1. Descent with Modification: A Darwinian View of Life
Chapter 22
Descent with Modification: A Darwinian View of Life

2. Concept 22.3: Evolution is supported by an overwhelming amount of scientific evidence
New discoveries continue to fill the gaps identified by Darwin in The Origin of Species

3. 1. Direct Observations of Evolutionary Change
Two examples provide evidence for natural selection: the effect of differential predation on guppy populations and the evolution of drug-resistant HIV

4. Predation and Coloration in Guppies : Scientific Inquiry
John Endler has studied the effects of predators on wild guppy populations
Brightly colored males are more attractive to females
However, brightly colored males are more vulnerable to predation
Guppy populations in pools with fewer predators had more brightly colored males

5. Area of colored spots (mm2) colored spots Number of
Fig
EXPERIMENT
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”
RESULTS 12 12
Figure Can predation result in natural selection for color patterns in guppies? 10 10 8 8
Area of colored
spots (mm2)
colored spots
Number of 6 6 4 4 2 2
Source
population
Transplanted
population
Source
population
Transplanted
population

6. EXPERIMENT Experimental transplant of guppies
Fig a
EXPERIMENT
Predator: Killifish; preys
mainly on juvenile
guppies (which do not
express the color genes)
Experimental
transplant of
guppies
Pools with
killifish,
but no
guppies prior
to transplant
Guppies: Adult males have
brighter colors than those
in “pike-cichlid pools”
Figure Can predation result in natural selection for color patterns in guppies?
Predator: Pike-cichlid; preys mainly on adult guppies
Guppies: Adult males are more drab in color
than those in “killifish pools”

7. Area of colored spots (mm2) colored spots Number of
Fig b
RESULTS 12 12 10 10 8 8
Area of colored
spots (mm2)
colored spots
Number of 6 6 4 4 2 2
Figure Can predation result in natural selection for color patterns in guppies?
Source
population
Transplanted
population
Source
population
Transplanted
population

8. Endler transferred brightly colored guppies (with few predators) to a pool with many predators
As predicted, over time the population became less brightly colored
Endler also transferred drab colored guppies (with many predators) to a pool with few predators
As predicted, over time the population became more brightly colored

9. The Evolution of Drug-Resistant Viruses
The use of drugs to combat particular viruses selects for viruses resistant to these drugs
HOW?
By chance some viruses have mutations that allow them to survive the drug
Other viruses are killed off but the viruses with the mutation survive—and reproduce
In the next generation, more viruses are resistant

10. 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 population

11. 1a. Artificial Selection

12. 2. The Fossil Record
The fossil record provides evidence of the extinction of species, the origin of new groups, and changes within groups over time

13. 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

14. (a) Pakicetus (terrestrial)
Fig
(a) Pakicetus (terrestrial)
(b) Rhodocetus (predominantly aquatic)
Figure The transition to life in the sea
Pelvis and
hind limb
(c) Dorudon (fully aquatic)
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)

15. (a) Pakicetus (terrestrial)
Fig ab
(a) Pakicetus (terrestrial)
Figure The transition to life in the sea
(b) Rhodocetus (predominantly aquatic)

16. (c) Dorudon (fully aquatic)
Fig cd
Pelvis and
hind limb
(c) Dorudon (fully aquatic)
Figure The transition to life in the sea
Pelvis and
hind limb
(d) Balaena
(recent whale ancestor)

17. 3. Homology
Homology is similarity resulting from common ancestry

18. Humerus Radius Ulna Carpals Metacarpals Phalanges Human Cat Whale Bat
Fig
Humerus
Radius
Ulna
Carpals
Metacarpals
Figure Mammalian forelimbs: homologous structures
Phalanges
Human
Cat
Whale
Bat

19. Comparative embryology reveals anatomical homologies not visible in adult organisms

20. Pharyngeal pouches Post-anal tail Chick embryo (LM) Human embryo
Fig
Pharyngeal
pouches
Post-anal
tail
Figure Anatomical similarities in vertebrate embryos
Chick embryo (LM)
Human embryo

22. Vestigial structures are remnants of features that served important functions in the organism’s ancestors

23. Homologies and “Tree Thinking”
The Darwinian concept of an evolutionary tree of life can explain homologies
Evolutionary trees are hypotheses about the relationships among different groups
Evolutionary trees can be made using different types of data, for example, anatomical and DNA sequence data

24. Homologous characteristic
Fig
Branch point
(common ancestor)
Lungfishes 1
Amphibians
Tetrapods 2
Mammals
Tetrapod limbs
Amniotes 3
Lizards
and snakes
Amnion 4
Crocodiles
Figure Tree thinking: information provided in an evolutionary tree
Homologous
characteristic 5
Ostriches
Birds 6
Feathers
Hawks and
other birds

25. 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

26. NORTH AMERICA Sugar glider AUSTRALIA Flying squirrel Fig. 22-20
Figure Convergent evolution
Flying
squirrel

27. 4. 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

28. 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

29. What Is Theoretical About Darwin’s View of Life?
In science, a theory accounts for many observations and data and attempts to explain and integrate a great variety of phenomena
Darwin’s theory of evolution by natural selection integrates diverse areas of biological study and stimulates many new research questions
Ongoing research adds to our understanding of evolution

30. Individuals in a population vary in their heritable characteristics.
Fig. 22-UN1
Observations
Individuals in a population
vary in their heritable
characteristics.
Organisms produce more
offspring than the
environment can support.
Inferences
Individuals that are well suited
to their environment tend to leave
more offspring than other individuals
and
Over time, favorable traits
accumulate in the population.