Presentation on theme: "Ch. 23 The Evolution of Populations"— Presentation transcript:
1 Ch. 23 The Evolution of Populations Objective:Understand how allele frequencies can show evolution in a population.
2 OVERVIEW Individuals have variations but they do not evolve. The smallest scale (microevolution) shows change in allele frequencies in a population over time.Causes of evolution:Natural selection (fittest organisms)Genetic drift (random catastrophes change allele frequencies)Gene flow (movement of alleles between populations)
3 23.1: Genetic Variation Makes evolution possible Diverse, inheritable traits set the stage for evolutionVariation within a populationVariation between populationsSources of Genetic VariationFormation of new alleles by mutationChromosomal alterations (deletion, duplication, translocation, etc.)Sexual reproduction (crossing over, independent assortment, and fertilization).
4 23.2 The hardy-weinberg equation A gene pool is a summative of a population’s genes.Allele frequency is the number of times one allele appears in the gene pool.# of time allele is present/total # of allelesEx: 500 flowers = 320 Red (RR) pink (RW) + 20 White (WW)Total alleles = 1,000 (each individual has 2 alleles)= 800 red alleles= 200 white allelesFrequency of red = 800/1000 = 80%Frequency of white = 200/1000 = 20%
5 The Hardy-Weinberg Theorem A control to compare evolving populations to.H-W EquilibriumLarge populationNo movement into/out of populationNo mutationsRandom matingNo natural selection (no beneficial/lethal alleles)
6 H-W Equation p = dominant allele frequency q = recessive allele frequencyp + q = 1p2 + 2pq + q2 = 1AA + Aa + aA + aa = 1Applying the H-W EquationSee if evolution is happening (allele frequencies change.)This can be used to calculate the number of heterozygotes vs homozygous dominant individuals
7 23.3 Altering Population’s Gene Pool Natural SelectionNonrandom matingTraits allow you to have more offspring.Ex: sexual dimorphism: males are elaborately decorated to attract mates.
9 Random changes in allele frequencies over time, reducing variation. Genetic DriftRandom changes in allele frequencies over time, reducing variation.Bottleneck EffectAn event causes a loss of the majority of a population.Founder EffectA few individuals leave to start a new population
10 Gene FlowReduces differences between populations by sharing of gametes across them.
11 23.4 Natural Selection is the only mechanism that consistently causes Adaptive Evolution Modes of SelectionAt any moment, populations show a normal curve for most traits.This curve can change in 3 ways depending on how the environment selects for a trait.Phenotypes (fur color)Original populationFrequency of individuals
12 Directional Selection Common when environment changes. One of two extremes is favored.EX: Increase size of bears during ice agesOriginalpopulationEvolved
13 Both extremes are favored while average disappears. Disruptive SelectionBoth extremes are favored while average disappears.EX: beak size in finches (large for hard seeds and small for soft)
14 Stabilizing Selection Extremes disappear increasing the intermediate. EX: birth weight of babies