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

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
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
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

5. (a) Similarity between different species
Fig. 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

6. Figure 24.3 Does gene flow occur between widely separated populations?
EXPERIMENT
Example of a gene tree for population pair A-B
Allele
Population
Gene flow event 1 B
Allele 1 is more closely related to
alleles 2, 3, and 4 than to
alleles 5, 6, and 7.
Inference: Gene flow occurred. 2 A 3 A 4 A 5 B
Alleles 5, 6, and 7 are more closely
related to one another than to
alleles in population A.
Inference: No gene flow occurred. 6 B 7 B
RESULTS
Pair of
populations
with detected
gene flow
Estimated minimum
number of gene flow
events to account for
genetic patterns
Distance between
populations (km)
Figure 24.3 Does gene flow occur between widely separated populations?
A-B 5
340
K-L 3
720
A-C
2–3
1,390
B-C 2
1,190
F-G 2
760
G-I 2
1,110
C-E
1–2
1,310

7. Reproductive Isolation
Reproductive isolation is the existence of biological factors (barriers) that impede 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. 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
Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

9. Figure 24.4 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 24.4 Reproductive barriers
(k)
Water-dwelling Thamnophis
Terrestrial Thamnophis
Eastern spotted skunk(Spilogale putorius)
Western spotted skunk (Spilogale gracilis)
Courtship ritual of blue-footed boobies
Bradybaena with shell spiraling in opposite
directions
g) Sea urchins
h) Ensatina hybrid
Donkey
Horse
Mule (sterile hybrid)
Hybrid cultivated rice plants with
stunted offspring (center)

10. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes
Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Mechanical isolation: Morphological differences can prevent successful mating
Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

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

12. Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development
Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile
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
Limitations: The biological species concept cannot
be applied to fossils or asexual organisms
(including all prokaryotes)

13. 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

14. 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
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

15. 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
Sympatric speciation

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

17. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
The definition of barrier depends on the ability of a population to disperse
Separate populations may evolve independently through mutation, natural selection, and genetic drift

18. Fig. 24-6
A. harrisi
A. leucurus
Regions with many geographic barriers typically have more species than do regions with fewer barriers
Figure 24.6 Allopatric speciation of antelope squirrels on opposite rims of the Grand Canyon

19. Millions of years ago (mya)
Fig. 24-7
Mantellinae
(Madagascar only):
100 species
Rhacophorinae
(India/Southeast
Asia): 310 species
Other Indian/
Southeast Asian
frogs
100 80 60 40 20 1 2 3
Figure 24.7 Allopatric speciation in frogs
Millions of years ago (mya) 1 2 3
India
Madagascar
88 mya
65 mya
56 mya

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

21. Degree of reproductive isolation
Fig. 24-8
2.0
1.5
Degree of reproductive isolation
1.0
0.5
Figure 24.8 Variation in reproductive isolation with distance between populations of dusky salamanders 50
100
150
200
250
300
Geographic distance (km)

22. 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

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

24. Polyploidy
Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division
An autopolyploid is an individual with more than two chromosome sets, derived from one species

25. 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.
Fig Sympatric speciation by autopolyploidy in plants

26. An allopolyploid is a species with multiple sets of chromosomes derived from different 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
Normal
gamete
n = 3
Species A
2n = 6

27. Polyploidy is much more common in plants than in animals
Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids
Ancestral species:
Fig. 24 AA BB DD
Wild
T. tauschii
(2n = 14)
Triticum
monococcum
(2n = 14)
Wild
Triticum
(2n = 14)
AA BB DD
Product:
T. aestivum
(bread wheat)
(2n = 42)

28. 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

29. 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

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

31. 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

32. 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

33. 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
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

34. 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)

35. 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

36. 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

37. 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

38. Fig
Sympatric male
pied flycatcher
Allopatric male
pied flycatcher 28
Pied flycatchers 24
Collared flycatchers 20 16
Figure Reinforcement of barriers to reproduction in closely related species of European flycatchers
Number of females 12 8 4
(none)
Females mating
with males from:
Own
species
Other
species
Own
species
Other
species
Sympatric males
Allopatric males

39. 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

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

41. 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

42. 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
Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data

43. 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

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

45. 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

46. Figure 24.18 Rapid speciation in a sunflower hybrid zone
(a) The wild sunflower Helianthus anomalus
H. anomalus
Chromosome 1
Experimental hybrid
H. anomalus
Chromosome 2
Figure Rapid speciation in a sunflower hybrid zone
Experimental hybrid
H. anomalus
Chromosome 3
Experimental hybrid
Key
Region diagnostic for
parent species H. petiolaris
Region diagnostic for
parent species H. annuus
Region lacking information on parental origin
(b) The genetic composition of three chromosomes in H.
anomalus and in experimental hybrids

47. 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
From Speciation to Macroevolution
Macroevolution is the cumulative effect of many speciation and extinction events

48. 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