1. NOTES - CH 23:
Population Genetics

2. DEFINITIONS: ● POPULATION: a localized group of individuals
belonging to the same species
● SPECIES: a group of populations whose
individuals have the potential to
interbreed and produce fertile
offspring
● GENE POOL: all alleles at all
gene loci in all individuals in a
population

3. We all belong to the same gene pool!!!
A population of flamingo’s
6 different species of flamingo

4. The Hardy-Weinberg Theorem:
● a tool that describes a gene pool of a non-evolving population
● states that allele frequencies and genotypes in a population’s gene pool remain constant over the generations unless acted upon by agents other than sexual recombination

5. ● for Hardy-Weinberg equilibrium to occur, the following conditions must be met:
1) Large population
2) No mutation
3) No gene flow (no immigration or emigration)
4) Random mating (no mating preference for
particular phenotype)
5) No natural selection (all genotypes have an =
chance of surviving & reproducing)

6. vs.

7. HOWEVER, in nature:
1) most populations are small & may mate with one another
2) there are always mutations
(chance with every DNA replication)
3) gene flow often occurs between
populations
4) mating is non-random
5) natural selection is always occurring
**Therefore, in nature there will always be
changes in populations (“microevolution”)

9. **So why study population genetics? Why use the H-W Theorem?
1) shows how genetics is related to evolution;
2) provides a benchmark genetic equilibrium against which change can be noted;
3) permits an estimation of gene frequencies; especially useful in estimating the number of carriers of lethal alleles in human populations.
Ex: Brachydactyly - fingers are abnormally short in heterozygotes; condition is fatal during infancy to homozygous recessive individuals due to major skeletal defects

10. Hardy-Weinberg Equation:
● p = frequency of dominant allele (A)
● q = frequency of recessive allele (a)
● p + q = 1
● frequency of possible diploid
combinations (AA, Aa, aa):
p pq q2 = 1
(AA)
(Aa)
(aa)

12. Example Problem:
● If the frequency of a recessive allele is 35% in a population of 1500 people, how many people would you predict would be carriers of this allele, but would not express the recessive phenotype?

13. Solution: q = 35% = 0.35 p = 1 - q = 1 - 0.35 = 0.65 p2 + 2pq + q2 = 1
freq. of Aa genotype = 2pq
= 2(0.65)(0.35)
= = 45.5%
# of carriers = (0.455)(1500)
= 683 people

14. Example Problem:
● In a population with 2 alleles for a particular locus, B and b, the allele frequency of B is If the population consists of 172 individuals, how many individuals are heterozygous? How many will show the recessive phenotype?

15. Solution: p = 0.78 q = 1 - p = 1 - 0.78 = 0.22 p2 + 2pq + q2 = 1
freq. of Bb genotype = 2pq
= 2(0.78)(0.22)
= = 34.3%
# of heterozygotes = (0.343)(172)
= 59 individuals

16. Solution: p2 + 2pq + q2 = 1 freq. of recessive phenotype =
freq. of bb = q2 = (0.22)2
= = 4.84%
# of recessive ind. = (0.0484)(172)
= 8.3 individuals
(8 ind.)

17. DEFINITIONS
● Microevolution = studies how pop’s of organisms change from generation to generation; changes in allele frequencies in a population’s gene pool
● Macroevolution = studies changes in groups of related species over long periods of geologic time; determines evolutionary relationships among species

19. Causes of Microevolution:
1) Natural Selection
2) Genetic Drift (changes in the gene pool of a small population due to chance)
Examples:
-Bottleneck Effect: results from drastic
decrease in population size
-Founder Effect: few individuals in a
population colonize a new habitat

20. Bottleneck Effect

23. 3) Gene Flow (migration of fertile individuals between populations)
4) Mutation (introduces new alleles into a population)
5) Nonrandom Mating (individuals choose mates based upon their traits)

25. Ways Natural Selection Acts on a Population:
1) Stabilizing Selection: eliminates individuals with extreme or unusual traits; existing population frequencies of common traits are maintained

26. *Example of Stabilizing Selection in humans:
*human babies most commonly weigh 3-4 kg; babies much smaller or larger have higher infant mortality rates.

27. 2) Directional Selection: favors traits at one extreme of a range of traits; common during periods of environmental change
Examples:
-insecticide resistance
-peppered moth

28. Peppered Moth example:
● 100 years after the first dark moth was discovered in 1848, 90% of moths were dark;
● the light variety continued to dominate in unpolluted areas outside of London.

29. 3) Diversifying (a.k.a. Disruptive) Selection: occurs when environment favors extreme or unusual traits while selecting against common traits

31. 4) Sexual Selection: differential mating of males in a population; leads to sexual dimorphisms
-females tend to increase their fitness by
increasing the quality of their offspring by
choosing superior male mates (and are therefore
“choosier” or more selective when finding a mate)

33. Sexual Selection (cont.)
-males increase their fitness by maximizing the quantity of offspring produced
**as a result, in vertebrate species,
the male is typically the
“showier” sex
-colorful plumage
-lion’s mane
-antlers

36. Sexual Selection:
● INTRASEXUAL SELECTION = direct competition among individuals of one sex (males use antlers, aggressive behavior, etc.)
● INTERSEXUAL SELECTION = “mate choice”; individuals of one sex are choosy (usually the females)