Descent with
Modification
and
Natural Selection
Darwin’s Inspirations
Geology
Earth’s current features are the result of billions of years
of gradual change
Darwin’s Inspirations
Fossil Record
Extant species are not the only ones to have existed on Earth
Darwin’s Inspirations
Fossil Record
Some fossils
indicate missing
links between
extant groups
Darwin’s Inspirations
Morphology
 Homologous
structures’ underlying similarities
indicate common ancestry
 Analogous structures have a similar function but are
likely coincidental
 Vestigial structures are remnants of ancestors that
have no apparent use in a particular current
organism.
Homologous Structures
Analogous Structures
Vestigial Structures
Beyond Darwin
 Embryological

Homologies
Similarity in early development indicates common
ancestry
Sponge
Jellyfish
Clam
Insect
Human
Beyond Darwin
Molecular Homologies
Molecular
comparisons indicate relations
(closer match = closer relation)
 DNA
Hybridization
A
B
A+B
Hybridization Data
85%
75%
60%
98.8%
96.9%
93%
Protein Homologies
A
B
C
Chickens
# of Cytochrome C amino acids
that differ from human Cytochrome C
18
Chimpanzees
0
Dogs
13
Rattlesnakes
20
Rhesus monkeys
1
Yeasts
56
Organism
Mechanisms of Change
How did giraffes get
such long necks?
Natural Selection
Environment determines success of traits
1.
Overproduction of offspring and scarcity of resources
Natural Selection
Environment determines success of traits
2. Genetic variation within populations
Natural Selection
Environment determines success of traits
3. Organisms with adaptations will survive and
reproduce in greater numbers.
Natural Selection
Environment determines success of traits
4. Subsequent generations will contain more orgs with these
adaptations.
Adaptation
Inherited trait
 Increases chance of survival/reproduction
 Environment-specific

Natural Selection
The environment
determines success of traits
1.Overproduction
2.Genetic variation
3.Struggle for survival
4.Differential reproduction
Natural Selection
Modes of Selection
Stabilizing
 Environment is
stable
 Initially adaptive form
becoming more
numerous from many
generations of natural
selection
MOST
ADAPTIVE
FORM
RANGE OF PHENOTYPES
Modes of Selection
Directional
 Environment is
changing (or has
changed)
 Formerly adaptive form
is no longer adaptive
 Pop evolves toward
newly adaptive form
MOST
ADAPTIVE
FORM
RANGE OF PHENOTYPES
Modes of Selection
Disruptive
(Diversifying)
 Intermediate form
is
undesirable and
decreases in #
RANGE OF PHENOTYPES
MOST
ADAPTIVE
FORMS
MOST
ADAPTIVE
FORMS