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Evolution by
Natural Selection
Change over time
What is evolution
by
natural selection?
Adaptations will spread throughout a population
(species) if:
 It helps an organism GET FOOD
 It helps an organism AVOID PREDATION
 If helps an organism ATTRACT A MATE
How does NATURAL
SELECTION WORK?
 VARIATIONS (don’t help survival, just
provide differences among organisms)
exist in a population
 Ex: Eye color: does it help you…
 GET FOOD?
 AVOID PREDATION?
 ATTRACT A MATE?
How does NATURAL
SELECTION WORK?
 Adaptations: These traits do help
you survive
 Examples: A peacock’s bright colors,
red cardinals, a rattle snake’s rattle,
a turtle’s shell……what else?
NATURAL SELECTION IS A
THEORY
What is a theory?
 A theory is a well supported testable
explanation of phenomena that have
occurred in the natural world
 Some common theories include:
Gravity
Heliocentric theory (earth
revolves around the Sun)
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Galapagos Islands, Ecuador
Darwin’s Observations
 Was fascinated at how WELL
ADAPTED the animals were to the
island
 Noticed that many of the animals
he saw on the island closely
resembled the animals on the main
land but were a BIT DIFFERENT!
Darwin’s Finches
Natural Selection
 The struggle for existence
= survival of the fittest
 Fitness = ability to survive
and reproduce; the result
of adaptations
 Members of the same
species compete for food,
living space, and mates
 Those that are faster or
better camouflaged, etc are
more likely to
survive…sometimes leading
to new traits!
Darwin’s Theory of Natural Selection
1. Overproduction:

more offspring are produced than can survive to maturity
2. Genetic variation:

No 2 organisms are exactly alike (so when competition exists,
some may win!).
3. Struggle to survive:

Organism BEST ADAPTED to the new environmental pressures
will survive to REPRODUCE (and pass on adaptation to
offspring)
4. Differential reproduction:

In time….. (millions of years)….. the traits in a population will
change (and may form new species).
Evidence: The Fossil Record
 Fossils are the
remains of once
living organisms
 Many different
species were
present on this
planet in the past
Comparative sizes of a
human male with aquatic
creatures that existed
between 460 and 255
mya:
a) Ancient sea scorpion
Jaekelopterus rhenaniae
b) Trilobite Isotelus rex
c) Dragonfly Meganeura
monyi
d) Millipede Arthropleura
armata
e) Reconstruction of a fossil
claw from the ancient sea
scorpion (390 mya)
Evidence: The Fossil Record
 Superposition
 If no major disturbance has occurred, a layer of
rock at one place was formed after the layer
below it
 Provides relative age of fossil
 Observe life forms change over time
Evidence: Homologous Structures
Homologous = similar
anatomical structures
in different species
 Ex: Forearms of a
human, cat, whale, and
bat are similar in
structure.
 Indicates that these
organisms shared a
common ancestor.
Evidence: Analogous Structures
 Analogous = anatomical structures that serve a
similar purpose, but are different in structure.
 Ex: The wings of a bat and a bird are not similar in
structure but are both meant for flying.
 Species with analogous structures do NOT
share a common ancestor.
 Just adapted to a similar environment (convergent
evolution).
Evidence: Vestigial Organs
 Vestigial = Organs that are no longer
useful to the organism
 Humans: coccyx, wisdom teeth, eye fold
 Eyes on the naked mole rat
 Were present and used in the past and
have since become useless.
 Not considered vestigial anymore: appendix
Evidence: Embryology
 Vertebrates all have similar structures in the
early developmental stages
 Ex: Gill slits
 Shows the similarities of the different
vertebrates; suggests common ancestor
Evidence: Biochemical Evidence
 By comparing the DNA, RNA, &
Proteins of organisms scientists
can quantify how alike or different
two organisms are.
 There is a high level of similarity
among all organisms.
 Can you believe it??? All life on this
planet is over 90% the same when
viewed at the DNA level