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Evolution
10.1 – Early Ideas About Evolution
 Key Concept
 There were theories of biological and geologic change
before Darwin.
Early scientists proposed
ideas about evolution.
 Evolution is the biological change over time.
 A species is a group of organisms that can reproduce
and have fertile offspring.
Theories of geologic change set the
stage for Darwin’s theory.
 There were three theories of geologic change:
 Catastrophism: natural disasters such as floods and volcanic
eruptions have shaped landforms and caused species to become
extinct.
 Gradualism: changes in landforms resulted from slow changes over
a long period of time
 Uniformitarianism: the geologic processes that shape Earth are
uniform through time
 Uniformitarianism is the prevailing theory of geologic change.
10.2 – Darwin’s Observations
 Key Concept:
 Darwin’s voyage provided insight on evolution.
Charles Darwin
 Known as the father of evolution
 Traveled around the world on the HMS Beagle
 Observed geological phenomena and adaptations in
species
 Published findings in his book Origin of Species
 1800’s

Darwin observed differences
among island species.
Variation: difference in a physical trait

Galapagos tortoises that live in areas with tall plants have long necks and
long legs

Galapagos tortoises that live in areas with low plants have short necks and
short legs

Galapagos finches (Darwin’s finches) that live in areas with hard-shelled nuts
have strong beaks

Galapagos finches that live in areas with insects/fruit have long, thin beaks
 Adaptation: feature that allows an organism to better
survive in its environment
 Species are able to adapt to
their environment
 Adaptations can lead to
genetic change in a population
Darwin observed fossil and geologic
evidence supporting an ancient Earth.
 Darwin found fossils of extinct animals that resemble
modern animals
 Darwin found fossil shells high up in the Andes
mountains
Glyptodon
Modern armadillo
 He saw land move from underwater to above sea level
during an earthquake
 Darwin extended his observations to the evolution of
organisms
10.3 – Theory of Natural
Selection
 Key Concept:
 Darwin proposed natural selection as a mechanism for
evolution.
Several key insights led to Darwin’s
idea for natural selection.
 Natural selection: mechanism by which individuals
that have inherited beneficial adaptations produce
more offspring on average than do other individuals
 Heritability: ability of a trait to be passed down
 There is a struggle for survival due to overpopulation
and limited resources
 Darwin proposed that adaptations arose over many
generations
Natural selection explains how
evolution can occur.
 Variation: heritable differences that exist in every population are the
basis for natural selection
 Overproduction: Having many offspring increases the chance of
survival but also results in competition for resources
 Adaptation: certain variation that allows an individual to survive
better than other individuals it competes against
 Descent with modification: Heritability of adaptations. More
individuals will have the trait in every following generation, as long
as the environmental conditions remain beneficial for the trait
 Fitness: ability to survive and reproduce
Natural selection acts on
existing variation.
 Natural selection can act only on traits that already
exist.
 Structures take on new functions in addition to their
original function.
five digits
wrist bone
10.4 – Evidence of Evolution
 Key Concept:
 Evidence of common ancestry among species comes
from many sources.
Fossils & the Fossil Record
 Shows how species changed their form/shape over time
 Ways of dating fossils:
 Relative dating: estimates the age of fossils by comparing fossil to
others in the same layer of rock
 Pro: can be used if there is no other way to tell the age of the fossil
 Con: layers of rock can be shifted by natural events (earthquakes,
mudslides, etc.) and this can mess up estimate
 Radiometric dating: uses the decay of radioactive isotopes (carbon-
14 changes into carbon-12)
 Pro: can give an accurate age
 Con: can’t give an age for really old fossils (if all isotopes have decayed)
Biogeography
 Island species most closely resemble nearest mainland
species
 Populations can show variation from one island to
another
 Example: rabbit fur vs. climate
Embryology
 Similar embryos,
diverse organisms
 Identical larvae,
diverse adult body forms
 Gill slits and “tails”
as embryos
Larva
Adult crab
Adult barnacle
Homologous Structures
 Similar in structure, different in function
 Evidence of a common ancestor
 Example: bones in the forelimbs of different animals (humans,
cat legs, whale fins, bat wings)
Vestigial Organs/Structures
 Remnants of organs or structures that had a function in an
early ancestor but have lost their function over time
 Evidence of a common ancestor
 Examples:
 Human appendix & tailbone
 Wings on flightless birds (ostrich, penguins)
 Hindlimbs on whales, snakes
Molecular Biology
 Common genetic code (A, T, C, & G)
 Similarities in DNA, proteins, genes,
& gene products
 Two closely related organisms will
have similar DNA sequences & proteins
 DNA fingerprints will also be very close if the species
are closely related
11.1 – Genetic Variation
Within Populations
 Key Concept:
 A population shares a common gene pool.
Genetic variation in a population increases the
chance that some individuals will survive.
 Genetic variation leads to phenotypic variation
 Necessary for natural selection
 Genetic variation is stored in a population’s gene pool
 Made up of all the alleles in a population
 Allele combinations form when organisms have offspring
Genetic variation comes from
several sources.
 Mutations
 Can form a new allele
 Passed to offspring
if in a gamete
 Recombination
 Usually occurs during meiosis
 Parents’ alleles rearranged during gamete formation
11.2 – Natural Selection
in Populations
 Key Concept:
 Populations, not individuals, evolve.
Microevolution
 Evolution within a population
 Observable change in allele frequencies
 Can result from natural selection
 Types:
 Directional selection
 Stabilizing selection
 Disruptive selection
Directional Selection
 Favors phenotypes at one extreme
Stabilizing Selection
 Favors the intermediate phenotype
Disruptive Selection
 Favors both extreme phenotypes
11.3 – Other mechanisms of
Evolution
 Key Concept:
 Natural selection is not the only mechanism through
which populations evolve.
Gene Flow
 Movement of alleles between populations
 Occurs when individuals
join new populations
and reproduce
 Keeps neighboring
populations similar
 Low gene flow
increases the chance
that two populations
will evolve into different
species
bald eagle migration
Genetic Drift
 Change in allele frequencies due to chance
 Causes a loss of genetic diversity
 Common in small populations
 Bottleneck Effect is genetic
drift after a bottleneck event
 Occurs when an event
drastically reduces population size
 Founder Effect is genetic drift that occurs after the
start of a new population
 Occurs when a few individuals start a new population
Sexual selection occurs when certain
traits increase mating success.
 Sexual selection
 Occurs due to higher cost of reproduction for females
 Males produce sperm continuously
 Females are more limited in potential offspring each cycle
 Two types:
 Intrasexual selection: competition among males
 Intersexual selection: males display certain traits to females
11.6 – Patterns in Evolution
 Key Concept:
 Evolution occurs in patterns.
Species can become extinct.
 Extinction: elimination of a species from Earth
 Background extinction
 Mass extinction
Background Extinction
 Occur randomly, but at a low rate
 Usually affect only a few species
in a small area
 Can by caused by local changes
in the environment
Mass Extinction
 Rare, but very intense
 Can operate at a global level
 Caused by a catastrophic event such as an ice age
 At least 5 mass extinctions in the last 600 million years
Extinction
 Species go extinct because they lack the variation
needed to adapt