1. Chapter 24
The Origin of Species

2. Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
Video: Galápagos Tortoise

3. Animation: Macroevolution
Speciation, the origin of new species, is at the focal point of evolutionary theory
Evolutionary theory must explain how new species originate and how populations evolve
Microevolution consists of adaptations that evolve within a population, confined to one gene pool
Macroevolution refers to evolutionary change above the species level. Ex: how groups of organisms such as mammals evolved.
Animation: Macroevolution

4. Concept 24.1: The biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance”
Biologists compare morphology, physiology, biochemistry, and DNA sequences when grouping organisms

5. The Biological Species Concept
The biological species concept states that a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations
Gene flow between populations holds the phenotype of a population together

6. Reproductive Isolation
The formation of new species hinges on reproductive isolation.
Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring
Such barriers block gene flow and limit the formation of hybrids.
Hybrids are the offspring of crosses between different species
Reproductive isolation can be classified by whether factors act before or after fertilization

7. Prezygotic barriers block fertilization from occurring by:
Impeding different species from attempting to mate
Preventing the successful completion of mating
Hindering fertilization if mating is successful

8. Prezygotic barriers include:
Habitat Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Gametic Isolation

9. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers
Two species of garter snake
occur in the same geographic
areas, but one lives in water
and the other on land.
Terrestrial Thamnophis
Water-dwelling Thamnophis

10. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
The ranges of these
two skunks overlap,
but the eastern skunk
mates in winter and the
western skunk mates
in the summer.
Eastern spotted skunk
(Spilogale putorius)
Western spotted skunk
(Spilogale gracilis)

11. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Female blue-footed boobies will
only choose a mate who performs
the correct courtship display. This
provides for the proper
“mate recognition”
Video: Albatross Courtship Ritual
Video: Giraffe Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual

12. Mechanical isolation: Morphological differences can prevent successful mating
As a result of their shells spiraling
in opposite directions, these two
species of snails cannot physically
mate. Their genital openings,
indicated by arrows, do not line up.
Bradybaena with shells spiraling in opposite directions

13. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
Sperm and eggs from sea urchins
are released into the water where they
fuse and form zygotes. However,
sperm and eggs from the red and purple
sea urchins seen here are unable
to fuse because their surface proteins
cannot bind to each other.
Sea urchins

14. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

15. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
Some salamander subspecies will
occasionally hybridize but the hybrids
do not usually complete their
development, and those that do are
usually very frail
Ensatina hybrid

16. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
The hybrid offspring of
a donkey and a horse is
a mule…it is sterile.

17. Hybrid breakdown: Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile
Both of the healthy hybrid parents
carry recessive alleles that show
up in the offspring.
Hybrid cultivated rice plants with
stunted offspring (center)

18. Recap Summary of Pre/Postzygotic barriers
Fig. 24-4
Recap Summary of Pre/Postzygotic barriers
Prezygotic barriers
Postzygotic barriers
Habitat Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Gametic Isolation
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Individuals of
different
species
Mating
attempt
Viable,
fertile
offspring
Fertilization
(a)
(c)
(e)
(f)
(g)
(h)
(i)
(l)
(d)
(j)
(b)
Figure 24.4 Reproductive barriers
(k)

19. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)
Thus, we have alternative species concepts that are useful in some situations.

20. Other Definitions of Species
Other species concepts emphasize the unity within a species rather than the separateness of different species
The morphological species concept defines a species by structural features
It applies to sexual and asexual species but relies on subjective criteria

21. The ecological species concept views a species in terms of its ecological niche
It applies to sexual and asexual species and emphasizes the role of disruptive selection (organisms adapting to different environmental conditions)
The phylogenetic species concept: defines a species as the smallest group of individuals on a phylogenetic tree
It applies to sexual and asexual species, but it can be difficult to determine the degree of difference required for separate species

22. Speciation can occur in two ways:
Concept 24.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation (with geographic separation)
Sympatric speciation (without geographic separation)
(In both cases: gene flow is interrupted)

23. (a) Allopatric speciation (b) Sympatric speciation
Fig. 24-5
Fig 24.5 Two main modes of speciation
(a) Allopatric speciation
(b) Sympatric speciation

24. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

25. The Process of Allopatric Speciation
The definition of barrier depends on the ability of a population to disperse. Ex: birds can travers the Grand Canyon but squirrels cannot..thus, two species of antelope squirrels have evolved on the rims of the canyon.
Separate populations may evolve independently through mutation, natural selection, and genetic drift

26. Evidence of Allopatric Speciation
Regions with many geographic barriers typically have more species than do regions with fewer barriers

27. Reproductive isolation between populations generally increases as the distance between them increases
Barriers to reproduction are intrinsic; separation itself is not a biological barrier

28. EXPERIMENT RESULTS Initial population Some flies raised on
Fig. 24-9
EXPERIMENT
Initial population
Some flies
raised on
starch medium
Some flies
raised on
maltose medium
Mating experiments
after 40 generations
RESULTS
Female
Female
Starch
Starch
Starch
Maltose
population 1
population 2
Figure 24.9 Can divergence of allopatric populations lead to reproductive isolation?
Starch
population 1
Starch 22 9 18 15
Male
Male
Maltose 8 20 12 15
Starch
population 2
Mating frequencies
in experimental group
Mating frequencies
in control group

29. Sympatric (“Same Country”) Speciation
In sympatric speciation, speciation takes place in geographically overlapping populations
It can occur if gene flow is reduced by such factors as polyploidy, habitat differentiation and sexual selection.

30. Polyploidy
Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division. A new species may originate if an accident during cell division produces a polyploid individual.
An autopolyploid is an individual with more than two chromosome sets, derived from one species

31. 2n = 6 4n = 12 Failure of cell division after chromosome
Fig
2n = 6
4n = 12
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Fig Sympatric speciation by autopolyploidy in plants

32. 2n = 6 4n = 12 2n Failure of cell division after chromosome
Fig
2n = 6
4n = 12 2n
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Fig Sympatric speciation by autopolyploidy in plants

33. 2n = 6 4n = 12 2n 4n Failure of cell division after chromosome
Fig
2n = 6
4n = 12 2n 4n
Failure of cell
division after
chromosome
duplication gives
rise to tetraploid
tissue.
Gametes
produced
are diploid..
Offspring with
tetraploid
karyotypes may
be viable and
fertile—creating
a new biological
species
Fig Sympatric speciation by autopolyploidy in plants

34. An allopolyploid is a species with multiple sets of chromosomes derived from different species:

35. Species B Unreduced 2n = 4 gamete with 4 chromosomes Meiotic error
Fig
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Meiotic
error
Normal
gamete
n = 3
Figure One mechanism for allopolyploid speciation in plants
Species A
2n = 6

36. Species B Unreduced 2n = 4 gamete with 4 chromosomes Hybrid with 7
Fig
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Figure One mechanism for allopolyploid speciation in plants
Species A
2n = 6

37. Species B Unreduced 2n = 4 gamete Unreduced with 4 gamete chromosomes
Fig
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Figure One mechanism for allopolyploid speciation in plants
Normal
gamete
n = 3
Species A
2n = 6

38. Allopolyploids are fertile when mating with each other,
Fig
Allopolyploids are fertile when mating with each other,
but cannot interbreed with either parent species; thus
they represent a new biological species.
Species B
2n = 4
Unreduced
gamete
with 4
chromosomes
Unreduced
gamete
with 7
chromosomes
Hybrid
with 7
chromosomes
Meiotic
error
Normal
gamete
n = 3
Viable fertile
hybrid
(allopolyploid)
2n = 10
Figure One mechanism for allopolyploid speciation in plants
Normal
gamete
n = 3
Species A
2n = 6

39. Polyploidy is much more common in plants than in animals
Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids

40. Habitat Differentiation
Sympatric speciation can also result from the appearance of new ecological niches
For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees
Natural selection has favored the apple-feeding populations and they are now beginning to show temporal isolation from each other. They are still considered subspecies—but gene flow is being reduced and they appear to be on their way to forming two new species.

41. Sexual Selection
Sexual selection can drive sympatric speciation
Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria

42. Monochromatic orange light
Fig
EXPERIMENT
Monochromatic
orange light
Normal light P.
pundamilia
Figure Does sexual selection in cichlids result in reproductive isolation?
P. nyererei

43. Allopatric and Sympatric Speciation: A Review
In allopatric speciation, geographic isolation restricts gene flow between populations
Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations
Even if contact is restored between populations, interbreeding is prevented

44. In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species
Sympatric speciation can result from polyploidy, natural selection, or sexual selection

45. Concept 24.3: Hybrid zones provide opportunities to study factors that cause reproductive isolation
A hybrid zone is a region in which members of different species mate and produce hybrids

46. Patterns Within Hybrid Zones
A hybrid zone can occur in a single band where adjacent species meet
Hybrids often have reduced fitness compared with parent species
The distribution of hybrid zones can be more complex if parent species are found in multiple habitats within the same region

47. Fig
EUROPE
Fire-bellied
toad range
Hybrid zone
Fire-bellied toad,
Bombina bombina
Yellow-bellied
toad range
Yellow-bellied toad,
Bombina variegata
0.99
0.9
Fig A narrow hybrid zone for B. variegata and B. bombina in Europe
Allele frequency (log scale)
0.5
0.1
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km)

48. Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Strengthening of reproductive barriers
Weakening of reproductive barriers
Continued formation of hybrid individuals

49. Gene flow Barrier to gene flow Population (five individuals are shown)
Fig
Gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)

50. Isolated population diverges
Fig
Isolated population
diverges
Gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)

51. Isolated population diverges
Fig
Isolated population
diverges
Hybrid
zone
Gene flow
Hybrid
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)

52. Isolated population diverges
Fig
Isolated population
diverges
Possible
outcomes:
Hybrid
zone
Reinforcement OR
Fusion
Gene flow
Hybrid OR
Figure Formation of a hybrid zone and possible outcomes for hybrids over time
Barrier to
gene flow
Population
(five individuals
are shown)
Stability

53. Reinforcement: Strengthening Reproductive Barriers
The reinforcement of barriers occurs when hybrids are less fit than the parent species
Over time, the rate of hybridization decreases
Where reinforcement occurs, reproductive barriers should be stronger for sympatric than allopatric species

54. Fusion: Weakening Reproductive Barriers
If hybrids are as fit as parents, there can be substantial gene flow between species
If gene flow is great enough, the parent species can fuse into a single species

55. Pundamilia pundamilia
Fig
Pundamilia nyererei
Pundamilia pundamilia
Figure The breakdown of reproductive barriers
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water

56. Stability: Continued Formation of Hybrid Individuals
Extensive gene flow from outside the hybrid zone can overwhelm selection for increased reproductive isolation inside the hybrid zone
In cases where hybrids have increased fitness, local extinctions of parent species within the hybrid zone can prevent the breakdown of reproductive barriers

57. Concept 24.4: Speciation can occur rapidly or slowly and can result from changes in few or many genes
Many questions remain concerning how long it takes for new species to form, or how many genes need to differ between species

58. The Time Course of Speciation
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

59. Patterns in the Fossil Record
The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear
Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change
The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence

60. (a) Punctuated pattern
Fig
(a) Punctuated pattern
Time
(b) Gradual pattern
Figure Two models for the tempo of speciation

61. Speciation Rates
The punctuated pattern in the fossil record and evidence from lab studies suggests that speciation can be rapid
The interval between speciation events can range from 4,000 years (some cichlids) to 40,000,000 years (some beetles), with an average of 6,500,000 years

62. Studying the Genetics of Speciation
The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation
Depending on the species in question, speciation might require the change of only a single allele or many alleles

63. From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events

64. Allopatric speciation Sympatric speciation
Fig. 24-UN1
Original population
Fig. 24-UN1
Allopatric speciation
Sympatric speciation

65. Ancestral species: AA BB DD Triticum monococcum (2n = 14) Wild
Fig. 24-UN2
Ancestral species: AA BB DD
Triticum
monococcum
(2n = 14)
Wild
Triticum
(2n = 14)
Wild
T. tauschii
(2n = 14)
Product:
Fig. 24-UN2
AA BB DD
T. aestivum
(bread wheat)
(2n = 42)

66. Fig. 24-UN3
Fig. 24-UN3

67. You should now be able to:
Define and discuss the limitations of the four species concepts
Describe and provide examples of prezygotic and postzygotic reproductive barriers
Distinguish between and provide examples of allopatric and sympatric speciation
Explain how polyploidy can cause reproductive isolation
Define the term hybrid zone and describe three outcomes for hybrid zones over time