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CH 13 – Microevolution - Modern
1. Recap
2. Definitions
3. Analyzing Gene Pools (Hardy Weinberg Equilibrium)
4. Mechanisms of Evolution
a) Genetic Drift
b) Gene Flow
c) Mutation
d) Natural Selection
5. Sexual Selection – a form of Natural Selection
6. General Outcomes of Natural Selection
 Directional, Disruptive, Stabilizing
7. Evolution Connection: Sickle Cell Anemia
The Modern Synthesis:
Darwinism Meets Genetics (Mendel)
– The modern synthesis is the fusion of genetics with
evolutionary biology.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Populations as the Units of Evolution
– A population
• Is a group of individuals of the same species living in the
same place at the same time.
• Is the smallest biological unit that can evolve.
– Population genetics
• Focuses on populations as the evolutionary units.
• Tracks the genetic makeup of populations over time.
Genetic Variation in Populations
– Individual variation abounds in populations.
• Not all of this variation is heritable.
• Only the genetic component of variation is relevant to
natural selection.
Sources of Genetic Variation
• Mutations
– Are changes in the DNA of an organism.
• Sexual recombination
– Shuffles alleles during meiosis. (independent
assortment, random fertilization, crossing over)
Gene pool
Analyzing Gene Pools
– The gene pool
• Consists of all alleles of all individuals making up a
– Alleles in a gene pool
• Occur in certain frequencies.
• Can be symbolized by p for the relative frequency of the
dominant allele in the population and q for the frequency
of the recessive allele in the population.
• p+q=1
The Hardy-Weinberg formula
criteria (non-evolving population):
– Large population – must be large to minimize
sampling error.
– Random mating – no mating preference
– No mutation- alleles must not change.
– No migration – exchange of genes between the
population and another population must not occur.
– No natural selection- natural selection must not
favor any individual.
– Genotype frequencies
– Can be calculated from allele frequencies.
– Are symbolized by the expressions p2, 2pq, and q2.
p2 + 2pq + q2 = 1
Figure 13.20
Sample Problems
• 1.You have sampled a population in which you know that the percentage of
the homozygous recessive genotype (aa) is 36%. Using that 36%, calculate the
• A. The frequency of the "aa" genotype.
• B. The frequency of the "a" allele.
• C. The frequency of the "A" allele.
• D. The frequencies of the genotypes "AA" and "Aa."
• E.
The frequencies of the two possible phenotypes if "A" is completely
dominant over "a."
• #2. Sickle-cell anemia is an interesting genetic disease. Normal homozygous
individials (SS) have normal blood cells that are easily infected with the
malarial parasite. Thus, many of these individuals become very ill from the
parasite and many die. Individuals homozygous for the sickle-cell trait (ss)
have red blood cells that readily collapse when deoxygenated. Although
malaria cannot grow in these red blood cells, individuals often die because of
the genetic defect. However, individuals with the heterozygous condition (Ss)
have some sickling of red blood cells, but generally not enough to cause
mortality. In addition, malaria cannot survive well within these "partially
defective" red blood cells. Thus, heterozygotes tend to survive better than
either of the homozygous conditions. If 9% of an African population is born
with a severe form of sickle-cell anemia (ss), what percentage of the
population will be more resistant to malaria because they are heterozygous
(Ss) for the sickle-cell gene?
• #3. There are 100 students in a class. Ninety-six did well in the
course whereas four blew it totally and received a grade of F.
Sorry. In the highly unlikely event that these traits are genetic
rather than environmental, if these traits involve dominant and
recessive alleles, and if the four (4%) represent the frequency of
the homozygous recessive condition, please calculate the
• A. The frequency of the recessive allele.
• B. The frequency of the dominant allele.
• C. The frequency of heterozygous individuals.
• #4. Within a population of butterflies, the color
brown (B) is dominant over the color white (b).
And, 40% of all butterflies are white. Given this
simple information, which is something that is
very likely to be on an exam, calculate the
• A. The percentage of butterflies in the
population that are heterozygous.
• B. The frequency of homozygous dominant
Microevolution as Change in a Gene
– Hardy-Weinberg equilibrium
• Describes a non-evolving population that is in genetic
– Microevolution is defined as
• A generation-to-generation change in a population’s
frequencies of alleles.
Mechanisms of Microevolution
– The main causes of microevolution are
Genetic drift.
Gene flow.
Natural selection.
Genetic Drift
– Genetic drift
• Is a change in the gene pool of a small population due to
The Bottleneck Effect
– The bottleneck effect
• Is an example of genetic drift.
• Results from a drastic reduction in population size.
Figure 13.23
– Bottlenecking in a population usually reduces genetic
The Founder Effect
– The founder effect
• Is genetic drift in a new colony.
The Founder Effect
Explains the relatively high frequency of certain inherited
disorders among some populations.
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
– Gene flow
Gene Flow
• Is genetic exchange with another population.
• Tends to reduce genetic differences between populations.
Figure 13.26
– Mutations
• Are changes in an organism’s DNA.
• Alone do not have much effect on a large population.
• Can have significant cumulative effects on a population.
Natural Selection: A Closer Look
– Of all causes of microevolution, only natural
selection promotes adaptation.
Darwinian Fitness
– Darwinian fitness
• Is the contribution an individual makes to the gene pool of
the next generation relative to the contributions of other
Sexual Selection – a form of Natural Selection
Figure 13.27
Three General Outcomes of Natural
– Directional selection
• Shifts the phenotypic “curve” of a population.
• Selects in favor of some extreme phenotype.
– Disruptive selection
• Can lead to a balance between two or more contrasting
morphs in a population.
– Stabilizing selection
• Maintains variation for a particular trait within a narrow
Figure 13.28
Evolution Connection: Population Genetics
of the Sickle-Cell Allele
– Sickle-cell disease
• Affects about one out of every 400 African Americans.
• Is more common among African Americans; but why?
Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings
– The sickle-cell allele
• Confers resistance to the disease malaria.
• Is adaptive in the African tropics where malaria is