Question on Neutral Mutations

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nomad656
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Question on Neutral Mutations

Post by nomad656 » Thu Dec 24, 2015 9:07 am

Throwing a hail-mary out there by posting this here at this time (4:05 am) and needed clarification so I can continue writing my paper, but I was hoping if someone can clarify something for me on the concept of neutral mutations. Also, this is my first post, HELLO ALL!

The definition of neutral mutations are changes in DNA sequence that are neither ***beneficial*** nor detrimental to the ability of an organism to survive and reproduce. In population genetics, mutations in which natural selection ***does not*** affect the spread of the mutation in a species are termed neutral mutations. The loss or fixation of the gene proceeds based on ***genetic drift***.

The bold's are whats confusing me in my scenario

Okay so my confusion is if a gene does have an affect on the organism, but comes about through genetic drift- is it still considered a neutral mutation?

Lets say the normal gene of this organism after some time "turns off" and the gene is used for the ability of being able to eat skittles.
So, after some time the organism, normally, would not be able to eat skittles anymore because the gene that allows it naturally stops working. But eating skittles is fun and delicious.

So the mutant gene gives the organism the ability to eat skittles even after when its supposed to turn off. And it is found that this mutant gene came about through genetic drift, and fixed in the population through genetic drift, meaning it was essentially random that it happened.

In this scenario, this gene was NOT SELECTED by natural selection, but rather genetic drift.

My confusion is this: if this mutant gene came about through genetic drift and fixed (not sure if fixation in this case matters to my question), would it be considered a neutral mutation, even though it does have a visible affect on the organism? (the affect would be: still being able to eat skittles when "normally" it's not supposed to)

Because in one part of the definition it says mutations that spread not because of natural selection are called neutral mutations. But in another part it says, neutral mutations are neither beneficial or detrimental to the organism. Herein lies my confusion.

Hope someone can help =[

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After writing this out I had a thought with one part of my confusion that may answer my question, but still hoping someone can confirm: the idea of the mutation being "beneficial" or "detrimental" is the key? Even if the organism can eat skittles when it shouldn't, if this ability doesn't help or harm the organisms survival and ability to reproduce, in other words its fitness, then it's considered a neutral mutation. And defining whats beneficial or not would be seeing how this mutation affects it's fitness.

So my follow up question would be (anyone who read this far, I appreciate your curiosity and time for even sparing a thought to this long winded confusing post):

What if a mutation IS considered beneficial, it positively affects the organisms fitness, but it spread through genetic drift, would it still be considered a neutral mutation?

Or is it by definition, if a mutation is beneficial (positive affect on fitness) and spreads through the population, it is always by definition because of natural selection, and therefore not a neutral mutation? And that there aren't any instances where it was seen otherwise?

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Thank you to anyone in advance who takes the time to read all this. And a big THANK YOU to anyone who can answer my question(s)!

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Luxorien
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Post by Luxorien » Wed Dec 30, 2015 6:38 pm

This is probably too late to help you with your paper, but the terms beneficial, detrimental and neutral refer to the effect the mutation has on the organism's success in reproducing. Since you generally have to be alive to reproduce, most traits that are positively selected have some impact on survival.

If a specific allele has an effect on survival, it will tend to increase/decrease because of natural selection. However, this does not mean that genetic drift plays no role in the frequency of such alleles. A beneficial allele could disappear from a population if the one individual who carried that allele died before reproducing for reasons unrelated to the presence of that allele. A detrimental allele could become fixed in a population if, for instance, some catastrophic event just happened to kill off every individual without the detrimental allele.

It sounds to me like you are connecting genetic drift and "neutral" mutations too closely. All alleles are subject to genetic drift, so the fact that a particular trait became fixed because of genetic drift does not necessarily say anything about that allele's status as a "neutral" mutation. Genetic drift will, however, be more likely to affect the frequencies of alleles that have no impact on survival. Genetic drift also plays a much larger role in smaller populations than larger ones.

It is also worth noting that a given allele's status as "neutral" or "beneficial" is entirely dependent on context. That same mutation that confers an advantage to an African swallow may not have any impact on the survival of a European swallow. So that same mutation could be considered beneficial in the one case, but neutral in the other.

I am not 100% sure I understand your skittle-eating example, but it sounds like you are saying that the allele has a visible effect on the organism (allows it to eat skittles) but because this effect was not related to survival, the allele became fixed as the result of genetic drift rather than active selection. In that case, I would consider the allele for skittle-eating to be a neutral mutation, because it did not affect the organism's survival or ability to reproduce. It could be the case that the allele was initially neutral, but became beneficial only after it was already fixed in the population. This is another example of how context (i.e. the environment) can change the status of a particular allele. Whether you consider the allele neutral or beneficial would depend on when you are looking at it. It seems plausible to me that there could be many beneficial traits that started out as neutral mutations that became prevalent through genetic drift.
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