Bio 203L- Lecture 4
Bio 203L- Lecture 4 BIOL 203L 005
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This 5 page Class Notes was uploaded by Karissa Sandoval on Tuesday January 26, 2016. The Class Notes belongs to BIOL 203L 005 at University of New Mexico taught by Dr. Kelly Miller and Dr. Scott Collins in Spring 2016. Since its upload, it has received 29 views. For similar materials see Ecology and Evolution Laboratory in Biology at University of New Mexico.
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Date Created: 01/26/16
BIO 203: Evolution Lecture 4 •Hardy-Weinberg Application – Hardy-Weinberg Equilibrium as null hypothesis – Example: MN blood group • Humans with two alleles (M and N), MM (homozygous), MN(heterozygous) or NN(homozygous) • Analysis: • Observe genotype frequencies • Calculate allele frequencies • Use allele frequencies to calculate expected genotypes • Compare observed and expected values – MN proteins are on the surface of red blood cells • CALCULATION EXAMPLE: MM MN NN People A .835 .156 .009 (Observed) CALCULATION EXAMPLE: People A • Allele frequencies • Frequency M=p=0.835 + ½(.156)=0.913 • Frequency N=q=0.0009+1/2(.156)= 0.087 • Expected • P =0.913 =0.834 • 2pq=2(.913)(.087)=0.159 2 2 • q =(.087) =.008 – Another Example • Sickle cell anemia (SS, Ss, ss) • SS, no anemia, no problem • ss, severe anemia and (typically) death • Ss, anemia, resistance to malaria • Malaria causing red cells to change in shape and they are eaten by the white blood cells so when the blood cell is eaten up the parasite is eaten up as well • Problem: If 9% of a population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene? 2 • Frequency of genotype ss = 0.09 = q • Frequency of allele s = q = √0.09 = 0.3 • p = 1 - 0.3 = 0.7 • Frequency of gamete Ss = 2pq = 2 x 0.3 x 0.7 = 0.42 = 42% population is heterozygous BIO 203: Evolution Lecture 4 •Balancing selection – Balancing selection – one is selecting for both alleles so heterozygotes are selected for which is referred to as the “heterozygote advantage” • Heterozygotes are selected for (higher fitness) • Homozygotes are selected against (lower fitness) *****Hardy Weinberg problems are going to be on test so use resources to practice these types of problems.****** •Hardy-Weinberg Assumptions – Several significant assumptions and conditions that must be met for equilibrium to hold – If assumptions are not true then the Hardy-Weinberg equilibrium will not be correct 1. No natural selection – ALL members of original generation survive and contribute gametes equally 2. No random allele frequency changes – Infinite population size 3. No movement of new alleles into or out of original population (gene flow) – All offspring derived only from original gene pool 4. No mutations – No new alleles are assumed to occur 5. Random mating – Summary • no evolution and random mating – Microevolution – generation to generation change in allele or genotype frequency in a population • A departure from Hardy-Weinberg Equilibrium • Mechanisms that cause change in allele frequencies – Evolution causes a shift from Hardy-Weinberg Equilibrium o Many ways this can happen, which include: 1. Natural selection – Natural selection is not random it is directed toward what features are selected for – Certain features are selected by the environment (adaptations) – the environment directs natural selection 2. “Random” evolution – Genetic drift – Gene flow – Mutation •Genetic Drift BIO 203: Evolution Lecture 4 – Any random change in allele frequency in populations – undirected by natural selection – Any change in allele frequencies in a population due to chance – Founder effect – Bottleneck – Genetic drift example • Suppose a population of 6 males, 6 females in a population • There is a gene with three alleles A , A and A 1 2 3 • Each randomly paired adults produces two children with alleles selected randomly from parental alleles •Random Sampling and Genetic Drift – If you are selecting gumballs out of a jar and you start with two even amounts of two different colors and you pick randomly out of the jar. For each generation that you pick you may pick more or less of a specific color to be passed on to the next generation. So, the pattern repeats and you may not end up with the same amount of gumballs for each color. •Genetic drift characteristics – Most evident in small populations • Small populations= small pool of genes • Any change done to this generation is going to result in a larger change • Every allele is susceptible to fixation or random selection – Can lead to extinction or fixation of alleles by chance – Random with respect to fitness •Sampling and Drift (go hand in hand) – Doesn’t matter how the random sampling occurs • Founder effect BIO 203: Evolution Lecture 4 • Few individuals from a main population are founding (establish) a new population somewhere else • Allele frequencies may be very different from original population because their environment may be different from where their original population was located therefore they develop new features (new alleles) • Sampling of small number of alleles from a larger population – limited gene pool • Founder effect – Loss of genetic variation when a new population is founded Lowfrequencyofa Higherfrequencyofa 18 • Founding • Can occur with any isolated habitats or peripheral isolated regions • Most islands in the world (e.g. Hawaii, Galapagos) • Mountain tops • Ponds and streams • Openings in forests / clusters of trees in prairies • Introduced species • Human example • Afrikaners and Huntington’s disease • Afrikaners are more likely to code for Huntington’s disease • Few colonists carrying relatively high frequency of allele coding for Huntington’s • Bottleneck BIO 203: Evolution Lecture 4 • Sudden reduction in size of a population 24 • Diseases • Catastrophes • Bottlenecks and Founder Effect • Result in increased frequency of some recessive alleles, and increased probability of homozygous recessives • Lead to reduced total diversity of alleles in population • Reduced diversity makes adaptation difficult
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