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Chapter 22
Descent with Modification (Evolution)
• What Evolution?
Change over time in the genetic composition of
populations
• What is Natural Selection?
Differential success in reproduction
Ch. 22 - Key Concepts
• 22.1. Darwin challenged traditional views of a
young Earth inhabited by unchanging species
• 22.2. Descent with modification by natural
selection explains the adaptations of organisms
and the diversity of life.
• 22.3. Evolution is supported by an
overwhelming amount of scientific evidence
Evolution of insecticide resistance in insect populations
22.9. Artificial selection: diverse vegetables derived from wild mustard
Evolution of drug resistance in HIV
Patients treated with anti-HIV drug 3TC. Percentage of drug-resistant
viruses increases and reaches 100% within a few weeks in all cases.
Inquiry 22.13. Can a change in a population’s food source result
in evolution by natural selection? (Carroll and Boyd, 1992)
See book page 461.
22.2. The context of Darwin’s ideas
Punctuated equilibrium
Trilobite, animals that lived in the seas hundreds of millions of years ago.
Paleontology: The study of fossils
(relics or impressions of organisms
from the past as mineralized in rock)
22.3. Formation of sedimentary rock and deposition of fossils from
different time periods
Strata of sedimentary rock at the Grand Canyon
22.5 The Voyage of HMS Beagle
Galapagos Islands
South America
22.6 Beak variation in Galápagos finches
Diversification of finches on the Galápagos Islands
Title page from The Origin of Species
22.7. “I think ….” In this 1837
sketch, Darwin envisioned the
branching pattern of evolution
22.8. Descent with
modification:
Evolutionary tree of
elephant family (based
on fossil evidence)
Based on this
phylogenetic tree, when
did the most recent
ancestor shared by
Mammuthus (woolly
mammoths), Asian
elephants and African
elephants live?
22.17. Tree thinking: Information provided in an evolutionary tree.
Evolutionary tree for tetrapods and their closest living relatives.
• Each branch point represents the common ancestor of the lineage beginning there and to the right of it
• A hatch mark represents a homologous characteristic shared by all the groups to the right of the mark
• If present, what could a horizontal or a vertical axis represent?
Logic of Darwin's theory of natural selection
• OBSERVATION 1: Species
can produce more offspring
than the environment can
support (great reproductive
potential).
• OBSERVATION 2: Most
populations are normally stable
in size over longer periods.
• OBSERVATION 3: Resources
are limited.
INFERENCE 1: Production of more individuals than the
environment can support (‘struggle for existence’); a fraction
of the offspring survives each generation.
• OBSERVATION 4:
Individuals in a population
vary both in phenotype and
in genotype.
• OBSERVATION 5: This
variation is heritable
INFERENCE 2: Survival in the struggle for existence is not
random. Those individuals who inherited the most fit
characteristics leave more offspring than less fit individuals.
INFERENCE 3: This produces a gradual change in population,
with favorable characteristics accumulating over the generations.
In evolutionary biology, when we say
that one organism has a greater fitness
than another organism in the same
population, we specifically mean that it
A. leaves more viable offspring than others in that
population.
B. competes for resources more successfully than
others of its species.
C. utilizes resources more efficiently than other
species occupying similar niches.
D. mates more frequently than others of its species.
E. lives longer than others of its species.
In evolutionary biology, when we say
that one organism has a greater fitness
than another organism in the same
population, we specifically mean that it
A. leaves more viable offspring than others in that
population.
B. competes for resources more successfully than
others of its species.
C. utilizes resources more efficiently than other
species occupying similar niches.
D. mates more frequently than others of its species.
E. lives longer than others of its species.
Chapter 22 – Key Concepts
 Darwin’s findings challenged traditional views of
a young Earth inhabited by unchanging species
that were individually created for a particular
habitat.
 Descent with modification (evolution) by natural
selection explains the adaptations of organisms
and the unity/diversity of life.
 Evolution is supported by an overwhelming
amount of scientific evidence.
22.18. Different geographic regions, different mammalian “brands”
Analogous structures due to convergent evolution.
Trace history in one evolutionary branch: The evolution of fruit
fly (Drosophila) species on the Hawaiian archipelago.
Evidence points a single founder that arrived
several million years ago on the oldest island
of Kauai, whose offspring radiated to more
than 500 species today!
Where would you expect the most species?
A transitional fossil linking past and present
Vestigial organs
22.16. Anatomical similarities in vertebrate embryos. At some
stage in their embryonic development, all vertebrates have a
post-anal tail and pharyngeal (throat) pouches.
22.15. Mammalian forelimbs - Homologous structures
Anatomical signs of descent with modification
Limpets (Mollusca)
are grazers and
live on rocky
shores in or near
the tidal zone.
Shape of the
shell is flat in
turbulent, shallow
water and more
cone-like in quiet,
deeper water.
Example question:
Which of the following observations helped Darwin
shape his concept of descent with modification?
a) Species diversity declines farther from the equator.
b) Fewer species live on islands than on the nearest
continents.
c) Birds can be found on islands located farther from
the mainland than the bird’s maximum non-stop
flight distance.
d) Earthquakes reshape life by causing mass
extinctions.
e) South American temperate plants are more similar
to the tropical plants of South America than to
temperate plants of Europe.
Example question:
Which of the following observations helped Darwin
shape his concept of descent with modification?
a) Species diversity declines farther from the equator.
b) Fewer species live on islands than on the nearest
continents.
c) Birds can be found on islands located farther from
the mainland than the bird’s maximum non-stop
flight distance.
d) Earthquakes reshape life by causing mass
extinctions.
e) South American temperate plants are more similar
to the tropical plants of South America than to
temperate plants of Europe.