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EVOLUTION; what influences allele frequency to change?

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EVOLUTION; what influences allele frequency to change?

Postby onetwostep » Thu Dec 04, 2008 7:07 am

It's a bio lab called ' agents of change' and the purpose of it is to find out how random mating, genetic drift and natural selection influence the allele frequency within a population.. and i'm kinda stuck. well first i gotta come up with hypothesis for each.. I think random mating would not affect the allele freq. since random mating itself is required for constancy of allele frequency according to hardyweinberg principle.
As for genetic drift, it would change the allele freq since it will result in evoultion thus violating hardy-weinberg priinciple? and also from generation to the next, one of alleles will become fixed. also that it reduces amnt of genetic variable and increase allele frequency. I'm not sure cause I don't quite get what genetic drift is :S in fact, i dont think i'm understanding all three of them perfectly.
lastly, natural selection will also result in changes in allele freq. since favoured trait will be passed on to its offspring? oh. and as for the last two, allele freq. changes because they both violate the hardy-weinberg principe. is that right? ahh i am so confused..!

and I'm also stuck on this one..
if single harmful mutant is introduced in a large population and small population, what would exactly happen in each cases? and if beneficial mutant is introduced in large and small populations?
I'm assuming that if harmful mutant is introduced in a large pop, it wouldn't affect the population as much but in small popultion, there'd be a great chance that it would wipe out the population?
as for beneficial mutant in large populations.. it's population will get larger? and same goes for the small population?

any help would be greatly appreciated. thank you in advance.
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Postby Darwin420 » Thu Dec 04, 2008 8:13 pm

Group your questions to make it easier for me to understand what exactly you want.

Genetic drift = when alleles are lost in the gene pool and yes it changes the allele frequency.

H-W principle shows what would happen if mating was random and no evolution factors were in play (ex natural selection, genetic drift, gene flow etc.). The allele frequencies should be the same for each generation.

If you added these evolution factors though, you will see a change in allele frequencies.

if a SINGLE harmful mutant was introduced into a pop it would most likely die because it most likely won't be favoured by the environmental conditions. HENCE, "harmful".

That being said, this harmful mutant would be wiped out in large pop and small pop, in this case the size of the population doesn't matter.
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Re: EVOLUTION; what influences allele frequency to change?

Postby onetwostep » Sat Dec 06, 2008 8:37 am

so to sum it up,
random mating -> no change in allele frequency
genetic drift -> change
natural selection -> change

as for random mating, would genotype frequency change and new genotypes be produced in recombination which influences evolution in a population? but if genotype frequency changes, doesnt that mean allele frequency would also change thus random mating would result in change in allele frequency?
or does that applies to nonrandom mating ? (genotype frequency change thing)
i KNow random mating wouldn't change allele freq. but i dont know where i got the info from about random mating would change genotype freq.

Also, from natural selection demonstration, rr number decreased and ended up in 0. RR freq was 0.85 and Rr freq was 0.15 .. does that mean there is fixation in a population?
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Postby Darwin420 » Sun Dec 07, 2008 4:24 am

For the last question we see that the genotype rr is approaching a frequency of zero, once it reaches zero this means there is a fixation in the population. The population has now evolved and the rr individuals have been trimmed off the population.

As for random mating - no, the allele frequencies would no change if they meet the HW criteria. Think about it this way, I have a population of 10 alleles. 6 are represented by a, giving it a .6 frequency and 4 are represented by b giving it a .4 frequency. Since the population is fixed and no genetic drift, gene flow, natural selection and mutations occur, then the frequencies will stay the same. It is only when these other evolution factors stated above are added, the allele frequencies will change.
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Re: EVOLUTION; what influences allele frequency to change?

Postby onetwostep » Sun Dec 07, 2008 7:36 am

mmhm yepp i get what you're saying. but
I forgot to add this..
eventhough rr number and frequency reached zero,
R allele frequency was 0.925 and
r allele frequency was 0.075
( got it from adding RR number and R in Rr number then divide by population which i doubled original pop since there is 2 alleles per gene.. done same for r allele frequency)
does this still mean R allele became fixed?

i'm confused because i can clearly see that genotype rr became fixed but when calculated R and r allele frequency, i assumed that r allele freq should be zero but instead, i got 0.075.. so i thought it has not yet become completely fixed since it's not zero :S

or is it that r allele freq. doesn't necessarily have to be zero? and does it just simply mean that recessive is gone for good but dominant and heterogenous genotypes still remain in population?
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Re: EVOLUTION; what influences allele frequency to change?

Postby Darwin420 » Sun Dec 07, 2008 8:13 pm

ok I just skimmed over your stuff, I am really busy studying for two chem finals tomorrow.

ok rr frequency is zero, and r frequency is .075.

This is because the genotype frequency (rr) has been fixed not the allele (r)frequency.
individuals with allele combination (genotype) of rr have been trimmed out of the population due to them not being suitable for the environment, but allele r is still around in heterozygotes (Rr) which were able to survive. Thus, the frequency of r should not be zero. Make sense?

What was the frequency of r before you conducted this experiment?

This is all I can help you with. Hope it makes things clear.
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Re: EVOLUTION; what influences allele frequency to change?

Postby onetwostep » Mon Dec 08, 2008 11:10 pm

alright,thank you!

btw, hope your chemistry exam went well
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