Chapter 22

12/19/11 Chapter 22
Descent with Modification: A Darwinian View of Life
Fig. 22-2
Linnaeus (classification)
Hutton (gradual geologic change)
Lamarck (species can change)
Malthus (population limits)
Cuvier (fossils, extinction)
Lyell (modern geology)
Darwin (evolution, natural selection)
Wallace (evolution, natural selection)
American Revolution
French Revolution
U.S. Civil War
1795 Hutton proposes his theory of gradualism.
1798 Malthus publishes “Essay on the Principle of Population.”
1809 Lamarck publishes his hypothesis of evolution.
1830 Lyell publishes Principles of Geology.
1831–1836 Darwin travels around the world on HMS Beagle.
1837 Darwin begins his notebooks.
1844 Darwin writes essay on descent with modification.
1858 Wallace sends his hypothesis to Darwin.
1859 The Origin of Species is published.
A Historical Perspective
•  The Greek philosopher Aristotle viewed species as
fixed and arranged them on a scala naturae
•  The Old Testament holds that species were
individually designed by God and therefore perfect
•  Carolus Linnaeus interpreted organismal
adaptations as evidence that the Creator had
designed each species for a specific purpose
•  Linnaeus was the founder of taxonomy, the branch of
biology concerned with classifying organisms
1 12/19/11 Fossils, Paleontology, &
The study of fossils helped to lay the
groundwork for Darwin’s ideas
•  Remains or traces of organisms from the past,
usually found in sedimentary rock, which
appears in layers or strata
•  Paleontology, the study of fossils, was largely
developed by French scientist Georges Cuvier
•  Cuvier advocated catastrophism, speculating
that each boundary between strata represents a
Geologists James Hutton and Charles Lyell
perceived that changes in Earth’s surface
can result from slow continuous actions
still operating today
•  Lyell’s principle of uniformitarianism states
that the mechanisms of change are constant
over time
•  This view strongly influenced Darwin’s
Not the First Theory
•  Lamarck hypothesized that species evolve
•  through use and disuse of body parts
•  the inheritance of acquired characteristics
•  The mechanisms he proposed are unsupported by
•  As a boy and into adulthood,
Charles Darwin had a
consuming interest in nature
•  Darwin first studied medicine
(unsuccessfully), and then
theology at Cambridge
•  After graduating, he took an
unpaid position as naturalist
and companion to Captain
Robert FitzRoy for a 5-year
around the world voyage on
the Beagle
2 12/19/11 Voyage of the Beagle
Collected specimens of South
American plants and animals
Observed adaptations of plants
and animals that inhabited many
diverse environments
Influenced by Lyell’s Principles of
Geology and thought that the
earth was more than 6000 years
Interest in geographic
distribution of species was
kindled by a stop at the
Galápagos Islands near the
equator west of South America
Focus on Adaptation
Darwin perceived adaptation to the environment
and the origin of new species as closely related
From studies made years after Darwin’s voyage, biologists
have concluded that this is indeed what happened to the
Galápagos finches
1844: Darwin wrote an essay on the origin of
species and natural selection
Did not introduce his theory publicly, anticipating an
June 1858: Darwin received a manuscript from
Alfred Russell Wallace
Darwin quickly finished The Origin of Species and
published it the next year
Developed a theory of natural selection similar to
In the Darwinian view, the history of life is like a tree with
branches representing life’s diversity
Darwin’s theory meshed well with the hierarchy of
Artifical Selection, Natural
Selection, and Adapation
•  Darwin noted that humans have modified other
species by selecting and breeding individuals with
desired traits
•  artificial selection
•  Darwin then described four observations of nature
and from these drew two inferences
3 12/19/11 Observation #1:
Members of a population often
vary greatly in their traits
Observation #2:
•  Traits are inherited from parents to offspring
Observation #3:
All species are capable of
producing more offspring than
the environment can support
4 12/19/11 Observation #4:
•  Owing to lack of food or other resources, many of
these offspring do not survive
Two Inferences
•  1: Individuals whose inherited traits give them a
higher probability of surviving and reproducing in a
given environment tend to leave more offspring than
other individuals
•  2: This unequal ability of individuals to survive and
reproduce will lead to the accumulation of favorable
traits in the population over generations
Natural Selection: A
Individuals with certain heritable characteristics
survive and reproduce at a higher rate than other
Natural selection increases the adaptation of
organisms to their environment over time
If an environment changes over time, natural
selection may result in adaptation to these new
conditions and may give rise to new species
•  Note that individuals do not evolve; populations
evolve over time
•  Natural selection can only increase or decrease
heritable traits in a population
•  Adaptations vary with different environments
•  Natural selection does not create new traits, but edits
or selects for traits already present in the population
•  The local environment determines which traits will be
selected for or selected against in any specific
5 12/19/11 Evidence: Direct Observation
John Endler has studied the effects of
predators on wild guppy populations
Brightly colored males are more attractive to
Brightly colored males are more vulnerable to
Guppy populations in pools with fewer
predators had more brightly colored males
Endler transferred brightly colored guppies
(with few predators) to a pool with many
Endler also transferred drab colored
guppies (with many predators) to a pool
with few predators
As predicted, over time the population became
less brightly colored
As predicted, over time the population became
more brightly colored
Evidence: The Fossil Record
•  The fossil record provides
evidence of the extinction
of species, the origin of new
groups, and changes within
groups over time
•  The Darwinian view of life
predicts that evolutionary
transitions should leave
signs in the fossil record
•  Paleontologists have
discovered fossils of many
such transitional forms
Evidence: Homologies
Homology is similarity resulting from
common ancestry
Homologous structures: anatomical
resemblances that represent variations
on a structural theme present in a
common ancestor
Comparative embryology reveals
anatomical homologies not visible in
adult organisms
Vestigial structures: remnants of
features that served important functions
in the organism’s ancestors
Examples of homologies at the
molecular level are genes shared among
organisms inherited from a common
6 12/19/11 Homologies & Tree Thinking
•  The Darwinian concept of
an evolutionary tree of life
can explain homologies
•  hypotheses about the
relationships among
different groups
•  Evolutionary trees can be
made using different types
of data
•  anatomical
•  DNA sequence data
Convergent Evolution
•  Convergent evolution is the
evolution of similar, or
analogous, features in
distantly related groups
•  Analogous traits arise when
groups independently adapt
to similar environments in
similar ways
•  Convergent evolution does
not provide information
about ancestry
Evidence: Biogeography
•  Darwin’s observations of biogeography, the geographic
distribution of species, formed an important part of his
theory of evolution
•  Islands have many endemic species that are often closely
related to species on the nearest mainland or island
•  Earth’s continents were formerly united in a single large
continent called Pangaea, but have since separated by
continental drift
•  An understanding of continent movement and modern
distribution of species allows us to predict when and where
different groups evolved
7 12/19/11 Fig. 22-UN1
Individuals in a population
vary in their heritable
Organisms produce more
offspring than the
environment can support.
Individuals that are well suited
to their environment tend to leave
more offspring than other individuals
Over time, favorable traits
accumulate in the population.
You Should Be Able To:
1.  Describe the contributions to evolutionary theory made by
Linnaeus, Cuvier, Lyell, Lamarck, Malthus, and Wallace
2.  Describe Lamarck’s theories, and explain why they have been
3.  Explain what Darwin meant by “descent with modification”
4.  List and explain Darwin’s four observations and two inferences
5.  Explain why an individual organism cannot evolve
6.  Describe at least four lines of evidence for evolution by natural