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The Evolution of
Chapter 21
Evolutionary changes within a population
Changes in allele frequencies in a population over
Population – all members of a species living
in the same area, interbreed, produce fertile
Example – industrial melanism and the
peppered moth
Genetic Variation
Differences among individuals in the
composition of the genes
Single gene influence (Mendel) or polygenic
Phenotype – physical traits, can be inherited
or influence by environment.
Sources of Genetic Variation
Formation of new alleles
Altering Gene number or position
Chromosomal changes – deletion, translocation,
inversion and duplication
Rapid reproduction – prokaryotes
Sexual Reproduction
Crossing over, independent assortment and
random fertilization
Hardy-Weinberg principle
p2 +2pq + q2
Used to calculate the genotype and gene
frequencies of a population
States: equilibrium of allele frequencies in a gene
pool will remain in effect in each generation of
sexually reproducing populations as long as:
No mutations
No gene flow
Random mating
No genetic drift
No selection
Hardy-Weinberg principle
Tells us what factors cause evolution
The 5 conditions are hardly ever met
Allele frequencies do change from one generation
to another
Evolution can be detected by seeing any deviation
from a Hardy-Weinberg equilibrium
Practice problems p.406
Causes of microevolution
Opposite of HWP
Genetic mutations – cause for multiple
alleles, can be adaptive and include
favorable phenotypes
Nonrandom mating – inbreeding or
breeding between relatives, decreases the
Causes of microevolution that alter
allele frequency directly
Genetic Drift – change in allele frequencies
due to chance
Bottleneck effect – natural disaster, reduce in
population prevents the majority of genotypes from
participating in the production of the next generation
Founder effect – rare alleles occur at a higher
frequency in a population isolated from a general
population ex. amish
Gene Flow – transfer of alleles into or out of a
population due to the movement of fertile
individuals or their gametes.
Natural selection
Not random – adaptive evolution
Most traits are polygenic, see bell curve in
allele frequency
3 major types of selection -
Directional – extreme phenotype favored
Stabilizing – intermediate phenotype is favored
Resistance to antibiotics and pesticides, malaria
Birth weight survival, sickle cell trait
Disruptive – 2 or more extreme phenotypes are
Sexual selection
Natural selection in which individuals with
certain inherited characteristics are more
likely than others to obtain mates.
Sexual dimorphism – differences in males and
females (i.e. size, color, …)
Balancing selection
Natural selection maintains two or more
forms in a population.
Heterozygote advantage – Malaria and sickle cell
Frequency-dependent selection – scale eating
fish. Right and left mouthed
Why doesn’t Natural Selection create
perfect organisms?
Selection can act only on existing variations
Evolution is limited by historical constraints
Adaptations are often compromises
Chance, natural selection and the
environment interact