Microevolution

What is the smallest unit that can evolve? a)Individual b)Species c)Genus d)Population Final Answer? d! Do you remember how evolution can be measured? Measure the allele frequency in the gene pool

Number of alleles: Calculating Allele Frequencies Phenotypes for flower color (R = red, r = white) 320 RR160 Rr20 rr 640 “R” 160 “R” and 160 “r”40 “r” Total alleles in gene pool: 1000 Allele frequencies:F R = 800/1000 =.8F r = 200/1000 =.2

To measure evolution, calculate the allele frequency before and after a given amount of time. If the frequency is the same or constant  no evolution has taken place A non-evolving population is said to be in “Hardy-Weinberg Equilibrium”

Hardy-Weinberg (HW) Equations Let’s revisit the previous example involving flower color…. Let p = allele frequency of dominant allele (R) Let q = allele frequency of recessive allele (r) Then… √ = 1 p + q = 1 HW Equation # 1

Hardy-Weinberg (HW) Equations If p = allele frequency of dominant allele (R), then p 2 = frequency of homozygous genotype (RR) If q = allele frequency of recessive allele (r), then q 2 = frequency of homozygous genotype (rr) If you complete the square, then the frequency of heterozygous genotype Rr must be 2pq So… p 2 + 2pq + q 2 = 1 HW Equation # 2 √ = 1

5 Assumptions for HW equilibrium 1)Large Population Size 2)No Migration 3)No Mutation 4)Random Mating 5)No Natural Selection If all 5 conditions are met, no evolution will occur Any deviation results in microevolution of the population

What is microevolution? A generation to generation change in a population’s allele frequency

What causes microevolution? 5 opposing conditions to HW equilibrium 1)Genetic DriftGenetic Drift 2)Gene FlowGene Flow 3)MutationMutation 4)Non-random matingNon-random mating 5)Natural SelectionNatural Selection

Genetic Drift In a small population, some individuals may, just by chance, leave behind a few more descendents (and genes, of course!) than other individuals. Genetic Drift always reduces diversity

2 Examples of Genetic Drift 1.Population Bottleneck –A natural disaster or event causes a population’s size to be greatly reduced Initial Diverse Population Event that causes the population to be reduced in size Final population (not diverse)

Over-poaching has bottlenecked the cheetah population, which is now susceptible to random changes in gene frequencies Reduction in diversity and inbreeding puts this species at risk of extinction

2 Examples of Genetic Drift 2.Founder Effect –Changes in gene frequencies that usually accompany starting a new population from a small number of individuals. Return to Causes of Microevolution

Individuals from one group move into another group. Makes certain genes more frequent in the population. Gene Flow Return to Causes of Microevolution

Mutation A change in a DNA sequence –usually occurring because of errors in replication or repair. Mutation is the ultimate source of genetic variation. Return to Causes of Microevolution

Non-Random Mating Individuals choose mates based on preferences for particular traits 2 examples – 1) Sexual Selection – mates are chosen based upon behavior/appearance – 2) Inbreeding – the choice to mate with related individuals Female peacocks choose mates based upon the male’s plumage display Return to Causes of Microevolution

Natural Selection Increases or decreases in allele frequencies due to environmental impact Natural Selection can act upon a population in a variety of ways… Directional Selection = favors traits that are at one extreme of a range of traits Strength of Selection Diversifying Selection = environmental conditions are varied such that individuals at both extremes are favored Stabilizing Selection = selection that acts against extreme phenotypes and favors intermediates