Population: 10 individuals Phenotype frequency: 30% blue eyes (or 0.3) 70% brown eyes (or 0.7)

Population 10 individuals Phenotype frequency: 30% blue eyes (or 0.3) 70% brown eyes (or 0.7) = bb = BB = Bb Genotype frequency: f(bb) = 30% (or 0.3) f(Bb) = 50% (or 0.5) F(BB) = 20% (or 0.2)

Population 10 individuals = bb = BB = Bb Genotype frequency: f(bb) = 30% (or 0.3) f(Bb) = 50% (or 0.5) f(BB) = 20% (or 0.2) bbbbbbBb Bb Bb Bb BbBBBB Number of alleles: b = 11 B = 9

Population 10 individuals Genotype frequency: f(bb) = 30% (or 0.3) f(Bb) = 50% (or 0.5) f(BB) = 20% (or 0.2) bbbbbb Bb Bb Bb Bb Bb BBBB Number of alleles: b = 11 B = 9 b b b b b b B b B b B b B b B b B B B B Allele frequency: f(b) = 11/20 = 0.55 f(B) = 9/20 = 0.45

b b b b b b B b B b B b B b B b B B B B = Gene pool of a population

= bb = BB = Bb bbbbbbBb Bb Bb Bb BbBBBB Possible gametes of each individual

b b b b b b B b B b B b B b B b B B B B = Possible gametes in the population

Population 10 individuals Genotype frequency: f(bb) = 30% (or 0.3) f(Bb) = 50% (or 0.5) f(BB) = 20% (or 0.2) bbbbbb Bb Bb Bb Bb Bb BBBB Number of alleles: b = 11 B = 9 b b b b b b B b B b B b B b B b B B B B Allele frequency: f(b) = 11/20 = 0.55 f(B) = 9/20 = 0.45

Genetic Drift (a) Founder effect (b) Genetic bottleneck High mortality strikes individuals at random Time 1Time 2 Bottlenecked population is likely to have different allele frequencies than original population, by chance New population is likely to have different allele frequencies than the source population, by chance Immigrants establish new population Homozygous for allele A 1 Homozygous for allele A 2 Heterozygous

Time 1 Population 1 At time 1, populations differ in allele frequencies Gene flow causes allele frequencies in the two populations to be more alike Time 2 Population 2 Population 1 Population 2 Homozygous for allele A 1 Homozygous for allele A 2 Gene flow Gene flow Gene Flow

(a) Directional selection changes the average value of a trait. Before selection During selection Normal distribution Number of individuals Low fitness High fitness Change in average value After selection Value of a trait

Figure 25-3Slide 6 Original population (N = 2880) Body size class For example, directional selection caused average body size to increase in a cliff swallow population. Percentage of birds Change in average value Survivors (N = 1027)

(a) Stabilizing selection reduces the amount of variation in a trait. Normal distribution Before selection During selection After selection Number of individuals Value of a trait Low fitness Low fitness Reduction in variation High fitness

(b) For example, very small and very large babies are the most likely to die, leaving a narrower distribution of birth weights. Percentage of newborn population Mortality Birth weight (pounds) Percentage of mortality Heavy mortality on extremes

(a) Disruptive selection increases the amount of variation in a trait. Normal distribution Before selection During selection After selection Number of individuals Value of a trait Highfitness High fitness Increase in variation Low fitness

(b) For example, only juvenile black-bellied seedcrackers that had very long or very short beaks survived long enough to breed. Only the extremes survived Only the extremes survived Number of individuals Beak length (mm)