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Natural Selection and the
Evidence for Evolution
THE THEORY OF
EVOLUTION
Section Objectives:

Summarize Darwin’s theory of
natural selection.

Explain how the structural and
physiological adaptations of
organisms relate to natural
selection.

Distinguish among the types of
evidence for evolution.
Charles Darwin and Natural Selection

The modern theory of evolution is the
fundamental concept in biology.

Recall that evolution is the change in
populations over time.
Charles Darwin and Natural
Selection
 Evolution
 Change in populations over time
 Charles Darwin
 H.M.S. Beagle
 Naturalist – study nature
 Galapagos Islands
 First started to realize that organisms
change over time
Darwin on HMS Beagle
 Worked on his ideas for over 2 decades
 Used Malthus’ idea that the human
population would grow too large for the
food supply
 Must struggle to compete for supplies
 Pigeon-breeding
 Artificial selection – similar to nat.selec.
 Breeding organisms w/ specific traits in
order to produce offspring w/ desired traits
Natural Selection
 Organisms w/ favorable variations
survive, reproduce, and pass their
variations to the next generation
 Alfred Wallace had similar ideas
 Advent of genetics further supports
evolution
Adaptations:
Evolution
Evidence for
 Adaptations develop over time
 Thorns or no thorns
 Distinctive coloring or plain
Ex: mole-rats
 Those that live underground are
blind have large teeth and claws
 Mimicry
Enables one species to
resemble another species
 Harmless species mimics a
harmful species
 Ex: harmless fly and wasp Fig.
15.4
 Camouflage
 Blend w/ surroundings Fig. 15.4C
 Most structural adaptations take
millions of years to develop
 Physiological adaptations can be
quicker
 Ex: Antibiotic resistant bacteria
 Penicillin no longer kills as many bacteria
as it once did
 Also observed in insects and weeds
(insecticides and pesticides)
Physiological adaptations can develop rapidly
The bacteria in a
population vary in
their ability to resist
antibiotics.

When the population is
exposed to an antibiotic,
only the resistant bacteria
survive.
The resistant bacteria
live and produce more
resistant bacteria.
Today, penicillin no longer affects as many
species of bacteria because some species
have evolved physiological adaptations to
prevent being killed by penicillin.
Other Evidence of Evolution
 Fossils
 Provide a record of early life and
evolutionary history
 Ex: ancestors of whales were probably landdwelling dog-like animals
 Record is incomplete
 Like a puzzle -> still understand the picture
 Table 15.1 evolution of camel
Camel Evolution
Age
Organism
Skull and
teeth
Limb
bones
Paleocene
65 million
years ago
Eocene
54 million
years ago
Oligocene
33 million
years ago
Miocene
23 million
years ago
Present
Other
Evidence
of
Evolution,
 Anatomy
cont’d
 Homologous structures
 Structural features w/ a common evolutionary
origin
 Ex: Fig. 15.6 forelimbs of crocodile, whale, and
bird
 Analogous structures
 Structures that do not have a common
evolutionary origin , but are similar in function
 Ex: bird wing and insect wing
Crocodile
forelimb
Whale
forelimb
Bird
wing
Analogous Structures

For example, insect and bird wings probably evolved
separately when their different ancestors adapted
independently to similar ways of life.
 Vestigial Structures
 Structure in a present day
organisms that no longer serves
its original purpose
 Ex: appendix, eyes of blind
mole-rats, pelvic bones of
whales
Other Evidence of Evolution,
cont’d
 Embryology
 Most embryos of different species
look similar to each other
 Fig. 15.9
Embryology
Pharyngeal
pouches
Pharyngeal
pouches
Tail
Fish
Tail
Reptile
Bird
Mammal
Other Evidence of Evolution,
cont’d
 Biochemistry
 Nearly all organisms share DNA, ATP,
and many enzymes (cytochrome c
found from bacteria to bison to
humans)
 Organisms that are more closely
related have more similar cytochrome
c Fig. 15.10

Organisms that are biochemically similar have
fewer differences in their amino acid
sequences.
Biochemical Similarities of Organisms
Comparison of Organisms
Percent Substitutions
of Amino Acids in
Cytochrome c Residues
Two orders of mammals
5 and 10
Birds vs. mammals
8-12
Amphibians vs. birds
14-18
Fish vs. land vertebrates
18-22
Insects vs. vertebrates
27-34
Algae vs. animals
57
Question 1
_______ is considered to be the fundamental
concept of biology.
A. genetics
B. the modern theory of evolution
C. artificial selection
D. structural adaptation
The answer is B.
Question 2
Breeding organisms with specific traits in order to
produce offspring with identical traits is called
_______.
A.
B.
C.
D.
natural selection
adaptation
mutation
artificial selection
The answer is D.
Question 3
What is the difference between artificial selection
and natural selection?
Answer
Artificial selection is the intentional breeding of organisms
with specific traits in order to produce offspring with
identical traits. Natural selection occurs when organisms
with favorable variations of traits survive in nature,
reproduce, and pass these favorable variations to
offspring.
Question 4
Mimicry and camouflage are NOT examples of
_________.
A.
B.
C.
D.
adaptation
natural selection
evolution
artificial selection
The answer is D.
Question 5
How does mimicry differ from camouflage?
Answer
Mimicry is an adaptation that allows one species to
resemble another species. Camouflage is an
adaptation that allows one species to resemble its
surroundings.
Mechanisms for Evolution
THE THEORY OF
EVOLUTION
Populations Genetics and
Evolution
 Populations evolve, not individuals
 Natural selection acts on the range of
phenotypes
 Gene pool
 All of the alleles in a pop
 Allelic frequency
 percentage of any specific allele in the gene
pool
 Calculating Allelic Frequency
 Same way a baseball player calculates batting average
 Add up all of the alleles/by total # alleles
 Genetic Equilibrium
 Frequency of alleles remains the same over generations
Changes in Genetic
Equilibrium
 Any factor that affects the genes in a
gene pool can change allelic frequencies
= disruption in equilibrium = evolution
 Mutations – can be harmful or beneficial
 Radiation
 Genetic drift
 The alteration of allelic frequencies by chance
events
 Can greatly affect small populations
 Only genes available to pass on to offspring
 Found in humans that have become isolated
 Amish population in Lancaster County,
Pennsylvania
 Pop = 12,000
 One of the original founders had the recessive
allele for short arms and legs and extra fingers
and toes. Fig. 15.12
 Gene Flow
Migration of individuals into
and out of a populations
 Leave a pop = loss of genes in
gene pool
 Enter a pop = gain genes in gene
pool
Natural Selection acts on
Variations
 Traits have variations
 Eye color, height, skin color
 Some variations increase or
decrease an organism’s chance
of survival in an environment
 3 types of natural selection that act
on variation
1. Stabilizing Selection
 Favors average individuals in a pop
 Ex: spider size
 Too big = predators can find easily
 Too small = can’t get to food
 Average = more likely to survive,
reproduce and pass on genes
2. Directional selection
 Favors one of the extreme variations
of a trait
 Ex:
woodpecker beak size
 Only those w/ long beaks can reach
the bugs that live deep in the tree
 Long beaks would be an advantage
over short beaks or average size
beaks
3. Disruptive Selection
 Individuals w/ either extreme of a trait’s
variation are selected.
 Ex: marine limpets
 Color from white, to tan, to dark brown
 Live attached to rocks which are light and
dark
 Both the white and dark brown are
camouflaged
Disruptive Selection
The Evolution of
 Speciation
Species
 Occurs when members of similar
populations no longer interbreed to
produce fertile offspring
 Creates new species
 Geographic Isolation
 Physical barrier divides a population
 Examples?
 Reproductive Isolation
 Occurs when formerly interbreeding
organisms can no longer mate and
produce fertile offspring
 Genes are so different that fertilization
or the production of a fertile offspring
does not occur
 Mating times differ
 Polyploidy
 Individual or species contains a
multiple of the normal set of
c’somes
 Caused by mistakes during cell
division
 May result in immediate
reproductive isolation
 Speciation Rates
 Rate at which new species arise
 Gradualism
 Gradual change of adaptations
 Ex: sea lilies
 Punctuated Equilibrium
 Occurs quickly, in rapid bursts
 Long periods of genetic equilibrium in
between
Patterns of
Evolution
 Occur
throughout the world
 Support evolution
 Divergent Evolution
 Where species become different from
each other
 Based on needs of particular
environment
 Adaptive Radiation, example
An ancestral species evolves
into an array of species to fit a
number of diverse habitats
Ex: Darwin’s finches,
honeycreepers
Adaptive Radiation
 Convergent Evolution
 Distantly related organisms
evolve similar traits
 Share similar environmental
pressures
 Ex: shark, penguin, dolphin
 Fish, bird, mammal
 Why do they look similar?
Question 1
Which type of natural selection does NOT
favor the evolution of new species?
A. divergent
B. disruptive
C. stabilizing
D. directional
Question 2
Which type of natural selection does NOT
favor the evolution of new species?
A. divergent
B. disruptive
C. stabilizing
D. directional
Question 3
Why are the Galapagos Islands rich in unique
species of organisms?
A. The islands are an area exhibiting an
abnormal number of mutations.
B. The islands are geographically isolated.
C. The island species have been subjected
to increased gene flow.
D. The island species have been subjected
to stabilizing selection.