Download 16-2 Evolution as Genetic Change

Chapter 16: Evolution of Populations (16-2)
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Natural selection does not act directly on genes. In
stead, it acts on phenotypes.
Even though natural selection does not operate
directly on genes, it can change the relative
frequencies of alleles in a population over time.
Evolution is any change in the relative frequencies
of alleles in a population’s gene pool
 Natural selection on single-gene traits can lead to
changes in allele frequencies and thus evolution
 See lizard story on page 397 (red v. black lizards)
When traits are controlled by more than one gene,
the effects of N.S. are more complex.
 N.S. can affect distributions of phenotypes in
three ways.
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Chapter 16: Evolution of Populations (16-2)
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occurs when individuals at
one end of the curve have
higher fitness than
individuals in the middle or
at the other end… evolution
causes an increase in the number of individuals
with the trait at one end.
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takes place when individuals
near the center of a curve
have higher fitness than
individuals at either
end…narrows the overall
graph.
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—when individuals at
the upper and lower ends of the
curve have higher fitness than
individuals near the middle…
selection acts most strongly
against individual of an
intermediate type.
Chapter 16: Evolution of Populations (16-2)
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In small populations, an allele can become more or
less common simply by chance.
Random change in alleles frequency is called
genetic drift.
A migration of a small group of a population is
known as the founder affect.
The Hardy-Weinberg principle states that allele
frequencies in a population will remain constant
unless one or more factors (5) cause those
frequencies to change.
 Genetic Equilibrium—allele frequencies remain
constant.
 Five conditions are required to maintain genetic
equilibrium:
 Random Mating: all members of the
population must have equal opportunity to
produce offspring … In natural populations
mating is rarely completely random.
 Large populations: genetic drift has less
effect on large populations.
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Chapter 16: Evolution of Populations (16-2)
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No Movement Into or Out of the
Population: the population’s gene pools must
be kept together and kept separate from other
populations.
No Mutations: If genes mutate, new alleles
may be introduced into the population, and
allele frequencies will change.
No Natural Selection: No phenotype can
have a selective advantage over another.
p is the frequency of the dominant allele
q is the frequency of the recessive allele
p2 =
2pq =
q2 =
TT
Tt
tt
p+q=1
2
(1 means 100%)
2
p + 2pq + q = 1