Week 4: Populations, Species and Evolution

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 F 1 (daughter generation) should be the same as the genotypes of F 0 (parent generation.

Hardy-Weinberg equations

• 1 = p + q (p = dominant, q = recessive) • 1 = p 2 + 2pq + q 2 (square both sides) • 1 = AA + 2Aa + aa • Genotype ratio of 1:2:1

0.5

0.5

0.5

Figure 03.02

aa = 0.5 X 0.5 = 0.25

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” (q 2 ) • 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” (q 2 ) • q 2 = 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 F 1 population 300 of which 27 blue eyed people • Since the allele for blue eyes is recessive then the blue eyed people are “aa” (q 2) • q 2 = 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 F 1 are not the same as in F 0 taking place (selection or mate choice has changed the distributions) therefore evolution is

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 ++ +--).

Tall Short

++++++ +++++ ++++- +++-- ++--- +---- ------

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

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

Speciation and Phylogeny

Macroevolution

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Species and Speciation

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Phylogenies: Evolutionary Trees

Biological Species

• Reproductive isolation – Gene flow within species – No gene flow between species • Allopathic speciation: results from geographic/environmental isolation – Selection or gene drift will eventually change each population in different directions and eventually they will be incapable of interbreeding

• Sympatric Speciation: Niche diversification – Different ways of making a living in the same place. – Specialization toward different resource gathering strategies leads to different selective forces

• Sympatric Speciation: Niche diversification – Different ways of making a living in the same place. – Specialization toward different resource gathering strategies leads to different selective forces

Time

– Darwin’s finches – Adaptive radiation Other isolation mechanisms – Mechanical isolation – Temporal isolation – Behavioral isolation Seagulls around the World