<p>The question reads:
Genetic drift has the largest effect on allelic frequencies when
(B) the population is small</p>
<p>-> I've learned that Genetic Drift reduces diversity. It occurs in two forms, namely the Founder Effect and the Bottleneck Effect. One example of Founder Effect is the Old Order of Amish of Lancester, a German immigrant group that had a high percentage of polydactyly. One example of Bottlneck Effect would be the Ashkenazic Jews from Central Europe who were the only surviving Jews after years of persecution: they had a high percentage of Tay-Sachs disease.. </p>
<p>From what I understand, genetic drift results in a population with traits that may not be characteristic of the original complete population before a diversity-reducing event occurred.</p>
<p>To me, the answer to the question above doesn't make sense. Rather, I chose the answer choice (D) there is no natural selection, since both natural selection and genetic drift cause evolution in different ways. Polydactyly and Tay-Sachs disease are both traits that are unfavorable regarding natural selection. It was because natural selection didn't intervene that genetic drift was able to make a mark on the population.</p>
<p>The other answer choices were:
(A) mating is random
(C) there is no migration, in or out
(E) there is no mutation</p>
<p>C never seemed like a choice to me because genetic drift already presupposes no migration. It's rather the circumstances after migration, referring back to the Amish of Lancester example.</p>
<p>Can someone help me understand why B is the correct answer? The answer explanation at the back didn't give me any substantiation... :) thanx in advance</p>
<p>Hello, I’m currently taking Biology Honors and had a question almost verbatim to this one.</p>
<p>I think the answer is (B) because mutations are expressed more in small groups.</p>
<p>For example, when a small group is geographically isolated from a large population, genetic drift is emphasized. In a big group, a mutation does not drastically change alllelic frequencies.</p>
<p>Well, aren’t you disregarding the “genetic drift” part? I dunno but, at least from how I understand genetic drift, there is an inherent “smallness” of population… How could it matter if the original population was big or small if the new population that has just branched off is already smaller than the previous population? …</p>
<p>Genetic drift is the effect of chance on gene frequencies. The smaller the population, the greater the effect of chance. The Founder Effect and the bottleneck effect are both examples of genetic drift - in each case, the trait frequency in the population you ended up with (a small one), was determined not by selection or mutation, but by being in the wrong place at the wrong time.</p>
<p>Suppose you had a herd of 500 sheep. 50 of them are black. A truck hits 5 sheep that wandered into the road. Even if all 5 were black, there would not be much of a shift in the gene frequency due to this chance event. Now use a herd of 50, 5 of them are black. Again, the truck hits 5 sheep, but this time, even if only 3 were black, there would be a significant change in the frequency of black sheep. The sheep were not selected against because they were black, they “disappeared” by “accident.”</p>
<p>This question is poorly worded, as C and E are possible, though weak responses. As a rule, genetic drift effects are independent of these, and also independent of natural selection. An OCD person may argue that migration in of organisms would counter any changes due to drift, but this would be a very outside chance.</p>
<p>B is best answer.</p>
<p>BTW, the high incidence of Tay-Sachs among Ashkenazy Jews is hypothesized to be a combination of genetic isolation, coupled with frequent exposure to cholera. An individual with a heterozygous genotype for Tay-Sachs is more likely to survive a cholera outbreak.</p>
<p>The Amish are considered to be an example of the founder effect couple with genetic isolation.</p>
<p>It’s not the traits that are necessarily different after the drift effect, it’s the distribution of the frequency of the various traits – polydactyly is found in the general population, but very rarely compared to the Amish.</p>
<p>Amish coupled with polydactyly was found in my AP Bio Barrons’ book… so don’t know the abstruse logistics of that but I guess you may be right.</p>
<p>I should’ve focused on the fact that genetic drift is the change in the allelic frequencies due to a chance event. Rather, I focused too much on the issue of migration of a population, which happens in the founder effect case. Any phenomenon that brings about a perceivably momentous change in allelic frequencies would have a bigger effect if the population were smaller. I think I missed that part.</p>
<p>Thank you for the sheep example, though OCD is a very strong term …</p>
…</p>