Download Chapter 15 Evolution

15.3 Shaping Evolutionary Theory
Main idea: The theory of evolution continues to
be refined as scientists learn new information.
Objectives:
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Discuss patterns observed in evolution.
Describe factors that influence speciation.
Compare gradualism with punctuated equilibrium.
Review Vocabulary
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Allele: alternative forms of a character trait that can be
inherited.
15.3 Shaping Evolutionary Theory
New Vocabulary
Hardy-Weinberg Principle
Genetic drift
Founder effect
Bottleneck
Punctuated Equilibrium
Sexual selection
Mechanisms of Evolution
Darwin’s theory of natural selection
remains the central theme of
evolution
Scientists of today know that natural
selection is not the only mechanism
of evolution
Evolution occurs at the population
level, with genes as the raw material.
Mechanisms of Evolution
Population Genetics
Hardy-Weinberg Principle states that when
allelic frequencies remain constant, a population
is in genetic equilibrium.

Even though the number of owls doubled, the ratio of
gray to red owls remained the same.
Mechanisms of Evolution
Population Genetics
p2 + 2pq + q2 = 1
This equation allows us to determine the
equilibrium frequency of each genotype in
the population:
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homozygous dominant: p2
Heterozygous: 2pq
homozygous recessive: q2
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Note the sum of these frequencies equals one.
Mechanisms of Evolution
Population Genetics
A population in genetic equilibrium must meet all 5
conditions
1.The population is very large
2.No immigration or emigration
3.Mating is random
4.Mutations do not occur (no new variations)
5.Natural selection does not occur
Mechanisms of Evolution
Genetic Drift
Genetic Drift is a change in the allelic
frequencies in a population that is due to
chance.
In smaller populations, the effects of
genetic drift becomes more pronounced,
and the chance of losing an allele
becomes greater.
Examples: Founder Effect & Bottleneck
Mechanisms of Evolution
Genetic Drift
Founder Effect occurs when a small
sample of a population settles in a location
separated from the rest of the population
Alleles that were
uncommon in the
original population
might be common
in the new
population.
Mechanisms of Evolution
Genetic Drift
Bottleneck occurs
when a population
declines to a very
low number and
then rebounds

Can decrease
genetic variety.
Mechanisms of Evolution
Gene Flow
No new genes enter the population and no
genes leave the population.
A population in genetic equilibrium
experiences no gene flow, however, few
populations are isolated
Random movement of individuals between
populations reduces differences between
populations
Mechanisms of Evolution
Nonrandom mating
Rarely is mating
completely random in a
population.
Usually individuals mate
with individuals in close
proximity.
This promotes inbreeding and could lead to a
change in allelic proportions favoring individuals
that are homozygous for particular traits.
Mechanisms of Evolution
Mutation
A mutation is a random
change in genetic
material.
Occasionally mutations
provide an advantage to
organisms and become
more common in
subsequent generations.
Basis upon which natural
selection works.
Mechanisms of Evolution
Natural Selection
Select the individuals that are best
adapted for survival and reproduction
Stabilizing selection
 eliminate extreme expressions
Birth Weights
Directional selection
 Increases the expression of the
extreme version of a trait.
Peppered Moths
Disruptive selection
 process that splits a population into
two groups.
Lake Erie Water Snakes
Mechanisms of Evolution
Natural Selection
Sexual selection operates in
populations where males and
females differ significantly in
appearance.
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Males are largest and most
colorful in the group
More likely target for predators
yet bodies enhance reproductive
success
Examples: Peacocks & Deer
Reproductive Isolation
Prezygotic Isolation
prevents reproduction
by making fertilization
unlikely.

Eastern and Western
meadowlarks similar
in appearance but
their mating songs
separate them
behaviorally.
Reproductive Isolation
Postzygotic Isolation
Occurs when
fertilization has
occurred but a
hybrid offspring
cannot develop
or reproduce.

A Liger is the offspring of a male lion
and a female tiger. Ligers are sterile.
Speciation
For speciation to occur, a population must
diverge and then be reproductively isolated.
Allopatric speciation
A physical barrier divides one population
into two or more populations.
Speciation
For speciation to occur, a population must
diverge and then be reproductively isolated.
Sympatric speciation
Species evolves into a new species
without a physical barrier

Apple maggot flies
lay eggs on apples or
hawthorns depending
on the type of fruit
the grew up on.
Patterns of Evolution
Adaptive Radiation (aka divergent evolution)
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 opportunity.
Follows
large-scale
extinction
events

Cichlid fish
of Lake
Victoria in
Africa.
Patterns of Evolution
Coevolution
The relationship between two species might be
so close that the evolution of one species affects
the evolution of the other species.
Mutualism – occurs when two species benefit
each other.
• Comet orchids and the moths
that pollinate them
Patterns of Evolution
Coevolution
Coevolutionary arms race – one species can
evolve a parasitic dependency on another
species.
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•
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A plant and an insect that is dependent on the
plant for food
The plant evolves a chemical
defense
The insects evolve the
biochemistry to resist the defense
Response continues to escalate.
Patterns of Evolution
Convergent Evolution
Unrelated species
evolve similar traits
even though they live
in different parts of
the world.
Occurs in
environments that are
geographically far
apart but have similar
ecology and climate.
Patterns of Evolution
Rate of Speciation
Punctuated
equilibrium explains
rapid spurts of
genetic change
causing species to
diverge quickly.
Gradualism: Evolution proceeds in small,
gradual steps.