1. The Origin of Species
Chapter 24

2. Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered unique plants and animals

3. Speciation
The origin of new species, the focal point of evolutionary theory
Evolutionary theory must explain how new species originate and how populations evolve
Microevolution - changes in allele frequency in a population over time
Macroevolution - broad patterns of evolutionary change above the species level
Macroevolution

4. Biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance”
Compare morphology, physiology, biochemistry, and DNA sequences

5. 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. (a) Similarity between different species
Figure 24.2
(a) Similarity between different species
Figure 24.2 The biological species concept is based on the potential to interbreed rather than on physical similarity.
(b) Diversity within a species

7. Reproductive Isolation
The existence of biological factors or barriers that prevent two species from producing viable, fertile offspring
Hybrids are the offspring of crosses between different species
Reproductive isolation can be classified by whether factors act before or after fertilization

8. Reproductive 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 24.3 Exploring: Reproductive Barriers
(k)

9. Prezygotic barriers Block fertilization from occurring by:
Prevent species from attempting to mate
Prevent successful completion of mating
Stop fertilization if mating is successful

10. Habitat isolation
Two species encounter each other rarely or not at all.
They occupy different habitats, even though not isolated by physical barriers
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11. Temporal isolation
Species that breed at different times of the day, season, or years
Cannot mix their gametes

12. Behavioral isolation
Courtship rituals and other behaviors unique to a species
Blue Footed Boobie dance

13. Mechanical isolation
Morphological differences can prevent successful mating
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14. Gametic Isolation
Sperm of one species may not be able to fertilize eggs of another species
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15. Postzygotic barriers
Prevent the hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

16. Reduced hybrid viability
Genes of different parent species may interact and impair the hybrid’s development

17. Reduced hybrid fertility
Even if hybrids are vigorous, they may be sterile
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18. Hybrid breakdown
Some first-generation hybrids are fertile, but when they mate with another species, offspring are feeble or sterile

19. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms
The biological species concept emphasizes absence of gene flow
However, gene flow can occur between distinct species
Grizzly bears + polar bears = “grolar bears”

20. Other Species Concepts
Other species concepts emphasize the unity within a species rather than the separateness of different species
Morphological species concept defines a species by structural features
Ecological species concept views a species in terms of its ecological niche
Phylogenetic species concept defines a species as the smallest group of individuals on a phylogenetic tree

21. Speciation can take place with or without geographic separation
Figure 24.5 Two main modes of speciation.
(a)
Allopatric speciation.
A population forms a
new species while
geographically isolated
from its parent population.
(b)
Sympatric speciation.
A subset of a population
forms a new species
without geographic
separation.

22. Allopatric Speciation
“Other Country”
Gene flow is interrupted/reduced when a population is divided geographically
The flightless cormorant of the Galápagos originated from a flying species on the mainland

23. The Process of Allopatric Speciation
The definition of barrier depends on the ability of a population to disperse
A canyon may create a barrier for small rodents, but not birds, coyotes, or pollen
A. harrisii
A. leucurus
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon.

24. Reproductive isolation may arise as a result of genetic divergence
Separate populations may evolve independently through mutation, natural selection, and genetic drift
Reproductive isolation may arise as a result of genetic divergence
Mosquitofish in the Bahamas comprise several isolated populations in different ponds
(a) Under high predation
(b) Under low predation
Figure 24.7 Reproductive isolation as a by-product of selection.

25. Evidence of Allopatric Speciation
15 pairs of sibling species of snapping shrimp (Alpheus) are separated by the Isthmus of Panama
These originated 9 to 13 Mya, when the Isthmus formed, separating the Atlantic and Pacific

26. Regions with many geographic barriers typically have more species than do regions with fewer barriers
Reproductive isolation between populations generally increases as the distance between them increases
For example, reproductive isolation increases between dusky salamanders that live further apart

27. Degree of reproductive isolation
Figure 24.9
2.0
1.5
Degree of reproductive isolation
1.0
0.5
Figure 24.9 Reproductive isolation increases with distance in populations of dusky salamanders.
Geographic distance (km)

28. Barriers to reproduction are intrinsic; separation itself is not a biological barrier

29. EXPERIMENT Initial population of fruit flies (Drosophila
Figure 24.10
EXPERIMENT
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies raised
on starch medium
Some flies raised on
maltose medium
Mating experiments
after 40 generations
RESULTS
Female
Female
Starch
population 1
Starch
population 2
Starch
Maltose
Figure Inquiry: Can divergence of allopatric populations lead to reproductive isolation?
Starch
population 1
Starch 22 9 18 15
Male
Male
population 2
Starch
Maltose 8 20 12 15
Number of matings
in experimental group
Number of matings
in control group

30. Sympatric Speciation “Same Country”
In sympatric speciation, speciation takes place in geographically overlapping populations

31. Polyploidy
The presence of extra sets of chromosomes due to accidents during cell division
More common in plants - oats, cotton, potatoes, tobacco, wheat
Autopolyploid - an individual with more than two chromosome sets, derived from one species
Allopolyploid – a species with multiple sets of chromosomes derived from different species

32. Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n
Normal
gamete
n = 3
Unreduced gamete
with 4 chromosomes
Figure One mechanism for allopolyploid speciation in plants.

33. Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
Figure
Species A
2n = 6
Species B
2n = 4
Meiotic error;
chromosome number not
reduced from 2n to n
Normal
gamete
n = 3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure One mechanism for allopolyploid speciation in plants.

34. Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n
Normal
gamete
n = 3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure One mechanism for allopolyploid speciation in plants.
Normal
gamete
n = 3
Unreduced gamete
with 7 chromosomes

35. Species A 2n = 6 Species B 2n = 4 Meiotic error; chromosome number not
reduced from 2n to n
Normal
gamete
n = 3
Unreduced gamete
with 4 chromosomes
Hybrid with
7 chromosomes
Figure One mechanism for allopolyploid speciation in plants.
Normal
gamete
n = 3
Unreduced gamete
with 7 chromosomes
New species:
viable fertile hybrid
(allopolyploid) 2n = 10

36. Habitat Differentiation
Sympatric speciation can also result from the appearance of new ecological niches
The North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

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

38. Monochromatic orange light
Figure 24.12
EXPERIMENT
Monochromatic
orange light
Normal light
P. pundamilia
Figure Inquiry: Does sexual selection in cichlids result in reproductive isolation?
P. nyererei

39. Allopatric and Sympatric Speciation: A Review
In allopatric speciation - geographic isolation restricts gene flow between populations
Reproductive isolation can then arise by natural selection, genetic drift, or sexual selection
Even if contact is restored, interbreeding is prevented
In sympatric speciation - a reproductive barrier isolates a subset of a population, without geographic separation
Sympatric speciation can result from polyploidy, natural selection, or sexual selection

40. Hybrid zones
A region in which members of different species mate and produce hybrids
Hybrids are the result of mating between species with incomplete reproductive barriers
A hybrid zone can occur in a single band where adjacent species meet
Two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone

41. B. variegata-specific allele
Figure 24.13
EUROPE
Fire-bellied
toad range
Fire-bellied toad, Bombina bombina
Hybrid zone
Yellow-bellied
toad range
0.99
Hybrid
zone
0.9
Figure A narrow hybrid zone for Bombina toads in Europe.
B. variegata-specific allele
Frequency of
0.5
Yellow-bellied
toad range
Fire-bellied
toad range
Yellow-bellied
toad, Bombina
variegata
0.1
0.01 40 30 20 10 10 20
Distance from hybrid zone center (km)

42. Hybrids often have reduced fitness compared with parent species
The distribution of hybrid zones can be more complex if parent species are found in patches within the same region

43. Hybrid Zones over Time
When closely related species meet in a hybrid zone, there are three possible outcomes:
Reinforcement
Fusion
Stability

44. Gene flow Population Barrier to gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Barrier to
gene flow

45. Isolated population diverges Gene flow Population Barrier to gene flow
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Barrier to
gene flow

46. Isolated population Hybrid diverges zone Gene flow Population Hybrid
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Hybrid
individual
Barrier to
gene flow

47. Possible outcomes: Isolated population Hybrid diverges zone
Reinforcement OR
Fusion OR
Figure Formation of a hybrid zone and possible outcomes for hybrids over time.
Gene flow
Population
Hybrid
individual
Barrier to
gene flow
Stability

48. 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
In populations of European Flycatchers, males are more similar in allopatric populations than sympatric populations

49. Females choosing between these males: Females choosing between 28
Sympatric pied male
Allopatric pied male 24
Sympatric collared male
Allopatric collared male 20 16
Number of females 12 8
Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers. 4
(none)
Own
species
Other
species
Own
species
Other
species
Female mate choice
Female mate choice

50. 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
Researchers think that pollution in Lake Victoria has reduced the ability of female cichlids to distinguish males of different species
This might be causing the fusion of many species

51. Pundamilia pundamilia
Figure 24.16
Pundamilia nyererei
Pundamilia pundamilia
Figure Fusion: The breakdown of reproductive barriers.
Pundamilia “turbid water,”
hybrid offspring from a location
with turbid water

52. 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

53. Does speciation occur rapidly or slowly
Does speciation occur rapidly or slowly? Does it 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

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

55. Patterns in the Fossil Record
The fossil record includes examples of species that appear suddenly, exist unchanged for some time, and then disappear
Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibria to describe periods of stasis punctuated by sudden change
The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence
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56. (a) Punctuated pattern Time (b) Gradual pattern
Figure Two models for the tempo of speciation.

57. Speciation Rates
Patterns in the fossil record and evidence from lab studies suggest that speciation can be rapid
Sunflower Helianthus anomalus originated from the hybridization of two other sunflower species
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58. chromosomes are shown)
EXPERIMENT
H. annuus
gamete
H. petiolarus
gamete
F1 experimental hybrid
(4 of the 2n = 34
chromosomes are shown)
RESULTS
H. anomalus
Figure Inquiry: How does hybridization lead to speciation in sunflowers?
Chromosome 1
Experimental hybrid
H. anomalus
Chromosome 2
Experimental hybrid

59. The interval between speciation events can range from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years

60. Studying the Genetics of Speciation
A fundamental question of evolutionary biology persists: How many genes change when a new species forms?
Depending on the species, speciation might require the change of only a single allele or many alleles

61. Examples
Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene
In monkey flowers (Mimulus), two loci affect flower color, which influences pollinator preference
Pollination that is dominated by either hummingbirds or bees can lead to reproductive isolation of the flower
In other species, speciation can be influenced by larger numbers of genes and gene interactions
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62. M. cardinalis flower-color allele
Typical
Mimulus
lewisii
(b)
M. lewisii with an
M. cardinalis flower-color
allele
Figure A locus that influences pollinator choice.
(c)
Typical
Mimulus
cardinalis
(d)
M. cardinalis with an
M. lewisii flower-color
allele

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