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15.3 How Does Evolution Occur?
What are the Mechanisms of Evolution?
 Population genetics
 Hardy-Weinberg principle states that when
allelic frequencies remain constant, a population
is in genetic equilibrium and evolution does not
What happens if changes occur in the
allele frequencies of a population?
Evolution is always occurring because these
conditions can never be met.
Genetic Drift
 A change in the allelic frequencies in a
population that is due to chance
 This is an issue in small populations. If an
individual with recessive alleles breeds more
than “normal” the frequency of the recessive
allele will increase quickly. This does not
happen in large populations, there are too
many individuals.
 Occurs when a population declines to a very
low number and then rebounds, the problems
is a loss of genetic variability.
What is Gene Flow?
 This is when organisms move from one
population to another. *Increases genetic
variation within a population
Nonrandom Mating
 Since organisms choose their mates,
this decreases variation in a population.
What is Natural
 The process by which
organisms with traits
well suited to their
environment survive
and reproduce at a
greater rate than less
organisms in the same
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
 Directional selection: selects for one
extreme such as tall giraffes. The population
average moves to that direction as those
individuals reproduce more often.
 Disruptive selection is a process that splits
a population into two groups. This is often the
cause of speciation. It selects against the
middle, like polar bears and Canadian Brown
What is Sexual Selection?
 Sexual selection: Organisms choose a mate
based on a behavior or appearance but that
behavior is often lethal to the organism.
Think: peacocks and peahens.
Analyze how reproductive isolating mechanisms
affect populations.
 Postzygotic isolation
occurs when an
organism that has
been produced can not
Prezygotic isolation
reproduction by
making fertilization
unlikely. Often
mating behavior.
Eastern meadowlark and Western meadowlark
What is Adaptive Radiation?
 Can occur in a relatively short time when one species
gives rise to
many different
species in
response to the
creation of new
habitat or some
other ecological
 Follows large-scale extinction events
 Geographic Isolation:
 A population is
separated from
another population by
the landscape
Chromosome number
becomes irrelevant in
mating since the two
populations do not
come in contact with
each other
 The relationship between two species might
be so close that the evolution of one species
affects the evolution of the other species.
 Mutualism
Convergent Evolution
 Unrelated species
evolve similar traits
even though they
live in different parts
of the world.
Chapter 15
 Large populations with lots of genetic variation! They
are least likely to change
 Small populations where many mutations occur are
most likely to evolve.