then every hairless person would have to accumulate almost every one of those mutations, i.e. everyone who didn't have "A" dies out, "A" fills the population, "B" mutates, everyone who doesn't have "B" dies out, "B" fills the population, "C" mutates, ... etc... etc...

Yes, but the point is that a lineage dying out is less likely than lineages merging.

One trait becomes universal when it is expressed by all individuals (give or take). This doesn't mean all those who didn't express the trait died out, but rather that they bred with someone with the trait, and so their offspring had the trait too.

Otherwise, as you say, multiple bottlenecks form.

But to address my question, if one mutation fills the entire population, those who don't have the mutation must not have successfully reproduced. i.e. died without reproducing (I should have clarified).

Think of it this way: when the mutation is first introduced, who did she mate with? Non-mutants are still capable of breeding, but eventually they're choice of partners becomes more and more dominated by mutants (as the generations go by and lineages continue to merge).

Basically, I can see where you're coming from, but it's a flawed conceptualisation of population dynamics: it's not that non-mutant lineages die out, but that non-mutants end up having mutant children.

Think of it like surnames: just because one surname dies out, doesn't mean the lineage did. It could very well be that the last pair had all daughters, and, when married, they took their husbands names.

Just fyi (apologies if you knew this already), Mitochondrial Eve is the most recent female from whom we are all descended. It's not that her peers failed to breed, or that she was the only woman on Earth, but rather that all our mitochondrial DNA is derived from hers, due to interweaving of lineages and the general flow of genes.
She lived ~170,000 years ago.
Her male counterpart, Y-chromosomal Adam, lived ~60,000 years ago.

 

[Nathan45]

They're still your kids. What do you mean "diagonally"?

it's sortof a made-up word... lol nevermind.


anyway, the point is the genes all get spread around within a population,It has to do with powers of two:

first, there are lots and lots of generations when you talk about the million year time spans that evolution takes place over.

20 year generations / 1 million years = 50,000 generations.

which means that everyone has 2^50,000 ( or 3.16 * 10^15051) non-distinct ancestors from the past million years. When i say non-distinct, i mean... your maternal paternal maternal maternal paternal paternal grandmother might be the same person as your paternal paternal maternal maternal paternal grandmother (but that still counts as two non-distinct ancestors). once you go back far enough you end up with the same bunch of ancestors no matter which direction you head up the family tree, if you know what i'm saying.

The genes, the new mutations, pretty much get spread around everywhere. If you go back a few thousand years, everyone on the planet is related to everyone else in thousands of different ways. The gene pool gets completely mixed. Over time the frequency of beneficial genes is going to increase in the population, and the frequency of deleterious genes is going to decrease. genes that make you faster or less hairy or smarter, are going to propogate in the gene pool... since everyone is related to everyone else, they're going to spread around to everyone. Furthermore, new mutations are always arising, which can spread around once they arise. if they're beneficial they'll become more common.

The key point is this:

The people who lacked the beneficial genes do not necessarily have zero living descendents. They simply have zero living descendents that lack the beneficial gene. In other words, they interbred with people who had the beneficial genes, and then natural selection took it's course. Just because you lack a beneficial gene, doesn't mean your great great great great great grandchild won't have picked it up, you'd basically have to marry into it.

On a macro scale over thousands or millions of years, we evolve more as a species than as individuals (species defined as groups that interbreed with eachother).

 

[Mystman]

The people who lacked the beneficial genes do not necessarily have zero living descendents. They simply have zero living descendents that lack the beneficial gene. In other words, they interbred with people who had the beneficial genes, and then natural selection took it's course. Just because you lack a beneficial gene, doesn't mean your great great great great great grandchild won't have picked it up, you'd basically have to marry into it.

See my awesome illustration. I could've included some more ancestors to cut down on the incest factor, but hey..

 

[Jester4kicks]

To all those bringing up meiosis and different chromosome arrangements - still missing the point. Recombination is fine, but ONE of your parents still MUST have the gene. To have a new mutation, you MUST be born from someone who has it. Bacteria can absorb DNA from their environment or transfect each other with it. Eukaryotes can't. If a new mutation "spreads" to fill a whole population, it must EVERYONE in the population has come from the ONE individual (albeit combined with many others). My problem with that is that it seems very unlikely, and creates genetic bottlenecks... (Alternatively, I just can't get my head around it )

I could be wrong here... but I think we're getting a little caught up on the difference between an inherited trait and a mutation.

From what I understand (and PLEASE correct me if this is wrong), a mutation occurs as a result of a "copying error". Now, once copied, the mutation must still result in functional DNA... otherwise the body recognizes and destroys it. If it IS functional, the same error could (and would) be copied as a regular part of the new organism's DNA.

Now, if that organism procreates, the mutation could be passed on to its offspring. However, at that point, it is not a mutation... it's just an inherited trait (as far as the offspring is concerned).

With that said, let's talk a little bit about these "bottlenecks".

Bombila - the point about speciation is interesting, but doesn't really help. My problem is, 6 billion humans must have come from one ancestor. Not once, but many times - in fact, almost as many times as the number of mutations, from the long journey from chimps to humans.


(Also Wiccan_Child said something similar.)
Doesn't that make the situation even more unlikely? If hairlessness occured due to a single mutation (let's call it A) that would create one bottleneck, i.e. everyone would have to be descended from the person who first got A. But if hairlessness was due to many gradual mutations (A, B, C, D, E ...), then every hairless person would have to accumulate almost every one of those mutations, i.e. everyone who didn't have "A" dies out, "A" fills the population, "B" mutates, everyone who doesn't have "B" dies out, "B" fills the population, "C" mutates, ... etc... etc...

Not quite.

In your example, you're basically viewing "hair" or "hairlessness" as a single yes/no (1 or 0) value on a single chromosome. However, consider if the amount of hair on the organism was actually governed by multiple chromosomes.

Let's say there are 3 chromosomes that determine how much hair will be on the body. (And this is PURELY for example purposes... otherwise I'm about to tick off all the biologists here )

Of our three sample chromosomes, let's say the "value" can be a 1 or a zero. If all three are "set" to "1", the body is completly covered with hair. If all three are set to "0", the body is completely hairless. Combinations of 1's and 0's result in varying degrees of hair/hairlessness.

Now take those possible "hairyness" combinations, multiple them across multiple organisms and MANY generations... and there's PLENTY of room for different distant cousins to range anywhere from completely hairy to completely hairless.

 

[plindboe]

To all those bringing up meiosis and different chromosome arrangements - still missing the point. Recombination is fine, but ONE of your parents still MUST have the gene. To have a new mutation, you MUST be born from someone who has it. Bacteria can absorb DNA from their environment or transfect each other with it. Eukaryotes can't. If a new mutation "spreads" to fill a whole population, it must EVERYONE in the population has come from the ONE individual (albeit combined with many others). My problem with that is that it seems very unlikely, and creates genetic bottlenecks... (Alternatively, I just can't get my head around it )

I'm not sure I understand you right, but saying that everyone comes from the same individual is not the case, as evolution deals with continually changing populations. Think of it like this instead: Every allele or every mutation must have come from one individual.

(Also Wiccan_Child said something similar.)
Doesn't that make the situation even more unlikely? If hairlessness occured due to a single mutation (let's call it A) that would create one bottleneck, i.e. everyone would have to be descended from the person who first got A. But if hairlessness was due to many gradual mutations (A, B, C, D, E ...), then every hairless person would have to accumulate almost every one of those mutations, i.e. everyone who didn't have "A" dies out, "A" fills the population, "B" mutates, everyone who doesn't have "B" dies out, "B" fills the population, "C" mutates, ... etc... etc...

You might think it's unlikely if you think that reduction of hair can only occur in a fixed order and that mutations are rare, none of which is true. Mutations arise all the time, continually providing raw material for evolution to act upon. Each individual has about 100 new mutations. Multiply this number with the population size and you roughly have the number of newly arisen mutations per generation. The vast majority of these mutations will disappear over time due to selection and drift, but a few will become fixed, and we'll be able to trace each back to a single individual.

But to address my question, if one mutation fills the entire population, those who don't have the mutation must not have successfully reproduced. i.e. died without reproducing (I should have clarified).

That is not correct. Remember that a reproducing individual only passes 50% of its genetic material on to each offspring. I have mutations that I received from my mother, that my father doesn't have. That doesn't mean my father didn't reproduce.

Thanks, I'll certainly peruse that at some point

Glad to hear it. It explains many of these things in great depth and detail (sometimes a bit excessively). I haven't read it all the way through yet though, as it's rather heavy reading, but luckily it's also very interesting and enlightening.

Peter

 

[lostaquarium]

Wait, I think I'm starting to get it...

 

[Jester4kicks]

Wait, I think I'm starting to get it...

 

[lostaquarium]

If everyone's ancestry was unique (that is, no one bred with a relative), then there should be trillions of humans living just a few generations ago (~40 generations, since 2[sup]40[/sup]=10[sup]12[/sup]). Clearly this isn't realistic. Over time, families interbreed. Family trees are more like elongated hedges, intertwining with one another.

One trait becomes universal when it is expressed by all individuals (give or take). This doesn't mean all those who didn't express the trait died out, but rather that they bred with someone with the trait, and so their offspring had the trait too.

Got it.

I think I was subconsciously thinking of spider-plants. I knew about meiosis obviously, but it didn't quite "click" until now.

Just fyi (apologies if you knew this already), Mitochondrial Eve is the most recent female from whom we are all descended. It's not that her peers failed to breed, or that she was the only woman on Earth, but rather that all our mitochondrial DNA is derived from hers, due to interweaving of lineages and the general flow of genes.
She lived ~170,000 years ago.
Her male counterpart, Y-chromosomal Adam, lived ~60,000 years ago.

She's one of the things that sparked off my question actually! Though I forgot all of her details. But I'm confused about why you say her peers didn't fail to breed. Surely none of her female peers have descendents alive today?
Also, how was the ~170,000 worked out? Did they take into account the fact that mitoch DNA mutate much more quickly than nuclear DNA?

See my awesome illustration. I could've included some more ancestors to cut down on the incest factor, but hey..
View attachment 119659

Very cool!

You might think it's unlikely if you think that reduction of hair can only occur in a fixed order

That doesn't necessarily matter; if there are several mutations needed, all of them have to be acquired in some sort of order... But it doesn't matter any more. My question is answered

Glad to hear it. It explains many of these things in great depth and detail (sometimes a bit excessively). I haven't read it all the way through yet though, as it's rather heavy reading, but luckily it's also very interesting and enlightening.

Peter

Btw I had a brief look at that site and could only find questions. Do I have to buy the book for the full version?

Unfortunately your post wasn't helpful or even on topic. Sorry.

Thanks to everyone else who answered my question though

 

[mpok1519]

Hey, I have another question about evolution by natural selection. Please bear with me, as I'm really trying to understand this topic.

Natural selection:
Mutations occur randomly. Beneficial mutations occur at very low frequency. Beneficial mutations are only 'propagated' through the population when other animals (who don't have the mutation) die from selection pressures. i.e. we don't share genes like bacteria do, we actually have to come from those parents.

Let's take the example of monkeys-to-humans. I was reading this morning about long latency reflexes. When a muscle is stretched, monkeys respond by co-contracting agonist and antagonist muscles. Humans only contract their agonist muscles. These are reflexes, driven by very simple circuits of neurons (probably just 5) which are genetically coded, not learnt.

Other differences between monkeys and humans: size, upright walking, no hair, larger forebrain, speech, etc... These are in ALL humans and NO monkeys.

So, in the simple transition from monkey to human, do these mutations occur sequentially? e.g. a monkey mutates to have no hair, finds this advantageous, all other monkeys die, this monkey reproduces. Then one of his offspring mutates his long-latency reflexes, all other monkeys die, this monkey reproduces to fill the population... etc. This would suggest a series of evolutionary bottlenecks, which (in my vague impression) is not advantageous, and not the case.

Or, do the mutations occur in parallel? If so, how? Two (or more) beneficial mutations occuring in the same monkey is even unlikelier.

Please explain things using solid examples. No analogies please. I don't cope well with abstract thoughts. Details would be most useful.

Thanks

Mutation is only one of the mechanics of evolution. Descent with modification is a better term to use when describing evolution.

also, don't think of it as monkey + mutations x long time period = human.

Monkeys don't have human speech but they do have a large vocabulary of vocalizations; the reason why humans have the ability to vocalize complex speech patterns has to do with the muscle attachments in the jaw, tongue and throat

also, human ancestors were monkey-like, but in no way similiar to modern monkeys we see today. So to compare humans to monkeys is in a way comparing cats to dogs. Cats and dogs USED to be the same animal, but broke apart into separate genuses and species; they used to look strangely like large racoon rat-dogs imo. So to say humans evolved FROM monkeys is inaccurate, as they evolved side-by-side. they share a common ancestor, but evolved side by side, not from and to.



[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Ardipithecus ramidusAustralopithecus anamensisAustralopithecus afarensisAustralopithecus africanus
Australopithecus garhiParanthropus aethiopicusParanthropus boiseiParanthropus robustus[/SIZE][/FONT]
[FONT=Arial, Helvetica, sans-serif][SIZE=-1]Homo habilisHomo rudolfensisHomo ergasterHomo erectusHomo heidelbergensisHomo neanderthalensisHomo sapiens[/SIZE][/FONT]



some of these species went extinct, some of them evolved.

diet, enviornment, mutation, geographical isolation, population isolation; these all are mechanics that drive evolution.

 

[plindboe]

Wait, I think I'm starting to get it...

Great! Often these things takes some discussion or additional reading before they really start to sink in. I've been confused about many of these details too, and I still have many things that I haven't properly understood yet.

She's one of the things that sparked off my question actually! Though I forgot all of her details. But I'm confused about why you say her peers didn't fail to breed. Surely none of her female peers have descendents alive today?

Why wouldn't they? Remember that the mitochondrial genome is inherited independently from the rest of the genome. Also remember that each chromosome is inherited independently from the other chromosomes. And due to cross-over, alleles on the same chromosome are not always inherited together either. So while you carry around Mitochondrial Eve's mitochondrial genome, you also carry around lots of fragments in the rest of your genome (i.e. nuclear genome) that the female peers had but which Mitochondrial Eve didn't have.

That doesn't necessarily matter; if there are several mutations needed, all of them have to be acquired in some sort of order... But it doesn't matter any more. My question is answered

Of course there will be an temporally acquired order of mutations when one examines them looking back, my point was that no specific order is required. In other words ABC, ACB, BAC, BCA, CAB, CBA all leads to the same result.

Btw I had a brief look at that site and could only find questions. Do I have to buy the book for the full version?

Yes, it's a real textbook (only chapters 27 and 28 are freely available online). The book is used in universities around the world, so if you really want to understand what evolution is about, that's the book for you. Since you're a 3rd year medical student I assume it will be on your level of understanding. You could try out your university library as well.

Btw, there are also figures and tables from the book you can check out on that website. You might find some of them interesting (though they don't all make sense without the text).

Peter

 

[sfs]

From what I understand (and PLEASE correct me if this is wrong), a mutation occurs as a result of a "copying error".

Error during copying is one way mutations can arise. They can also occur because of breaks to the DNA, or because of spontaneous chemical changes while the DNA is just sitting there.

Now, once copied, the mutation must still result in functional DNA... otherwise the body recognizes and destroys it. If it IS functional, the same error could (and would) be copied as a regular part of the new organism's DNA.

No. There is no requirement that a mutation result in functional DNA, and no mechanism to eliminate mutations based on how they affect function. Of course, if a vital gene is damaged by mutation, the cell carrying the mutation may die, or offspring that come from that cell may not be viable, but that's a different matter.

 

[Mystman]

No. There is no requirement that a mutation result in functional DNA, and no mechanism to eliminate mutations based on how they affect function. Of course, if a vital gene is damaged by mutation, the cell carrying the mutation may die, or offspring that come from that cell may not be viable, but that's a different matter.

Going from kinda-sorta-memory here, but can't apoptosis (the cell committing suicide) also be triggered by the cell detecting that it has some unwanted mutations on it's DNA, even if the mutation itself doesn't actually make the cell less viable?

e.g., mutations that would make a cell cancerous (make it grow uncontrollably) also trigger apoptosis. So for cancer to occur, the apoptosis pathway first needs to be destroyed via mutation, before the "overdrive growth"-mutation can happen.

(not exactly sure thow this is related to the topic at hand.. but I think it is )

 

[lostaquarium]

Why wouldn't they? Remember that the mitochondrial genome is inherited independently from the rest of the genome. Also remember that each chromosome is inherited independently from the other chromosomes. And due to cross-over, alleles on the same chromosome are not always inherited together either. So while you carry around Mitochondrial Eve's mitochondrial genome, you also carry around lots of fragments in the rest of your genome (i.e. nuclear genome) that the female peers had but which Mitochondrial Eve didn't have.

Aha. I get it. Mitochondrial Eve's female peers may have had sons, who didn't pass on their mitochondrial DNA...

Hence she wasn't even a real bottleneck Arrrgh!! Too many new concepts to cope with!

Yes, it's a real textbook (only chapters 27 and 28 are freely available online). The book is used in universities around the world, so if you really want to understand what evolution is about, that's the book for you. Since you're a 3rd year medical student I assume it will be on your level of understanding. You could try out your university library as well.

Thanks, I'll look it up when these exams are over.

Going from kinda-sorta-memory here, but can't apoptosis (the cell committing suicide) also be triggered by the cell detecting that it has some unwanted mutations on it's DNA, even if the mutation itself doesn't actually make the cell less viable?

e.g., mutations that would make a cell cancerous (make it grow uncontrollably) also trigger apoptosis. So for cancer to occur, the apoptosis pathway first needs to be destroyed via mutation, before the "overdrive growth"-mutation can happen.

Yep, apoptosis can be triggered by many things, one of them being breakages in DNA. But, breakages in DNA are pretty major, and I don't think they lead to cancer anyway. The mutations that lead to cancer are usually smaller ones, like point mutations that knock out the signals for apoptosis.

(not exactly sure thow this is related to the topic at hand.. but I think it is )

No, but it's ok My main question has been answered, so we're going off on all sorts of tangents now It's a cool topic!

 

[R3quiem]

Well, a number of things.

The most important thing to realise here is that you have chromosomes. As a woman, you have 23 pairs.

Often (not always), a beneficial mutation occurs on only 1 specific chromosome.

The second important thing (or most important, depending on who you ask ) is sex. Sex allows us to mix chromosomes, and that in a way allows for "parallel" beneficial mutations.

Say that there are 4 mutations:
Beneficial mutation B1 on chromosome 1
Beneficial mutation B2 on chromosome 2
Detrimental mutation D3 on chromosome 3
Detrimental mutation D4 on chromosome 4

I carry 1 copy of B1 and 1 copy of D3
My wife carries 1 copy of B2 and 1 copy of D3
All the other chromosomes in us both are normal.

Now, we proceed to make lots of kids. Since a child randomly chooses 1 copy of each chromosome from each of their parents, you will get children ending up with every possible combination of the aforementioned mutations or normal chromosomes.

So, you will get children having both the D3 and D4 mutations.. but you will also get children having the B1 and B2 mutations.

Kids having the B1 and B2 mutations will probably get more children of their own. Those children will again get more children, etc.

Now, lets say that in another town the same thing has been happening... only there children carrying B5/B6/B7 mutations have been produced. If those kids from another town mate with my grandchildren, they will produce B1/B2/B5/B6/B7 children. So yeah, parallel evolution.

BUT

If different mutations need to take place on the same chromosome, the story becomes a whole lot trickier. And sequential evolution also comes into play. But also 'weird', non-classical genetics start to enter the picture. Genes can be moved from one chromosome to the other, they can be copy/pasted between the two copies that you have of each gene, etc..

This post was probably the most informative and direct way of answering the OP's question that I have seen.

The one concept that I can add is the fact that not every mutation we have is a result of that mutation being the most fit. Sometimes, we can inherit traits that do not affect our ability to survive and mate because those traits come packaged with other traits that do.

As one example, I remember reading about a professor who was studying mutations and traits in foxes. He had a certain population of foxes, and choose to breed those that were most docile. Because he controlled the environment, and only bred the most docile of foxes, docile foxes were the most "fit" and aggressive ones were the least fit for survival and reproduction. After a number of generations, his population consisted mainly of docile foxes.

However, he also noticed a number of differences between his current population of foxes and his previous population of foxes. For example, his newer generations of foxes had rounder snouts, less rigid ears, and a number of other minor physical differences. Genetic testing showed that these traits seem to have been packaged on the same chromosomes as those that determine the level of aggression. In other words, certain traits that may not be advantageous (or even some that are slightly disadvantageous) can be propagated through a population because they are packaged with other, advantageous traits.

 

[peadar1987]

Nothing like a tangent...

Tangent 1- I read a while back that everyone with blue eyes today is descended from one mutation that probably took place in the region to the north of the Black Sea. It's kind of cool to look at someone with blue eyes and know that you have a common ancestor who was a hunter-gatherer in the Crimea! It's also an interesting illustration of the fact that a mutation doesn't actually have to be an advantage to be passed on, it just has to avoid killing you!

Tangent 2- One theory about why we walk upright is that it gradually occurred as out brains got bigger. A human skull with all the squishy stuff inside is significantly heavier than another ape's skull, so if we walked on all-fours with our head stuck out in front of us it would put a huge amount of strain on our neck. Having the head on top of the body means there is no lever arm and the weight can be taken as direct compressive stress in the vertebrae.

 

[Wiccan_Child]

.

 

[sfs]

Aha. I get it. Mitochondrial Eve's female peers may have had sons, who didn't pass on their mitochondrial DNA...

Or their great-great-great-granddaughters had sons.

Hence she wasn't even a real bottleneck

Right. For every bit of DNA in the genome (including mitochondrial DNA), all of the copies in the population are related, i.e. they ultimately descend from a single ancestral bit of DNA. But there is no reason that the ancestral bits for different parts of the genome had to be in the same person.

It's easier to think in terms of the pieces of chromosome, rather than in terms of individuals. Because of sexual reproduction and recombination, the genealogy of an individual's DNA is horribly complex, but the genealogy for each little piece is very simple: a tree, with the branch tips representing all of the modern copies and the root of the tree representing the common ancestral DNA.

Yep, apoptosis can be triggered by many things, one of them being breakages in DNA.

Yes, it's the damage that triggers apoptosis, rather than any changes (mutations) caused by the damage, or the loss of function. (Although I'm sure there are some mutations that will cause enough cellular disruption to trigger apoptosis that way.)

But, breakages in DNA are pretty major, and I don't think they lead to cancer anyway.

They can when they've been repaired. If a break is repaired incorrectly, which can easily happen, you can get fusion genes (a new gene created by joining pieces of two genes) or multiple copies of a gene, or loss of gene regulation due to a gene moving to a new neighborhood. All of these can cause or at least contribute to cancer.

 

[sfs]

Tangent 1- I read a while back that everyone with blue eyes today is descended from one mutation that probably took place in the region to the north of the Black Sea. It's kind of cool to look at someone with blue eyes and know that you have a common ancestor who was a hunter-gatherer in the Crimea! It's also an interesting illustration of the fact that a mutation doesn't actually have to be an advantage to be passed on, it just has to avoid killing you!

While mutations don't have to be advantageous to spread, the one that caused blue eyes probably did spread by positive selection. There is now strong evidence that mutations in multiple genes that caused lighter skin pigmentation were selected for after modern humans left Africa; one of these has the side-effect of changing pigmentation in the iris as well.

 

[juvenissun]

While mutations don't have to be advantageous to spread, the one that caused blue eyes probably did spread by positive selection. There is now strong evidence that mutations in multiple genes that caused lighter skin pigmentation were selected for after modern humans left Africa; one of these has the side-effect of changing pigmentation in the iris as well.

Could you briefly describe the evidence?

 

[Hespera]

Nothing like a tangent...

Tangent 1- I read a while back that everyone with blue eyes today is descended from one mutation that probably took place in the region to the north of the Black Sea. It's kind of cool to look at someone with blue eyes and know that you have a common ancestor who was a hunter-gatherer in the Crimea! It's also an interesting illustration of the fact that a mutation doesn't actually have to be an advantage to be passed on, it just has to avoid killing you!

Tangent 2- One theory about why we walk upright is that it gradually occurred as out brains got bigger. A human skull with all the squishy stuff inside is significantly heavier than another ape's skull, so if we walked on all-fours with our head stuck out in front of us it would put a huge amount of strain on our neck. Having the head on top of the body means there is no lever arm and the weight can be taken as direct compressive stress in the vertebrae.

The advantage of light skin / hair / blue eyes is related to the low light and vit D requirements for northern people.

As for the heavy skull, well, if that was the deal then we'd probably have bipedal rhinos and triceratops!

A lot of the dinosaurs became bipeds. It clearly worked for them. (alligatoars are descended from bipeds... kind of funny, went back to four legs and went back to the water)

Any notion why dinosaurs went bipedal, would it be in any way comparable to why people did?