Sewall Wright presentation PDF

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So Sewall Wright is born in December 21, 1889 in the US. So he is an American geneticist, and one of the founder of population genetics, proving effect of selection on change of allele frequencies. That’s why, youve probably seen this name during the week 4 lectures when mentioning about Fisher-Haldane-Wright equation, which is an equation that demonstrates how a population allele frequency in next generation will be affected due to different selections. So what is he famous of and how is his work contributed in forming the Fisher-HaldaneWright equation. He is famous for proving in particular the selection concept of “genetic drift”, published during the 1931, also known as Sewall wright effect. His work focuses on the fact that theres always a potential of change in allele frequency in a population due to random chance events (luck), unlike natural selection, or selection breeding, its totally random. So as an example, there’s 10 bugs on the wall with 2 different genetic make ups (blue and red), but they are the same species. You then use something to hit the wall trying to get rid of the bugs, what Sewall Wright is trying to prove is that the probability of the bugs on the wall dying is all the same no matter what their genetic make-up is, it all depends on their luck of whether they will be killed. Say you only hit the wall once, and leave the alive bugs, this causes an allele frequency change in the population and also in next generation because there might be less blue or red bugs alive for reproduction of next generation. Thus, we can also say that, the smaller the population the stronger the genetic drift effect will be because losing species will have an overall greater impact on the population allele frequency. In more detail, theres actually 2 types of genetic drift. Firstly, bottleneck effect. Bottleneck effect indicates that there will be a significant allele frequency change in next generation when there’s a natural disaster killing large portions of population because range of alleles will decrease. For example, lets say we start with 9 brown individuals with genotypes BB and Bb and only 1 white with genotype bb. Initially the allele frequency is: . However due to a forest fire, only the 2 circled individuals in the population survived due to luck. Now, as the 2 individuals is both BB, they will only produce BB rabbits and so the allele frequency will be changed to: , and as according to Hardy Weinberg equilibrium, next generation will have the same allele frequency. The disappear of white rabbit also show that when only few individuals carry certain rare genetics, its easier for them to extinct, as they have higher probability to fail to transmit to next generation, leading to emergence of new species. Secondly, there is Founders effect. Founders effect indicates that when population gets separated from their original population forming a new colony, say to another island. For example, butterfly flying away from original population or wind blowing seed away. The new colony will form a new population, but might not represent the original population as seen from the picture beside, because there will be allele frequency difference. Thus, as individuals leaves the population, the original population will have allele frequency change. In conclusion, Sewall Wright is famous for his work of “genetic drift”, showing that there will always be allele frequency changes due to random events such as natural disaster or forming a new colony in another location. Most importantly, it acted as a selection factor in FisherHaldane-Wright equation, to help us predict future generation allele frequency in the real world nowadays....