Hardy Weinberg Principles The frequency and assortment of a given allele in a given population remains constant IF o Random mating occurs o Neither gene is selected against o No mutations happen o There is no gene flow In other words allele frequency is only constant in Utopia Hardy Weinberg Principle Two alleles A and a o p is the frequency of A o q is the frequency of a o By definition p q 1 Genotype Frequencies p2 2pg q2 1 o p2 is frequency of AA o 2pq is frequency of Aa o q2 is frequency of aa These 2 equations are the keys to Utopia p2 2pg q2 1 and p q 1 o They let us use observed allele frequencies to predict genotype frequencies we d get in Utopia o We can then compare and contrast So what Example Conditions that violate Hardy Weinberg assumptions can be mechanisms of evolution It s up to us to figure out which of our 4 assumptions was violated Only natural selection leads to adaption Seed color in American lotus is determined by one gene with two alleles B gives a dark color and is dominant over b which confers a light seed coat If a population contains 145BB 97Bb and 124bb what are the frequencies of each allele o BB 145 o Bb 97 o Bb 124 o Total B alleles 145 x 2 97 387 o Total b alleles 124 x 2 97 345 o Total alleles 387 345 732 From this we can determine the observed frequencies Observed frequency of B 387 732 0 53 Observed frequency of b 345 732 0 47 o Use p and q to predict genotype frequencies Does this population appear to be in Hardy Weinberg Equilibrium o p2 145 366 0 396 Observed 0 281 Predicted o 2pq 97 366 0 265 Observed 0 498 Predicted o q2 124 366 0 339 Observed 0 220 Predicted Use B 0 53 and b 0 47 to predict what population genotypes SHOULD be in Utopia No the actual genotype frequencies do not match the expected frequencies obtained from p2 2pq q2 1 Types of Natural Selection 1 Directional shifts in a single direction reduces genetic variation 2 Stabilizing reduces extremes reduces variation average remains stable 3 Disruptive lose the middle favor extremes Variation is maintained Can lead to new species 4 Balancing no allele has advantage Each is balanced relative to the other Heterozygote advantage frequency dependent selection 1 Heterozygote Advantage sickle cell anemia 2 Frequency Dependent Selection rare allele gives an edge the edge is lost when allele Balancing Selection becomes common Genetic Drift Change in allele frequency due to chance alone Random with respect to fitness Most pronounced in small populations Can lead to random loss of alleles o Founder Effect Genetic drift due to colonization event Ex Sending 5 random people to a new planet to establish new population new population is likely to have different allele frequencies than the sources population by chance o Bottleneck target selection Gene flow due to sudden reduction in population size that is RANDOM in High mortality strikes individuals at random bottlenecked population is likely to have different allele frequencies than original population by chance Gene Flow Time 1 Populations differ in allele frequencies Time 2 Gene flow causes allele frequencies in the two populations to be more alike o Ex Interbreeding between two different allele frequencies Mutations Ultimate source of genetic variation Without mutations evolution would stop no diversity Alone insufficient to change allele frequencies Nonrandom Mating Inbreeding Sexual Selection o Occurs when o Increases homozygosity o Does not cause evolution since frequencies are constant o Inbreeding depression loss of fitness due to heterozygote advantage Heritable variation leads to differential mating success o Predictions of Sexual Selection Theory Female fitness is limited by availability of resources to use in producing offspring Male fitness is limited by opportunities to mate o Strategies of Sexual Selection Females choose Ex African Red Collared Widowbird females looks for a long enough tail that is not too short that it is inefficient but long enough to not use a lot of energy long tails more nests o Nature Tradeoffs long tails less healthy Males fight Fitness Trade offs Number of eggs vs size of eggs Energetically costly traits for mating success Aggression in social spiders Case Study Rapid evolution of flowering time by an annual plant in response to a climate fluctuation Franks S J et al PNAS 2007 104 1278 1282 Seeds of wild field mustard plants Brassica rapa were collected before 1997 and after 2004 What were the 3 experimental approaches that were possible 1 Old data vs new data 2 Survey at present in 2 environments 3 Collect seeds pre and post drought do controlled experiment in lab a Allows for much more control over conditions b Can do parallel experiments and rule out acclimation Season Treatments Water all to saturation for 22 days Short no water day 33 onward Medium no water day 51 onward Long no water day 81 onward
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