![]() This shows that the proportion of heterozygotes decreases asdrift proceeds (this also occurs when there is inbreeding which can alsobe thought of as a sampling error phenomenon). When all populations inthe array have fixed or lost the allele, there can be no heterozygotes(i.e., 0%). This is whygenetic drift can be an important force in evolution.Īt the start of this drift process in our array of populations, p =0.5 and there are 2pq = 0.5 = 50% heterozygotes. ![]() 2) genetic divergenceof populations entirely by chance! (no selection). Main Points: 1) total variation does not change variation goes from within populations (no variation between populations) to betweenpopulations (no variation within populations). If the initial frequency was p = 0.7, then 70% of the populationswould be fixed for the A allele (again, assuming no selection, migration,mutation). If each populationstarts at p = 0.5, then at the end, when all populations have lost theirvariation, 50% of the populations will be fixed for the A allele and 50%will be fixed for the a allele (latter = "loss" for the A allele,get it?). ![]() Through time each population will experience genetic drift dueto random sampling and the frequencies in each population will diverge.The distribution of frequencies changes over time from a tight distribution(all 0.5), to a flat distribution (some populations at p = 0.1, some at0.9 and all frequencies in between), to fixation (p =1.0) or loss (p = 0.0) of the alleles in all populations (see figure below).Fixation is when all alleles in the population are A this necessarilyimplies loss of the a allele ("fixation" or "loss"should only be used with reference to a specific allele). Wewill introduce the idea of population structure by showing how geneticdrift and inbreeding can change the frequencies of genotypes in populations.Ĭonsider a grid of small populations (e.g., ponds in Minnesota), allwith the same small population size and all starting at time t with p =q= 0.5. This structure is determinedby the combined effect of deterministic and stochastic forces. To illustrate this we need to understand Populationstructure, which describes how individuals (or allele frequencies)in breeding populations vary in time and space. To illustrate the consequences of genetic drift we will consider whathappens when drift alone is altering the frequencies of alleles among manysmall populations. Genetic driftis not a potent evolutionary force in very large randomly mating populations. Ifyou pulled out all the marbles in the bag (= large population) then thefrequency would be maintained exactly in the next generation. A second smallhandful will randomly shift the frequency to yet another frequency. Let that handful determine the frequency in a newpopulation that grows back to the original population size. The same sort of random fluctuationin allele frequencies can occur in small populations: consider abag full of red and green marbles each in equal frequency pull out a smallhandful and the frequency in your hand will probably not equal the frequencyin the original bag. In ten tosses you might easilyget seven heads in 1000 tosses, however, you would never get 700 headswith a "fair" coin. While it might seem that a random force would be of littlesignificance to evolutionary "progress" (we'' confront this loadedterm later), genetic drift is an extremely important force in evolution.However, its strength depends on the size of the population, as a simpleexercise in coin tossing will illustrate. ![]() Genetic drift is a stochastic (random)force that can scramble the predictable effects of selection, mutation,and gene flow. The previous lectureshave all dealt with deterministic (predictable) evolutionary forces oftenreferred to as linear pressures. Genetic drift refers to random fluctuations in allele frequencies dueto chance events (see figure 6.4, pg. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |