Bio 1B Lecture Outline (please print and bring along) Fall, 2007B.D. Mishler, Dept. of Integrative Biology 2-6810, [email protected] lecture #11 -- Hardy Weinberg departures: genetic variation & drift -- Nov. 30th, 2007459-464 (ch. 23) in 7th ed.450-456 (ch. 23) in 6th 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 populationsHow 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 wild-type allele. In addition, each individual is heterozygous for rare deleterious alleles, and occasionallyheterozygous 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. 1The 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, withselection 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. 2Phenotypic 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
View Full Document
Unlocking...