10-30 Notes - Population genetics: Hardy Weinberg PDF

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Population genetics: Hardy Weinberg...

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10/30 Lecture: Locus is a particular genotype sequence that codes for something; a different version of a gene at a specific locus is an allele A population may be homogenous for an allele, but then a mutation might occur so one of its chromosomes now has a mutated allele so it is now a heterozygote since it is has one original allele and one mutated. So when new alleles arise, they initially are always hidden from expression b/c new mutants are always heterozygotes (so it can spread among the population before it can be selected against since it is carried in heterozygotes). Once homozygotes form with the mutant alleles, then it can be selected for or against. Initial spread is always by heterozygotes then random mating must be done between them to form a homo for the allele so it can be manifested in the phenotype. Dominant alleles, though, affect the phenotype regardless of who they are paired with, while recessive alleles can lurk, sheltered in the heterozygous state as long as they are on in low frequency. So lots of genetic variation in natural populations exists as rare recessive alleles that hide in heterozygous states. So it is a bad thing to mate with relatives because it is more likely you both have same recessive mutations you are hiding as a heterozygote and so offspring will be homo for rare deleterious mutations. Population geneticists view populations as collections of alternative alleles Random mating means that it is random with respect to alleles at a certain locus, although it is not random in general (we obviously choose our mates, animals do by feather length, color, etc.), but these are only a few loci in a genome for attraction, all other loci are just random for mating. Population evolution refers to actual change of allele frequencies (p and q), not genotype or phenotype frequencies. Without an outside force (selection) evolution will not occur. But Hardy-Weinberg only is true though if there is:    

Infinite population (if finite, sampling error—genetic drift—makes frequencies change), drift All genotypes are equally likely to survive and produce offspring (equal performance), selection No migration (no individuals can leave or come into the population) No mutation

Genetic Drift: Fixed population are stuck b/c there is no more variation for that loci. In the fly experiment it was shown that over time most populations were becoming fixed so there was evolution; problem was that there was not an enormous population. He created genetic drift (sampling error) b/c the populations were so small.

Evolution is the sorting of individuals so small sample size of population leads to sampling error. Populations are not infinitely large, so there is always some genetic drift, but it is more significant on small populations and is less significant on large populations. On small populations it is going to make a big difference. Mutations are so rare, it is unlikely for them to randomly occur ever again once they are lost. Even very positive mutations can be eliminated due to genetic drift since sampling error can lead them to not being present in the new population since their initial frequency is so low it is unlikely they will be represented in the new sample b/c of sampling error. As population size gets smaller, the genetic drift of allele frequencies becomes much more extreme and fixation tends to happen (which does not really occur in larger populations). Even a very brief period of small population size can lead to catastrophic losses of genetic variation. Small sample size leads to evolution b/c tons of alleles are lost due to fixation and new frequencies are very different than previous generations. Brief reductions in population size are called “bottlenecks”, it is genetic drift. Example is elephant seals in 1800s where they were getting hunted for ivory so there were only a few left (like 30) so there was a huge decline from hundreds of thousands, so there is almost no genetic variation. Elm trees were all started from just a few trees so when a disease came over there was not enough variation and they basically all died. Potato famine in Ireland is another example, b/c potatoes had been grown from clones for generations so they could not resist the virus/algae that destroyed them. Farmers accidently created a bottleneck by using clones and created a sampling error. Founder effect is genetic drift caused by just a few members of a population end up on an island and now there is a tiny new population that is a subset of the previous larger one. For example, pseudoscorpions latch on to other animals and get transported. Usually the individuals who make it to the island die, but when it works you have a bottleneck effect going on. Most endangered species or speciation is because of genetic drift following an original bottleneck/founder effect that leads to huge changes in allele frequencies....