Talk:Genetic drift

From Wikipedia, the free encyclopedia

WikiProject Genetics This article is part of WikiProject Genetics, an attempt to build a comprehensive and detailed guide to genetics on Wikipedia. If you would like to participate, you can edit this page, or visit the project page to join the project and/or contribute to the discussion.
??? This article has not yet received a rating on the quality scale.
??? This article has not yet received an importance rating.
A summary of this article appears in evolution.
Charles Darwin This article is part of WikiProject Evolutionary biology, an attempt at building a useful set of articles on evolutionary biology and its associated subfields such as population genetics, quantitative genetics, molecular evolution, phylogenetics, evolutionary developmental biology. It is distinct from the WikiProject Tree of Life in that it attempts to cover patterns, process and theory rather than systematics and taxonomy. If you would like to participate, there are some suggestions on this page (see also Wikipedia:Contributing FAQ for more information) or visit WikiProject Evolutionary biology.
Start rated as start-Class on the assessment scale
High rated as high-importance on the assessment scale
This article is within the scope of WikiProject Human Genetic History, a collaborative effort to improve Wikipedia's coverage of genetic genealogy, genetics-based population history, and associated theory and methods. If you would like to participate, please visit the project page, where you can join the project and/or contribute to the discussion.


Archives: Archive 1, Archive 2, Archive 3, Archive 4

See also: Talk:Evolution/Genetic drift

The first sentence of this article, viz: "In population genetics, genetic drift is the statistical effect that results from the influence that genetic drift confers to become more common or more rare over successive generations.", is incomprehensible.Joescallan 13:44, 26 March 2007 (UTC)

Contents

[edit] Some points

1. Need to point out Anglo-American feud (Sewall Wright v Ronald Fisher and E.B. Ford)over the relative merits of selection and drift. See scarlet tiger moths for link to Fisher Ford 1950 paper. This probably needs its own page. I think that drift is relatively unimportant at levels that matter, though it is important at the molecular level. Simulations show this. Wright had some awfully wacky ideas about the goodness of molecules, when selfish genes are the real cause.

2. Need some maths.

probablity of fixation over an infinite time equals allele frequency. Time to fixation... I'm not sure the introduction is expressed as well as it could be either.

Idea of drift can be used by picking cubes out of a bag. Pick two cubes at a time, for loci. and adjust allele frequency.

(references Wright 1931/8) or is that F-statistics? Duncharris 11:36, May 3, 2004 (UTC)

3. Nonsense

"But the average is never tallied, because each generation parents the next one only once. Therefore the frequency of an allele among the offspring often differs from its frequency in the parent generation."

This is nonsense. Everyone knows birds are not raised by their grandparents. And what does this have to do with allele frequency anyway?


omg this is cool...writing on this :] —Preceding unsigned comment added by 76.94.30.1 (talk) 23:33, 9 April 2008 (UTC)

[edit] Needs a simplified explanation

There needs to be a simplified explanation for what genetic drift is, in addition to the specifics given in this topic. It appears that genetic drift occurs through neutral mutations that aren't selected against (or for)? I wouldn't know, because the article starts with what genetic drift does, "change the characteristics of species over time," but never explains what is actual is. Then the article explains how the process happens... but again you don't explain what the process is. What is it? I feel that the answer is probably in the article but contained within the more complicated sections. This sections shouldn't be removed by a more simple explanation should be included in the introductory section.

What do you think of the expanded version I just put in? --FOo 00:57, 6 September 2005 (UTC)
I think this does well to clarify the topic for those not already familiar with the process. thanks.

[edit] Example needed

There needs to be a simple example in this article. I don't think anything too elaborate is necessary—no charts or graphs—but perhaps someone could write a narrative example based on a particular named animal and some non-vital traits it has which have varied with genetic drift. —Psychonaut 16:40, 9 September 2005 (UTC)

I agree. Two or three examples would really help. Maybe one in humans and a couple from other species. Is hair color in humans an example of genetic drift? -Jeff Worthington 00:12, 24 October 2006 (UTC)

I read this article and did not understand it. Which was frustrating as I would rather have liked to. It needs a simplified explaination and some examples (please). —Preceding unsigned comment added by 81.157.149.12 (talk) 15:26, 22 December 2007 (UTC)

[edit] Merging

Suggesting mutation rate be merged with this. I am not a genetics expert though and if such a merge is based on flawed logic, say so. It appears that this article has no links to "mutation rate" and that article has no links to here, so probably they were written in isolation. To me they seem like the same thing except that genetic drift is like a "rate of mutation rate". Anyway, the concepts are so closely related it should be all in one article I think. What do you think? Donama 06:10, 7 June 2006 (UTC)

Oppose. They are only remotely related topics and books have been written on both subjects. Both are big enough to not only warrant their own page, but eventually to be split into subpages. --Aranae 07:08, 7 June 2006 (UTC)
Oppose. You misunderstand. Genetic drift is a change in allele frequency. Genetic drift happens AFTER a mutation. They are completely separate and different processes - in fact, they are two of the four microevolutionary forces, where the other ones are selection and migration (aka gene flow). This alone should justify them having separate articles. - Samsara (talk • contribs) 11:26, 7 June 2006 (UTC)
Oppose. They are not the same. The effects are quite different, as well. The reason you don't see mutation mentioned in drift is that the process is generally quite fast, and there are few individuals in the population, so the chance of mutation occurring is very small. There are possible candidates for merge, but this isn't one of them.Ted 13:55, 7 June 2006 (UTC)
Oppose - as Samsara said, they are quite different things. Guettarda 13:57, 7 June 2006 (UTC)
Oppose - Not the same thing. -PhDP 21:05, 7 June 2006 (UTC)
Ok, we'll leave it then. — Donama 22:08, 8 June 2006 (UTC)

[edit] Still too technical

I'm a college graduate, and the intro is two technical. I would think that genetic drift could be explained without words like "Allele" and "stochastic". The rest of the article can be technical, but the introduction needs to be mostly free of science words.

[edit] Needs Referencing

The article is well-written. However, there is a need to quote sources and give references.

Regards Sandy Chhajjusandeep 07:00, 23 November 2006 (UTC)

[edit] intro

I made some changes to the intro - my sole goal was to increase readibility; I think I did so without sacrificing accuracy. I wonder whether it woul be orth saying more about the relationship/difference between drift and natural selection as a way of saying something about drift's place in the modern synthesis, but I leae it to someone else to figure out how and where to do so in an elegant way. Slrubenstein | Talk 14:21, 16 December 2006 (UTC)

I think it is an improvement :) - cohesion 02:58, 17 December 2006 (UTC)

Thanks!! Slrubenstein | Talk 14:43, 17 December 2006 (UTC)

[edit] Clarification requested

I'm working through the concept & can see the general idea, but I'm perplexed by so long as it does not comprise all or none of the distribution. in the introduction, could someone explain please?

Paxoid 23:27, 26 February 2007 (UTC)

Removed. It made no sense.Professor marginalia 17:22, 18 October 2007 (UTC)

[edit] What's wrong with this sentence?

In population genetics, genetic drift is the statistical effect that results from the influence that genetic drift confers to become more common or more rare over successive generations.

Hint: It's supposed to be a definition of "genetic drift." —The preceding unsigned comment was added by 75.36.158.86 (talk) 10:19, 21 March 2007 (UTC).

[edit] Figure

The first figure in the definition is nice, but irrelevant... furthermore, it indicates that animals came before plants (in Eukaria) which is fairly unlikely...

[edit] Assessment

This is a very important topic in evolutionary biology. My two suggestions for raising to B class is to reduce the size of the lead section (too long relative to the article size) and add references, the latter being the most important. Richard001 21:56, 5 June 2007 (UTC)

[edit] Definition

I thought that the first sentence of the article should better be a definition of the term. The first sentence says:

In population genetics, genetic drift (or more precisely allelic drift) is the statistical effect that results from the influence that chance has on the survival of alleles (variants of a gene).

I can see that genetic drift is a statistical effect. But what kind of statistical effect is it? That question is more relevant to a definition than the question "From what does it result?" or "How does it come about?" Also, once an effect is statistical, the statement that chance plays a role in it adds little information, as it logically follows from the definition of statistical. A better definition would be welcome. --Dan Polansky (talk) 15:00, 8 December 2007 (UTC)

You are right-it's something of a mess. Professor marginalia (talk) 15:24, 8 December 2007 (UTC)
Made a first go. And the term "chance" does warrant mention in some way--the "chance" in genetic drift needs emphasis because it is distinguished from the other significant evolutionary mechanism, natural selection, where selection is based on adaptive advantage, ie not random chance at all. In natural selection, the "samples" which over time successfully reproduce in a population are not randomly selected. In genetic drift, they are. Professor marginalia (talk) 17:30, 8 December 2007 (UTC)

[edit] Analogy

Genetic drift is changes in allele frequencies that come entirely from chance. As an analogy, imagine you have a bag containing 60 marbles, half of them red marbles and half blue marbles. The bag represents a population of bacteria and the different colors the two types of bacteria in that population, which carry two different alleles of a gene. Each generation you randomly reach into the bag and pick out half (30) of the marbles, and are then given new marbles with the same colors as the ones you selected: if you pick out 18 red and 12 blue, you're given 18 new red marbles and 12 new blue marbles, producing a new population containing 60 marbles. These selections and replacements represent the bacteria that manage to divide and reproduce. The marbles remaining in the bag are thrown away - bacteria that don't reproduce die and leave no offspring. If you keep repeating this process, since the numbers of red and blue marbles you pick out will fluctuate by chance, the most common color in the population of marbles will change over time, sometimes more red: sometimes more blue. It is even possible that you may, purely by chance, lose all of one color and be left with a bag containing only blue or red marbles. This is genetic drift - random fluctuations in which organisms survive and reproduce, leading to random fluctuations in the allele frequencies of the population.

TimV, I think an analogy is invaluable, but this analogy is inexact - you're not throwing out marbles first and then sampling, you're sampling from the whole bag N times. I.e., I start out with N marbles in a sack, some red, some blue. I want to create N new marbles. I do this by reaching into the sack and picking a marble. If it is red, I make a red marble. If it is blue, I make a blue marble. Replace the marble in the sack, repeat N times. This describes the dynamics of the process better than your analogy (which implies a much faster collapse in variation - yours implies that every generation, low-frequency variation will disappear). Except this analogy sucks. Graft | talk 18:30, 14 March 2008 (UTC)

I've rewritten it a bit, what do you think? Tim Vickers (talk) 18:43, 14 March 2008 (UTC)
Nein, doesn't work. Is your population size 60? Or 30 (=12+18)? Since the strength of drift is governed by population size, the number of marbles in the bag is crucial. You MUST have sampling with replacement, since that's the way your real population works - more or less. Where the analogy breaks down I think is that you're using marbles to represent both the parental generation and the gametes - but I can't think of a non-gross analogy for generating gametes. Graft | talk 22:09, 17 March 2008 (UTC)

Actually, I'm thinking about an asexual species that divides by binary fission, which is easier to deal with. I've tried making this clearer. Tim Vickers (talk) 16:37, 18 March 2008 (UTC)

I may be missing it, but doesn't this new attempt still lack replacement? That's important, because of course one organism can reproduce numerous times, another zero. I am also confused because it sounds to me like the bacteria splitting into two in the scenario is no longer a parent-offspring duo but a set of twin offspring with the parent somehow transmogrified into one of the twins.Professor marginalia (talk) 17:45, 18 March 2008 (UTC)

Rewritten again. Tim Vickers (talk) 21:08, 18 March 2008 (UTC)

Sorry, I think it confuses "replacements". The "replacement" I was speaking of was "sampling with replacement". In this revision, the process describes "sampling without replacement", which is incorrect for this analogy. It also continues to conflate parent and offspring generations. The organism which splits once has one offspring, not two. If it splits a second time, then there would be two. But in this example, the marble which reproduces is, weirdly, immortal, living on and on in each newly reproduced generation, which is why it's confusing. The organisms in Generation A can reproduce any number of times, within the limits their reproductive lifespan. The way to represent this in the analogy is to take each reproductive event in the analogy as a single independent sample pulled from the same starting population--after its "match" is created, each marble pulled is then returned to the population in the bag before drawing the subsequent sample to reproduce. You can't draw 30 as a set. You can only draw one at a time, each one from the same complete starting population. When that population stops reproducing, then all are thrown away or at least ignored, not just those that didn't reproduce. Professor marginalia (talk) 21:54, 18 March 2008 (UTC)
What about something like this? :
The following simulation represents how genetic drift takes place in a population: First imagine a small jar holding 20 marbles, half of them red marbles and half blue marbles. This jar represents a small population of organisms, and the two colors represent the two different alleles of a single gene found in the population. And also imagine an initially empty second jar which will contain the future offspring reproduced by the first generation. Say the next generation produced in this population will also number 20 organisms. Draw one marble randomly from the first jar. If the marble drawn is red, place a new red marble in the second jar. If the marble drawn is blue, place a new blue marble in the second jar. Return the drawn marble to the first jar. There is now one randomly reproduced organism in the new generation. Repeat the process 19 more times to reproduce a total of 20 new marbles to represent the next generation of the population which is now contained in the second jar.
Since in this process, each of the red and blue marbles chosen to reproduce from the first jar were picked randomly, the number of red and blue marbles now contained in the second jar are unlikely to be equal, as was the case with the red and blue marbles in the original population in the first jar. Represent the eventual death of the original parent population by emptying and discarding all the marbles in the first jar, and re-use it to hold a third new generation of marbles reproduced from the second generation in the same manner. This process can be repeated for a fourth new generation of marbles, and so on. With each new generation of marbles, the number of red and blue marbles in the jar holding the offspring population can fluctuate from the number of reds and blues in the jar holding the parent population: sometimes there will be more red, sometimes more blue. Because the number of marbles in this population is small, there will be a tendency over time for one color to eventually become predominant throughout the population. This is the process seen in genetic drift, in which random fluctuations determine which organisms are reproduced, resulting in changes over time in the allele frequencies in the population.
Professor marginalia (talk) 17:34, 18 March 2008 (UTC)

Genetic drift is changes in allele frequencies that come entirely from chance. As an analogy, imagine having a large number of marbles, half of them red and half blue, with the two colors representing organisms with two different alleles of a gene. Put 10 red and 10 blue marbles in a jar, representing a small population of these organisms. Each generation the organisms will reproduce at random and the old generation will die. To see the effects of this, imagine randomly picking a marble from the jar and putting a new marble of the same color as the one you picked into a second jar. After your selected marble has "reproduced", put it back, mix the marbles, and pick another. After you've done this 20 times, the second jar will contain 20 "offspring" marbles of various colors. This represents the next generation of organisms.
Now throw away the marbles remaining in the first jar - since the older generation of organisms eventually die - and repeat this process over several generations. Since the numbers of red and blue marbles you pick out will fluctuate by chance, the most common color in the population of marbles will change over time, sometimes more red: sometimes more blue. It is even possible that you may, purely by chance, lose all of one color and be left with a jar containing only blue or red offspring. This is genetic drift - random variations in which organisms manage to reproduce, leading to changes over time in the allele frequencies of a population.

Tim Vickers (talk) 23:00, 18 March 2008 (UTC)

I think it's really good. And by isolating a sub-population (the 20 in the jar) from the larger population in the analogy, and describing how the allele frequency in the sub-population's next generation randomly diverges from the 50-50 in the larger population, you've also simulated in a sense the origins of these small isolated populations that undergo drift.Professor marginalia (talk) 23:22, 18 March 2008 (UTC)

[edit] Sampling error

The mathematical model of sampling error can be used also for modeling genetic drift. But sampling error carries substantial meaning that is not at hand in evolution. The statistician wants to measure the overall frequency and uses a sample to get an approximation. This approximation has an error, a difference between the frequency in the sample and the frequency in the population. In the breeding of a natural population there is no intention to measure the overall frequency, so there is no error. --Etxrge (talk) 08:38, 18 May 2008 (UTC)

Okay, can you suggest a more correct term? Graft | talk 09:17, 18 May 2008 (UTC)
One of the synonyms for genetic drift is genetic sampling error. Professor marginalia (talk) 14:11, 18 May 2008 (UTC)
This approximation has an error, a difference between the frequency in the sample and the frequency in the population - and that is why the analogy is made. The population "samples" its genetic pool. The fact that there is a difference between the allele frequency of the population and the allele frequency of the sample creates a "sampling error". There is an "attempt" to sample the overall gene frequency because each individual attempts to pass on its genes. Of course it's an analogy, but it matches pretty well.
More importantly though, this is how it is generally represented by our sources. Guettarda (talk) 15:25, 18 May 2008 (UTC)

[edit] Better analogy of Genetic Drift

The marbles analogy of genetic drift in this article is not illuminating. I don't know that much about genetics, so I think I am in a fair position to claim so. Here is a better definition of genetic drift from Bryan Sykes. It's taken from his book "The Seven Daughters of Eve". ISBN 0-393-02018-5. 2001. W.W.Norton & Company. New York, NY. Pg. 43

"Anthony Edwards explained his thinking [about genetic drift] in an ingenious article in New Scientist in 1965. He imagines a tribe that carries with it a pole along which are arrayed 100 discs which are either black or white. Every year, one disc, chosen at random, is changed to the other colour. When the tribe splits into two groups, each group takes with it a copy of the pole with the discs in their current order. The following year they each make one of the random changes to the discs. The next year they each make another, the next year another and so on, continuing the custom of one random change every year. Since the changes they make are completely random, the order of the discs on the two poles becomes more and more dissimilar as each year passes. If follows that if you were to look at the poles carried by the two tribes you could estimate how long ago, in a relative sense, they separated from each other by the differences in the order of the black and white discs. Providing an absolute date was very difficult from the gene frequency alone, but the comparative separation between the two tribes, known as "genetic distance", was a useful measure of their common ancestry. The bigger the genetic distance between them, the longer they had spent apart. This was a clever image of the process of "genetic drift", brought about by the random survival and extinction of genes as they pass from one generation to the next."

I didn't put this in the main article because I don't know if it infringes copyright or not, but I wanted to give an analogy from which I was able to make sense of genetic drift. HeWasCalledYClept (talk) 05:09, 23 May 2008 (UTC)

I find all of these analogies equally outlandish. This one manages to confuse drift with plain old divergence - if the array of discs represents the tribe's "genome", then the analysis makes sense, but does not represent drift. If the array of discs represent individuals with specific alleles, the analysis represents drift, but doesn't make sense, since you can't determine how two populations diverged just by looking at a single locus. Graft | talk 07:21, 23 May 2008 (UTC)
I think the description of drift given in Sykes' book is better to picture how genetic distance between two populations is measured, and that's getting a bit ahead. The analogy ignores completely the effect probability plays on the frequency distribution--how random sampling from small populations impact genetic frequencies in a population over time. It introduces random mutation-which only occurs in individuals-, but also fails to connect it in any way to the population, probability or gene frequencies. It also completely ignores heredity which is the key "event" sampling taking place in the dynamic or process of genetic drift, not spontaneous mutation. Graft's correct, the analogy is more apt for divergence, and I'm not sure I understand how the pole-discs analogy wouldn't apply to comparing selected alleles just as much as changes to allele frequency due to drift, ie from "chance" in sampling. I don't get it. I think Sykes was using the analogy to illustrate genetic distance. But the book's focus is on research in mtDNA differences, and this strange analogy maybe does make rough sense when talking about a "population" of mtDNA. Professor marginalia (talk) 16:55, 23 May 2008 (UTC)