15.3 Shaping Evolutionary Theory
Main idea: The theory of evolution continues to
be refined as scientists learn new information.
Objectives:



Discuss patterns observed in evolution.
Describe factors that influence speciation.
Compare gradualism with punctuated equilibrium.
Review Vocabulary

Allele: alternative forms of a character trait that can be
inherited.
15.3 Shaping Evolutionary Theory
New Vocabulary
Hardy-Weinberg Principle
Prezygotic isolating mechanism
Genetic drift
Allopatric speciation
Founder effect
Postzygotic isolating mechanism
Bottleneck
Sympatric speciation
Stabilizing selection
Adaptive radiation
Directional selection
Gradualism
Disruptive selection
Punctuated Equilibrium
Sexual selection
Mechanisms of Evolution
Evolution occurs at the population level, with genes as
the raw material.
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).
Hardy-Weinberg Principle
p2 + 2pq + q2 = 1
This equation allows us to determine the
equilibrium frequency of each genotype in
the population: homozygous dominant
(p2), heterozygous (2pq), and homozygous
recessive (q2). Note the sum of these
frequencies equals one.
Hardy-Weinberg Principle
A population in genetic equilibrium must meet five
conditions – there must be no genetic drift, no gene flow,
mating must be random, mutation, and there must be no
natural selection.
Genetic Drift
A change in the allelic frequencies in a
population that is due to chance and
random mating.
In smaller populations, the effects of
genetic drift become more pronounced,
and the chance of losing an allele
becomes greater.
Examples: Founder Effect & Bottleneck
Chapter
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Evolution
15.3 Shaping Evolutionary Theory
Genetic Drift
 A change in the allelic frequencies in a
population that is due to chance and random
mating
 In smaller populations, the effects of genetic
drift become more pronounced, and the chance
of losing an allele becomes greater.
 Marble Example
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.
Examples: Amish & French Canadians
Chapter
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Evolution
15.3 Shaping Evolutionary Theory
Founder Effect
 The loss of genetic variation that occurs
when a new population is established by a
very small number of individuals from a
larger population
Island 1
Island 2
Mainland
Island 3
Bottleneck
Occurs when a population declines to a very low
number and then rebounds
Evolution
15.3 Shaping Evolutionary Theory
Bottleneck
 a significant percentage of a population or
species is killed or otherwise prevented from
reproducing and can rebound later
 Often caused by a natural disaster
Gene Flow
A population in genetic equilibrium
experiences no gene flow, however, few
populations are isolated
Random movement of individuals between
populations, or migration increases
genetic variation within a population and
reduces differences between populations
Gene Flow
Genes entering or leaving a population through
migration


Emigration Genes LEAVING a population
Immigration INCOMING genes in a population
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
Mutations
A random change in genetic material.
Collective mutations violate genetic
equilibrium.
Occasionally mutations provide an
advantage to organisms and become
more common in subsequent generations.
Basis upon which natural selection works.
What is a gene pool?
Gene pool – combined genetic
info of all members of a
population
 Contains two of more alleles
(genes) for the same trait

Allele frequency – number of
times an allele occurs in a
gene pool compared to the
number of times another allele
occurs (expressed in percents)
Section 16-1
Relative Frequencies of Alleles
allele for
brown fur
allele for
black fur
Sample Population
48%
heterozygous
black
36%
homozygous
brown
16%
homozygous
black
Frequency of Alleles
Natural Selection
Acts to select the individuals
that are best adapted for
survival and reproduction
Stabilizing selection operates
to eliminate extreme
expressions of a trait when
the average expression leads
to higher fitness. (Birth
Weights)
Directional selection makes
an organism more fit.
(Peppered Moths)
Disruptive selection is a
process that splits a
population into two groups.
(African Finches)
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15.3 Shaping Evolutionary Theory
 Stabilizing selection operates to eliminate
extreme expressions of a trait when the
average expression leads to higher fitness.
 Ex. Siberian Husky
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Evolution
15.3 Shaping Evolutionary Theory
 Directional selection makes an organism
more fit.
 Favors the extremes
 Ex. Greyhound Dog
Sexual Selection
Sexual selection operates in populations
where males and females differ
significantly in appearance. (Males are
largest and most colorful in the group).
Qualities of sexual attractiveness appear
to be the opposite of qualities that might
enhance survival. (More likely target for
predators yet bodies enhance reproductive
success).
Examples: Peacocks
Reproductive Isolation
Two types of reproductive isolating
mechanism prevent gene flow among
populations:


Prezygotic isolating mechanisms – operate
before fertilization occurs.
Postzygotic isolating mechanisms – operate
after fertilization has occurred to ensure that
the resulting hybrid remains infertile.
Prezygotic Isolation
prevents reproduction by
making fertilization
unlikely.
prevents genotypes from
entering a population’s
gene pool through
geographic, ecological,
behavioral, or other
differences
Eastern and Western
meadowlarks similar in
appearance but their
songs separate them
behaviorally.
Postzygotic Isolation
Occurs when
fertilization has
occurred but a hybrid
offspring cannot
develop or reproduce.
Prevents offspring
survival or
reproduction.
A Liger is the
offspring of a male
lion and a female
tiger. Ligers are
sterile.
Speciation
A population must diverge and then be
reproductively isolated for speciation to occur.
Two types of speciation:
 Allopatric speciation - A physical barrier
divides one population into two or more
populations. (The Grand Canyon separates
the Abert and Kaibab squirrels).
 Sympatric speciation - A species evolves into
a new species without a physical barrier. The
ancestor species and the new species live
side by side during the speciation process.
(Occurs frequently in plants due to
polyploidy).
Patterns of Evolution
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 opportunity.
Follows large-scale extinction events
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. (Moth and the comet orchid it
pollinates exist in a mutualistic relationship).
Coevolutionary arms race – one species can
evolve a parasitic dependency on another
species.

Example: A plant and an insect pathogen that is
dependent on the plant for food. The plant evolves a
chemical defense; the insect evolves a biochemistry to
resist the defense; response continues to escalate.
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.
Rate of Speciation
Evolution proceeds in small, gradual steps
according to a theory called gradualism.
Punctuated equilibrium explains rapid spurts of
genetic change causing species to diverge
quickly.