Chapter 3: The Modern Synthesis

Chapter 3: The Modern Synthesis

Hardy-Weinberg equilibrium • If no selection and mating is random (i.e., no processes acting to change the distribution of genotypes), then the genotypes of F1 (daughter generation) should be the same as the genotypes of F0 (parent generation.

Hardy-Weinberg equations • 1 = p + q (p = dominant, q = recessive) • 1 = p2 + 2pq + q2 (square both sides) • 1 = AA + 2Aa + aa • Genotype ratio of 1:2:1

Figure 03.02 aa = 0.5 X 0.5 = 0.25 0.5 0.5 0.5 Aa = 0.5 X 0.5 = 0.25 aA = 0.5 X 0.5 = 0.25 0.50 0.5 0.5 0.5 AA = 0.5 X 0.5 = 0.25

Example: Eye color • In a population of 100 we have 25 blue eyed people • Since the allele for blue eyes is recessive then the blue eyed people are “aa” (q2) • 25% or .25 are “aa” • The frequency of the allele for blue eyes = the square root of .25 = .50 (50%) • Since p + q = 1, the allele for Brown also = .50 (50%)

Example 2: Eye color • In a population of 100 we have 16 blue eyed people • Since the allele for blue eyes is recessive then the blue eyed people are “aa” (q2) • q2 = 16/100 or .16 • The frequency of the allele for blue eyes = the square root of .16 = .40 • Since p + q = 1, the frequency of the allele for Brown eyes is .60 (60%)

Figure 03.02

Example 2: Eye color • The F1 population 300 of which 27 blue eyed people • Since the allele for blue eyes is recessive then the blue eyed people are “aa” (q2) • q2 = 27/300 = .09 • The frequency of the allele for blue eyes = the square root of .09 = .30 (30%) • Since p + q = 1, the allele for Brown = .70 (70%) • The frequencies of the alleles for eye color in F1 are not the same as in F0 therefore evolution is taking place (selection or mate choice has changed the distributions)

Can Look Like Blending • If the phenotype is result of multiple genes each having an additive effect. • Example: tallness is controlled by 3 different genes each with 2 alleles (one for tall and one for short) • If you get the tall allele in from all 3 genes then you get 6 tall (++++++) and you are the tallest, if you get all short you get 6 short (------) and you are the shortest. • But if you get half of each you are in the middle (+-+-+-), you are also in the middle if you get ++-+--).

Since all allele effects are additive (all get expressed), the you can have any combination of 3 tall and 3 short and have the same phenotype ++++++ +++++- ++++-- +++--- ++---- +----- ------ Tall Short

Hidden Variation • Multiple gene effects mean that natural selection and/or mate choice is not always favoring or selecting against the same genes • Genes can hide in the recessive state when phenotype is dependent on multiple genes (loci).

Phenotypic Plasticity • Soapberry bugs and mate guarding • Frequency dependent strategy (Oklahoma population), trait is plastic • In Florida sex ratios are stable and trait is canalized

Other terms • Pleiotrophy = genes effect more than one trait • Correlated response = phenotype is dependent on more than one gene, therefore selection for a trait effects frequency of multiple genes at the same time • Maladaption • Gene drift: sampling effect, not natural selection or mate choice but random sampling variation • Fixation

Chapter 3: The Modern Synthesis