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Genes and Evolution
EVOLUTION AS GENETIC CHANGE
Natural Selection and Genes
 Evolution occurs when
there is shift in the
relative frequency of
an allele in a gene pool.
 This occurs because
natural selection
results in individuals
with certain traits and
certain alleles surviving
more easily and
passing their alleles on
to more offspring.
Single-Gene Traits
 Over time one phenotype of the single-gene traits might
be more advantageous.
 As time passes, more individuals will show that
phenotype, and the alleles needed for it will have higher
frequency.
 With single gene traits it is nearly impossible to select
away an individual allele.
Polygenic Traits
 Directional Selection
 This occurs when one of the
extremes becomes most
advantageous.
 Stabilizing Selection
 This happens when the
middle of the curve is most
fit.
 Disruptive Selection
 This occurs when both
extremes are more
advantageous than the
middle.
Small Populations
 Sometimes chance can change the frequency of an
allele, rather than natural selection.
 This is called genetic drift.
 Typically, the founder effect, or when a small
population gets separated from the larger
population, creates this.
Hardy-Weinberg Principle
 Allele frequencies in a
population will stay
constant unless some
factor(s) force the
population to change
 When the allele
frequency stays
constant, this is known
as genetic equilibrium.
Requirements for Genetic
Equilibrium
1. Random Mating
2. Large Population
3. No movement in or
out of the population
4. No mutations
5. No Natural Selection
Answer These
1. What kind of trait can undergo disruptive selection?
1. Polygenic Trait 2. Single-gene trait
2. Describe an event that could cause stabilizing selection?
1. Something that makes being 6’3” advantageous
2. Something that makes being 5’7 advantageous
3. What is it called when the gene frequencies in gene pool
remain constant?
1. Genetic Stabilization 2. Genetic Equilibrium
4. What are the five requirements to maintain genetic
equilibrium?
 Random Mating, Large population, no movement
in/out, no mutations and no natural selection
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