Bio 1B Lecture Outline please print and bring along Fall 2008 B D Mishler Dept of Integrative Biology 2 6810 bmishler berkeley edu Evolution lecture 11 Hardy Weinberg departures genetic variation drift Dec 1st 2008 475 479 ch 23 in 8th ed 459 464 ch 23 in 7th ed Genetic variation genetic drift summary of topics Extent of genetic variation in natural populations Examine the extent of genetic variation in natural populations and understand the concepts of the neutral balancing selection and evolutionary lag schools to explain this variation Deviations from Hardy Weinberg HW Explain the consequences of violating each of the assumptions of the HW law non random mating mutation migration and genetic drift selection is for next lecture Genetic drift Understand the short and long term effects of genetic drift on the genetic structure of populations and the consequences of founder effects and bottlenecks Sexual versus asexual reproduction Contrast sexual and asexual reproduction in terms of the generation of genetic variability Extent of genetic variation in natural populations How much genetic variation is there in natural populations Before 1966 there were two disparate views on the extent of overall genetic variation in natural populations classical and balance The classical view assumes that at nearly every locus every individual is homozygous for a wildtype allele In addition each individual is heterozygous for rare deleterious alleles and occasionally heterozygous for a selected allele maintained in the population by balancing selection The balance view in its extreme form on the other hand assumed that there was a lot of genetic variation in populations so that most individuals will be heterozygous for alternative alleles at very many of their loci This genetic variation was believed to be maintained by some form of balancing selection Evolution 11 pg 1 The year 1966 is important in population genetics as it marks the use of an objective test to measure the extent of genetic variation in populations gel electrophoresis The initial and later studies showed that more than approximately 30 of loci and this is an underestimate exhibit variation in natural populations So we now know and more recent DNA based technologies have confirmed this that a great deal of variation does exist in natural populations In humans approximately 1 1 000 DNA base pairs is polymorphic referred to as a SNP single nucleotide polymorphism In contrast humans differ from chimpanzees approximately every 1 100 base pairs From these observations it would seem that the balance school wins out However the classical theory has been retained in terms of the so called neutral or neo classical theory Also to consider is that some or much of the variation in natural populations may represent a transient polymorphism the evolutionary lag school The argument is that there will ultimately be changes in a species ecosystem via environmental changes or evolutionary advances by other species and consequently if a species is to survive it must evolve continually and rapidly to catch up to the latest changes in its ecosystem neutral school much of the genetic variation in populations is evolutionary noise and the allelic variants are selectively equivalent balance school most variation has adaptive significance and is maintained by some form of balancing selection evolutionary lag school much of the variation in a population is transient variation as advantageous alleles replace other alleles Even if an allele is selected it will take a long time to become established in the population unless the selection is extremely strong for example with selection of 1 it takes 2 000 generations to fix an allele in a population which equates to about 45 000 years for humans Which school is right There is controversy as to which is the predominant factor creating the high level of genetic variation seen in most natural populations While selection certainly operates nevertheless much genetic variation is probably neutral All three factors probably play an important role Apportionment of genetic variation initial studies of the degree of genetic differentiation of human populations and ethnic groups using allozyme data from gel electrophoresis studies showed that most genetic variation in humans is found within populations 85 with the remaining variation equally divided 7 5 each between populations within ethnic groups and between ethnic groups Similar results have been found with RFLPs microsatellites and HLA data Evolution 11 pg 2 Phenotypic plasticity keep in mind that much variation observed in nature is non genetic Experiments and sophisticated genetic studies are needed to determine the basis for variation Deviations from Hardy Weinberg HW evolution Deviations from Hardy Weinberg assumptions the strength of the Hardy Weinberg HW law is that one can deviate from the assumptions quite a bit and the data will still approximate Hardy Weinberg proportions HWP The weakness of the HW test is that the deviation from the assumptions has to be very strong in order to detect the effect of this evolutionary force e g selection Deviations from HW assumptions involve 1 Non random mating e g inbreeding mate choice 2 Mutation The effects of mutation in populations are usually negligible as mutation rates are low but mutation is an important force in creating new variation 3 Migration is important if the migration rate is high and the two population are very distinct genetically 4 Genetic drift due to small population size chance effects genetic drift effects are important in both small and large but finite populations in terms of short and long term effects of changes in allele frequencies over generations due solely to drift effects note that the finite size of a sample taken from a population is taken into account in the statistical tests for HWP and finite population size itself does not cause significantly detectable deviations from HWP 5 Selection has to be strong to cause deviations from HWP e g it can be detected with sickle cell anemia selection is the topic for the next lecture More details about the first four of these 1 Non random mating individuals with certain genotypes sometimes mate with one another more commonly than would be expected on a random basis When like mates more often with like we term this positive assortative mating e g height IQ Positive assortative mating increases the proportion of homozygous individuals but does not
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