The document covers population genetics, focusing on gene frequencies, equilibrium, and mechanisms affecting gene variations such as mutation, selection, and genetic drift. It also discusses concepts of race and species formation, including reproductive isolation mechanisms and speciation processes. Key principles include the Hardy-Weinberg model for allele frequency estimation and the impact of environmental factors and mating systems on genetic diversity within populations.

1. Bio 106
Lecture 11
Genes in Populations

2. A. Population Genetics
B. Gene Frequencies and Equilibrium
1. Gene Frequencies
2. Gene Pool
3. Model System for Population
Stability (Hardy – Weinberg Law)
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3. C. Changes in Gene Frequencies
1. Mutation
2. Selection
2.1 Relative Fitness
2.2 Selections and Variability
2.3 Selection and Mating
3. Systems
4. Migration
5. Genetic Drift
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4. D. Race and Species Formation
1. The Concept of Races
2. The Concept of Species
2.1 Reproductive Isolating Mechanisms
2.2 Rapid Speciation
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5. Population: a group of individuals of the
same species that live in the same
area and interbreed (interbreeding
causes production of fertile offspring)
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6. POPULATION GENETICS
the study of genetic variation within
populations
involves the examination and modelling
of changes in the frequencies of genes
and alleles in populations over space
and time.
FOCUS: species or population
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7. Gene frequencies & equilibrium
GENE POOL: the
collection of all
the alleles of all
of the genes
found within a
freely
interbreeding
population
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8. Gene frequencies & equilibrium
GENE FREQUENCY (allele frequency)
The proportion of all alleles in all
individuals in the group in question
which are of a particular type.
Ex: 40 individuals which are AA
47 individuals which are Aa
13 individuals which are aa
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9. Gene frequencies & equilibrium
GENOTYPE
AA Aa aa TOTAL
# of individuals 40 47 13 100
# of A alleles 80 47 0 127
# of a alleles 0 47 26 73
Total # of alleles 200
Allele frequency of A: 127/200 = 0.635
pA=0.635
pa = 73/200 = 0.365 = 1- pA
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10. Gene frequencies & equilibrium
GENOTYPE FREQUENCY:
The proportion of individuals in a group
with a particular genotype.
40 AA 47 Aa 13 aa = 100 Total individuals
pAA = 40/100 = 0.4
pAa = 47/100 = 0.47
paa = 13/100 = 0.13
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11. Gene frequencies & equilibrium
Hardy-Weinberg Equation:
used to estimate frequency of alleles
in a population
p = the frequency of the dominant allele (A)
q = the frequency of the recessive allele (a)
For a population in genetic equilibrium:
p + q = 1.0
(The sum of the frequencies of both alleles is 100%.)
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12. Gene frequencies & equilibrium
(p + q)2 = 1 so
p2 + 2pq + q2 = 1
where
p2 = frequency of AA
2pq = frequency of Aa
q2 = frequency of aa
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13. Gene frequencies & equilibrium
Assumptions of the HW model:
1. Organism is diploid.
2. Reproduction is sexual.
3. Generations are non-overlapping.
4. Mating occurs at random.
5. Population size is very large.
6. Migration is zero.
7. Mutation is zero.
8. Natural selection does not affect the gene in
question.
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14. Changes in Gene frequencies
Factors that affect Gene Frequency:
1. Mutation
2. Natural selection
3. population size
4. genetic drift
5. environmental diversity
6. Migration
7. non-random mating patterns
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15. Changes in Gene frequencies
MUTATION is the primary source of
new alleles in a gene pool.
15cces2015

16. Changes in Gene
frequencies
Natural Selection: the
differential
reproduction of
genotypes
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17. Changes in Gene frequencies
Relative Fitness:
ability to survive in
an environment
long enough to
reproduce
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18. Changes in Gene frequencies
Selections and Variability:
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19. Changes in Gene frequencies
Selections and Variability:
selects against the
average individual in a
population.
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20. Changes in Gene frequencies
Selections and
Variability:
Favors the intermediate
variants
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21. Changes in Gene frequencies
Selections and Variability:
favors the
intermediate
variants
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22. Changes in Gene frequencies
Directional selection:
an extreme phenotype is
favored over other
phenotypes, causing the
allele frequency to shift
over time in the direction
of that phenotype.
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23. Changes in Gene frequencies
Selection and Mating
Sexual selection occurs when
individuals within one sex
secure mates and produce
offspring at the expense
of other individuals within
the same sex.
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24. Changes in Gene frequencies
Mating systems:
RANDOM: mate choice is independent of
phenotype and genotype
POSITIVE ASSORTMENT: mate choice is
dependent on similarity of phenotype
NEGATIVE ASSORTMENT: …..on dissimilarity
of phenotype
INBREEDING: mating with relatives at a rate
greater than expected by chance
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25. Changes in Gene frequencies
Mating
Systems
Positive
assortative
mating
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26. Changes in Gene frequencies
Mating
Systems
Negative
assortative
mating
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27. Changes in Gene frequencies
Positive assortment:
- increases homozygosity (prevents HW
equilibrium)
- does not affect allele frequency
- dominance dilutes its effect
- affects only those genes related to
the phenotype by which mates are
chosen
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28. Changes in Gene frequencies
Negative Assortment/disassortative
mating:
- yields an excess of heterozygotes
(compared to HW)
- does not affect allele frequencies *
- dominance dilutes its effect
- Increases the rate to equilibrium of
alleles among loci (because linkage phases are
disrupted by recombination in double homozygotes).
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29. Changes in Gene frequencies
Inbreeding alone does
not change allele
frequencies, but
inbreeding does
change genotype
frequencies.
Inbreeding can affect
allele frequencies, by
changing how selection
operates.
29cces2015

30. Changes in Gene frequencies
Inbreeding
- Can result to excess homozygotes
- Inbred individuals usually have lower fitness
than outbred individuals. (inbreeding
depression)
- 2 possible reasons for inbreeding depression:
(1) deleterious recessive alleles
(2) overdominance
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31. Changes in Gene frequencies
Population Size:
Increase in
population
causes
increase in
gene
frequencies.
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32. Changes in Gene frequencies
GENETIC DRIFT occurs as the result
of random fluctuations in the
transfer of alleles from one
generation to the next, especially in
small populations formed, as a result
of bottleneck effect and founder
effect
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33. Changes in Gene frequencies
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34. Changes in Gene frequencies
Bottleneck effect:
adverse
environmental
conditions
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35. Changes in Gene frequencies
Founder effect: geographical separation
of a subset of the population
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36. Changes in Gene frequencies
Random genetic drift can continue until
one allele is fixed or lost
As genetic drift progresses,
- heterozygosity decreases.
- genetic variance within populations
decreases.
- genetic variance among populations
increases.
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37. Changes in Gene frequencies
Migration: movement of individuals from
one population to another; translated
as gene flow
Can equalize gene frequency. 37cces2015

38. Changes in Gene frequencies
Gene flow
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39. Race and Species Formation
Race: geographically isolated breeding
population that shares certain
characteristics in higher frequencies
than other populations of that species,
but has not become reproductively
isolated from other populations of the
same species.
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40. Race & Species Formation
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41. Race & Species Formation
Species: members of populations that
actually or potentially interbreed in
nature, not according to similarity of
appearance
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42. Race & Species Formation
Reproductive Isolating Mechanisms
prezygotic isolating mechanisms
- prevent the formation of
viable zygotes
postzygotic isolating mechanisms
- prevent hybrids from passing on their
genes
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43. Race & Species Formation
Reproductive Isolating
Mechanisms
Prezygotic Isolating
Mechanisms
Ecological Isolation:
The geographic ranges
of two species overlap,
but their ecological
needs or breeding
requirements differ
enough to cause
reproductive isolation.
43cces2015

44. Race&SpeciesFormation Reproductive Isolating
Mechanisms
Prezygotic Isolating
Mechanisms
Temporal Isolation:
two species whose ranges
overlap have different
periods of sexual activity
or breeding seasons
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45. 45cces2015

46. Race & Species Formation
Reproductive Isolating Mechanisms
Prezygotic Isolating Mechanisms
Behavioral Isolation:
signals to attract mates, elaborate behaviors, courtship
rituals differ between species
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47. Race & Species Formation
Reproductive Isolating Mechanisms
Prezygotic Isolating Mechanisms
Mechanical Isolation:
Morphological differences prevent mating/pollination.
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48. Race & Species Formation
Reproductive Isolating Mechanisms
Prezygotic Isolating Mechanisms
Gametic Isolation:
sperm and ova of the two species are chemically
(genetically) incompatible, and will not fuse to form a
zygote.
48cces2015

49. Race & Species Formation
Reproductive Isolating
Mechanisms
Postzygotic
Isolating
Mechanisms
Hybrid inviability:
The hybrid offspring is
either weaker than the
parent species, or totally
inviable. This could be
caused by minor or major
genetic defects, and even
slightly reduced viability
can cause big decreases
in reproduction. 49cces2015

50. Race & Species Formation
Reproductive Isolating
Mechanisms
Postzygotic Isolating
Mechanisms
Hybrid sterility:
Viable hybrid is produced but
is unable to reproduce due to
meiotic problems
(mare) (jack)
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51. Hybrid sterility
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52. Race & Species Formation
Reproductive Isolating
Mechanisms
Postzygotic Isolating
Mechanisms
Hybrid breakdown:
successive generations of
hybrids suffer greatly
lowered fertility --> sterility.
Eventually, they are selected
out of the population..
52cces2015

53. Race and Species Formation
Speciation:
Hybrids may actually be reproductively superior to
parent populations, and if they tend to breed with
each other, this can result in what could be
termed hybrid speciation.
Speciation can occur as a result of hybridization
between two related species, if the hybrid
• receives a genome that enables it to breed with other
such hybrids but not breed with either parental species;
• can escape to a habitat where it does not have to compete
with either parent;
• is adapted to live under those new conditions.
53cces2015