# Ch. 23 The Evolution of Populations

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Ch. 23 The Evolution of Populations
Objective: Understand how allele frequencies can show evolution in a population.

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)

23.1: Genetic Variation Makes evolution possible
Diverse, inheritable traits set the stage for evolution Variation within a population Variation between populations Sources of Genetic Variation Formation of new alleles by mutation Chromosomal alterations (deletion, duplication, translocation, etc.) Sexual reproduction (crossing over, independent assortment, and fertilization).

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 alleles Ex: 500 flowers = 320 Red (RR) pink (RW) + 20 White (WW) Total alleles = 1,000 (each individual has 2 alleles) = 800 red alleles = 200 white alleles Frequency of red = 800/1000 = 80% Frequency of white = 200/1000 = 20%

The Hardy-Weinberg Theorem
A control to compare evolving populations to. H-W Equilibrium Large population No movement into/out of population No mutations Random mating No natural selection (no beneficial/lethal alleles)

H-W Equation p = dominant allele frequency
q = recessive allele frequency p + q = 1 p2 + 2pq + q2 = 1 AA + Aa + aA + aa = 1 Applying the H-W Equation See if evolution is happening (allele frequencies change.) This can be used to calculate the number of heterozygotes vs homozygous dominant individuals

23.3 Altering Population’s Gene Pool
Natural Selection Nonrandom mating Traits allow you to have more offspring. Ex: sexual dimorphism: males are elaborately decorated to attract mates.

Animation: Causes of Evolutionary Change
Right-click slide / select “Play” © 2011 Pearson Education, Inc. 8

Random changes in allele frequencies over time, reducing variation.
Genetic Drift Random changes in allele frequencies over time, reducing variation. Bottleneck Effect An event causes a loss of the majority of a population. Founder Effect A few individuals leave to start a new population

Gene Flow Reduces differences between populations by sharing of gametes across them.

23.4 Natural Selection is the only mechanism that consistently causes Adaptive Evolution
Modes of Selection At 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 population Frequency of individuals

Directional Selection Common when environment changes.
One of two extremes is favored. EX: Increase size of bears during ice ages Original population Evolved

Both extremes are favored while average disappears.
Disruptive Selection Both extremes are favored while average disappears. EX: beak size in finches (large for hard seeds and small for soft)

Stabilizing Selection Extremes disappear increasing the intermediate.
EX: birth weight of babies