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 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).

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 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%

5. 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)

6. 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

7. 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.

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

9. 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

10. Gene Flow
Reduces 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 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

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

13. 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)

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