1) The document discusses evolution and Charles Darwin's theory of natural selection. It provides background on evolution, defines key terms like adaptation and speciation. 2) It examines various evidence that supports evolution, including fossils, homologous and vestigial structures, embryological evidence, biogeography, and molecular biology. 3) The mechanisms of speciation are described, including allopatric, sympatric, and adaptive radiation speciation. Reproductive isolating mechanisms that lead to speciation are also outlined.

1. 1
Chapter 18:
Evolution and The
Origin of Species

2. 18.1: Introduction
• What does evolution mean to you?
• All species of living organisms, from
bacteria to baboons to blueberries,
evolved at some point from a different
species.
• Evolution is an ongoing process for
living things.
2

3. Figure 18.1
• All organisms are products of evolution adapted to their environment. (a) Saguaro (Carnegiea gigantea) can soak up 750 liters of water
in a single rain storm, enabling these cacti to survive the dry conditions of the Sonora desert in Mexico and the Southwestern United
States. (b) The Andean semiaquatic lizard (Potamites montanicola) discovered in Peru in 2010 lives between 1,570 to 2,100 meters in
elevation, and, unlike most lizards, is nocturnal and swims. Scientists still do no know how these cold-blood animals are able to move
in the cold (10 to 15°C) temperatures of the Andean night. (credit a: modification of work by Gentry George, U.S. Fish and Wildlife
Service; credit b: modification of work by Germán Chávez and Diego Vásquez, ZooKeys)

4. 4
• Evolution
– Definition: How an entity changes through time
“Through time, species accumulate differences;
as a result, descendants differ from their
ancestors. In this way, new species arise
from existing ones.”
– Charles Darwin
Genetic Variation and Evolution

5. Figure 18.2
• Darwin observed that beak shape varies among finch species. He postulated that
the beak of an ancestral species had adapted over time to equip the finches to
acquire different food sources.

6. • Darwin was not the first to propose a
theory of evolution, but what WAS new
was the mechanism he proposed.
– Darwin proposed “natural selection” as the
mechanism of evolution.
– Alfred Wallace studied wildlife in South
American and Asia and decided to seek
Darwin’s help in publishing his own ideas
on evolution.
6

7. Figure 18.3
• Both (a) Charles Darwin and (b) Alfred Wallace wrote scientific papers on natural
selection that were presented together before the Linnean Society in 1858.

8. 8
Reproduction
Reproduction
Individuals pass on their traits to next generation.
Darwin’s theory:
natural selection
leads to
evolutionary
change.
Over many generations, longer-necked individuals are more
successful, perhaps because they can feed on taller trees, and
pass the long-neck trait on to their offspring.
Natural
selection
Some individuals born happen to have longer necks due to
genetic differences.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

9. Figure 18.4
• A field biologist tranquilizes a polar bear for study. (credit: Karen Rhode)

10. 10
Genetic variation
– Definition: Differences in alleles of genes found within
individuals in a population
– Variation must be already present before natural
selection can occur
How does genetic variation arise in a population?
– New alleles arise from existing alleles by mutation
– Sexual reproduction creates new combinations of
alleles in organisms
• recombination of alleles in meiosis and fusion of 2 unique
gametes in fertilization
Processes and Patterns of
Evolution

11. Processes and Patterns of
Evolution
• Adaptation: a heritable trait that helps the
survival and reproduction of an organism in
its present environment.
– How have we as humans adapted to our
environments?
• Divergent evolution: when two species evolve
in diverse directions from a common
ancestor. (i.e. Figure 18.5).
• Convergent evolution: when similar traits
evolve independently in a species that do not
share a common ancestor (i.e. flying).
11

12. Figure 18.5
• Flowering plants evolved from a common ancestor. Notice that the (a) dense
blazing star (Liatrus spicata) and the (b) purple coneflower (Echinacea purpurea)
vary in appearance, yet both share a similar basic morphology. (credit a:
modification of work by Drew Avery; credit b: modification of work by Cory Zanker)

13. 13
5 agents of evolutionary change

14. Selection
–Some individuals leave behind more
progeny than others, and the rate at
which they do so is affected by
phenotype, which is affected by
genotype
–Two types of selection
• Artificial selection
• Natural selection - the main driver of
adaptive evolution!
14

15. Evidence of Evolution
• Fossils
• Anatomy and Embryology
• Biogeography
• Molecular Biology
15

16. Fossil Evidence of Evolution
• Fossils are the preserved remains of once-
living organisms
– The age of fossils can be estimated
• Rock fossils are created when three events
occur
– Organism buried in sediment
– Calcium in bone or other hard tissue
mineralizes
– Surrounding sediment hardens to form rock
• Process of fossilization is rare event 16

17. Figure 18.6
• In this (a) display, fossil hominids are arranged from oldest (bottom) to newest (top).
As hominids evolved, the shape of the skull changed. An artist’s rendition of (b)
extinct species of the genus Equus reveals that these ancient species resembled
the modern horse (Equus ferus) but varied in size.

18. Anatomical Evidence for
Evolution
• Homologous structures
–Structures with different appearances
and functions that all derived from the
same body part in a common
ancestor
–The bones in the forelimb of
mammals are homologous structures
–May have different functions (or the
same), but evolved from the same
ancestral structure 18

19. Figure 18.7
• The similar construction of these appendages indicates that these organisms share
a common ancestor.

20. • Vestigial structures
– Have no apparent function, but resemble
structures their ancestors possessed
• Human post-anal tail
• Hip bones in boa constrictors and whales
• Wings on flightless birds; leaves on cacti 20

21. • Early embryonic development
–Strongest anatomical evidence
supporting evolution comes from
comparisons of how organisms
develop
–Embryos of different types of
vertebrates, for example, often are
similar early on, but become more
different as they develop
–Early vertebrate embryos possess
pharyngeal pouches that develop into
• In humans: glands and ducts
• In fish: gill slits
21

22. 22

23. 23
Turtle
Chick
Mouse Human

24. Convergent evolution
• Organisms that share similar
environments and similarities occur
because of similar selective pressures
—benefits of not being seen by
predators.
–For example: Artic fox and Ptarmigan
both live in the Artic region and have
been selected for white phenotypes.
24

25. Figure 18.8
• The white winter coat of the (a) arctic fox and the (b) ptarmigan’s plumage are
adaptations to their environments. (credit a: modification of work by Keith
Morehouse)

26. Biogeography
• Study of the geographic distribution of
species
• Reveals that different geographical
areas sometimes exhibit groups of
plants and animals of strikingly similar
appearance, even though the
organisms may be only distantly related
• Natural selection appears to have
favored parallel evolutionary
adaptations in similar environments
26

27. Molecular Biology
• Evidence of common ancestor for all life
is found in DNA.
• DNA sequences have also shed light on
some of the mechanisms of evolution.
–i.e. new functions of proteins.
27

28. Misconceptions of Evolution
• Evolutionary theory is nearly universally
accepted by biologists
• Source of controversy for some in the general
public (Often caused by lack of knowledge
regarding what the theory of evolution states,
and what it does not state!)
• You need to visit this site (hyperlink below)
and read it all the way through to the “Quick
Quiz.” Some basic questions about this will
appear on Exam 1.
• http://www.evolution.berkeley.edu/evosite/misconce
28

29. 18.2: Formation of New
Species
• A species is composed of populations
whose members mate with each other and
produce fertile offspring
– One specie is distinguished from another
when, in nature, it is not possible for
matings between individuals from each
species to produce fertile offspring.
• The more DNA they have in common
29

30. Figure 18.9
• The (a) poodle and (b) cocker spaniel can reproduce to produce a breed known as
(c) the cockapoo. (credit a: modification of work by Sally Eller, Tom Reese; credit b:
modification of work by Jeremy McWilliams; credit c: modification of work by
Kathleen Conklin)

31. Hybrids
• Hybrids are sterile
–Abnormal sex organs
–Failure to form gametes
(i.e. African fish eagle and bald eagle
appear similar in appearance but if
humans were to artificially intervene
these two species, the offspring
would be a hybrid).
31

32. Figure 18.10
• The (a) African fish eagle is similar in appearance to the (b) bald eagle, but the two
birds are members of different species. (credit a: modification of work by Nigel
Wedge; credit b: modification of work by U.S. Fish and Wildlife Service)

33. Speciation
• Speciation is a 2-part process
–The formation of two species form
one original species.
• They must evolve in a way that it
becomes impossible for individuals from
the two new populations to interbreed.
–Darwin envisioned this process as a
branching event (see Figure 18.11a).
33

34. Figure 18.11
• The only illustration in Darwin's On the Origin of Species is (a) a diagram showing
speciation events leading to biological diversity. The diagram shows similarities to
phylogenetic charts that are drawn today to illustrate the relationships of species. (b)
Modern elephants evolved from the Palaeomastodon, a species that lived in Egypt 35–
50 million years ago.

35. Allopatric speciation
• Geographically separated, or allopatric,
populations appear much more likely to
have evolved substantial differences
leading to speciation
• The northern spotted owl and the
Mexican spotted owl
–Isolated populations are strikingly
different from each other
35

36. Figure 18.12
• The northern spotted owl and the
Mexican spotted owl inhabit
geographically separate locations
with different climates and
ecosystems. The owl is an example
of incipient speciation. (credit
“northern spotted owl”: modification
of work by John and Karen
Hollingsworth; credit “Mexican
spotted owl”: modification of work by
Bill Radke)

37. Adaptive radiations
• Closely related species that have
recently evolved from a common
ancestor (founder species) by adapting
to different parts of the environment
• Occurs
–In an environment with few other
species and many resources
• The Hawaiian honeycreeper birds
–Catastrophic event leading to
extinction of other species
37

38. Figure 18.13
• The honeycreeper birds illustrate adaptive radiation. From one original species of
bird, multiple others evolved, each with its own distinctive characteristics.

39. Sympatric speciation
• Can divergence occur if no physical barriers
are in place to separate individuals who
continue to live and reproduce in the same
habitat?
• One species splits into two at a single locality,
without the two new species ever having
been geographically separated
• One type occurs commonly as the result of
polyploidy
– Individuals that have more than two sets of
chromosomes
39

40. • Ways polyploidy occurs:
– Aneuploidy
• Error in chromosome separation and the end cell product
has too many or too few individual chromosomes.
– Autopolyploidy
• Error resulting in two sets of chromosomes
• Error in cell division produces a tetraploid (4n)
– Allopolyploidy
• Two species hybridize
• Resulting offspring have one copy of the
chromosomes of each species
• Infertile: cannot reproduce with either species –
can’t produce gametes (example: mule)
• Can reproduce asexually OR can become
fertile if chromosomes spontaneously doubled
(polyploidy) 40

41. Figure 18.14
• Aneuploidy results when the gametes have too many or too few chromosomes due
to nondisjunction during meiosis. In the example shown here, the resulting offspring
will have 2n+1 or 2n-1 chromosomes

42. Figure 18.15
• Autopolyploidy results when mitosis is not followed by cytokinesis.

43. Figure 18.16
• Alloploidy results when two species mate to produce viable offspring. In the
example shown, a normal gamete from one species fuses with a polyploidy gamete
from another. Two matings are necessary to produce viable offspring.

44. The biological species concept focuses
on the ability to exchange genes.
Members of separate species remain
separate because gene exchange
between the 2 species is somehow
blocked.
–Prezygotic isolating mechanisms
• Mechanisms that prevent formation of a
zygote
–Postzygotic isolating mechanisms
• Mechanisms that prevent development
into a fertile adult 44

45. • Reproductive isolating mechanisms
–Prezygotic isolating mechanisms
• Habitat isolation-species are moved to a
new habitat and rarely encounter one
another
• Behavioral isolation-species differ in
mating rituals
• Temporal isolation-differences in
breeding schedules
• Gamete barrier-prevent fertilization from
taking place
–Postzygotic isolating mechanisms
• Hybrid inviability or infertility 45

46. Figure 18.17
• These two related frog species exhibit temporal reproductive isolation. (a) Rana
aurora breeds earlier in the year than (b) Rana boylii. (credit a: modification of work
by Mark R. Jennings, USFWS; credit b: modification of work by Alessandro
Catenazzi)

47. Figure 18.18
• Speciation can occur when two populations occupy different habitats. The habitats need
not be far apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil, and the
cricket (b) Gryllus firmus prefers loamy soil. The two species can live in close proximity,
but because of their different soil preferences, they became genetically isolated.

48. Figure 18.19
• The shape of the male reproductive organ varies among male damselfly species,
and is only compatible with the female of that species. Reproductive organ
incompatibility keeps the species reproductively isolated.

49. Figure 18.20
• Some flowers have evolved to attract certain pollinators. The (a) wide foxglove
flower is adapted for pollination by bees, while the (b) long, tube-shaped trumpet
creeper flower is adapted for pollination by humming birds.

50. Figure 18.21
• Cichlid fish from Lake Apoyeque, Nicaragua, show evidence of sympatric speciation.
Lake Apoyeque, a crater lake, is 1800 years old, but genetic evidence indicates that the
lake was populated only 100 years ago by a single population of cichlid fish.
Nevertheless, two populations with distinct morphologies and diets now exist in the lake,
and scientists believe these populations may be in an early stage of speciation.

51. 18.3: Reconnection and Rates
of Speciation
• Two species may recombine or even
continue interacting indefinitely.
–Hybrid zone- an area where two
closely related species continue to
interact and reproduce, forming
hybrids.
51

52. Figure 18.22
• After speciation has occurred, the two separate but closely related species may
continue to produce offspring in an area called the hybrid zone. Reinforcement,
fusion, or stability may result, depending on reproductive barriers and the relative
fitness of the hybrids.

53. 53
The Pace of Evolution
• Gradualism
–Accumulation of small changes
–Standard view for a long time
• Punctuated equilibrium
–Niles Eldredge and Stephen Jay Gould
coined the term punctuated equilibrium
–Long periods of stasis followed by rapid
change
• Gradualism and punctuated equilibrium are
two ends of a continuum

54. Figure 18.23
• In (a) gradual speciation, species diverge at a slow, steady pace as traits change
incrementally. In (b) punctuated equilibrium, species diverge quickly and then
remain unchanged for long periods of time.