BIOL300c- Microevolution

1. Four Major Forces

   • Mutation: Primary source of new genetic variation caused by alterations in the DNA sequence.

   • Gene Flow: Transfer of genetic material (alleles) between populations through the movement of individuals or gametes.

   • Natural Selection: Process where organisms with higher fitness survive and reproduce at higher rates, passing beneficial traits to offspring.

   • Genetic Drift: Chance fluctuations in allele frequencies across generations, which can cause allele fixation or loss independently of fitness.

2. Population Genetics

   • Units of Evolution: Natural selection acts directly on individual organisms based on their phenotypes, but evolution occurs at the population level through changes in gene pools over generations.

   • Hardy-Weinberg Equilibrium: Theoretical baseline model used to test whether a population is undergoing evolution.

     • Assumptions: Infinite population size, random mating, no mutation, no gene flow (migration), and no natural selection.

     • Genotypic Frequency Equations:

       • Homozygous Dominant: $A_1A_1 = p^2$

       • Heterozygous: $A_1A_2 = 2pq$

       • Homozygous Recessive: $A_2A_2 = q^2$

     • Allele Frequency Equations:

       • $p^2+q^2+2pq=1$

       • $p = \text{freq}(A_1A_1) + \frac{1}{2}\text{freq}(A_1A_2)$

       • $q = \text{freq}(A_2A_2) + \frac{1}{2}\text{freq}(A_1A_2)$

3. Population Example & Hardy-Weinberg Analysis

   • Population A

     • Observed Genotype Frequencies:

       • $MM = 0.835$

       • $MN = 0.156$

       • $NN = 0.008$

     • Calculated Allele Frequencies:

       • $p = \text{freq}(M) = 0.835 + \frac{1}{2}(0.156) = 0.913$

       • $q = \text{freq}(N) = 0.008 + \frac{1}{2}(0.156) = 0.087$

     • Expected Genotype Frequencies under HWE:

       • $\text{Expected } MM = p^2 = (0.913)^2 = 0.834$

       • $\text{Expected } MN = 2pq = 2(0.913)(0.087) = 0.159$

       • $\text{Expected } NN = q^2 = (0.087)^2 = 0.008$

     • Interpretation: Observed frequencies ($MM = 0.835$ , $MN = 0.156$, $NN = 0.008$) match expected values ($0.834$ , $0.159$, $0.008$) very closely. Population A is in Hardy-Weinberg Equilibrium, indicating random mating and negligible evolutionary forces acting on this locus.

   • Population B

     • Observed Genotype Frequencies:

       • $MM = 0.431$

       • $MN = 0.142$

       • $NN = 0.427$

     • Calculated Allele Frequencies:

       • $p = \text{freq}(M) = 0.431 + \frac{1}{2}(0.142) = 0.502$

       • $q = \text{freq}(N) = 0.427 + \frac{1}{2}(0.142) = 0.498$

     • Expected Genotype Frequencies under HWE:

       • $\text{Expected } MM = p^2 = (0.502)^2 = 0.252$

       • $\text{Expected } MN = 2pq = 2(0.502)(0.498) = 0.500$

       • $\text{Expected } NN = q^2 = (0.498)^2 = 0.248$

     • Interpretation: Observed frequencies deviate drastically from expected values ($MN$ observed is $0.142$ vs $0.500$ expected). Population B is not in Hardy-Weinberg Equilibrium due to a strong heterozygote deficit and homozygote excess, pointing to non-random mating (such as inbreeding or positive assortative mating), selection against heterozygotes, or population structure (Wahlund effect).

SS-no anemia

Ss-anemia

ss-severe anemia

4% born with anemia ss 0.04

52% Ss is malria resistance 0.52

44%- have no anemia SS 0.44

Hardy Weinberg Again

p=0.3

p²=0.09

2pq= 0.42

q=0.7

q²= 0.49

ss=0.09

Ss=0.42

SS=0.49

Naturals selection is directed by fitness

Absolute fitness= kids

relative fitness= relative to population mean

relative>1 is adaptive and favored in section

relative<1 is detrimental deleterious

relative=1 neutral

Types of selection

directional- favors change in phenotype or allele. ex. less resistant → more resistant. reduces genetic diversity

stabilizing- intermediate phenotypes

disruptive-favors extremes

balancing -favors more than one allele in a population at the same time