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2. Chapter 24
Speciation

3. Overview: That “Mystery of Mysteries”
In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth
In Indonesia, Wallace found the same thing
Video: Galápagos Tortoise

4. Animation: Macroevolution
The book’s sloppy definitions
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

5. 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
Book left out fossils, apparent intermediates, and breeding

6. 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
Most popular definition – Ernst Mayer

7. (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

8. 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

9. EXPERIMENT Example of a gene tree for population pair A-B Allele
Fig. 24-3a
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
Figure 24.3 Does gene flow occur between widely separated populations? 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

10. 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 (pre-zygotic) or after (post-zygotic) fertilization

11. 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

12. Thamnophis Fig. 24-4a Prezygotic barriers Habitat Isolation
Temporal Isolation
Behavioral Isolation
Mechanical Isolation
Individuals of
different
species
Mating
attempt
(a)
(c)
(e)
(f)
(d)
Thamnophis
Figure 24.4 Reproductive barriers
(b)

13. Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers

14. The book is WAY oversimplifying. Hybrid fertility is complicated
Fig. 24-4i
Prezygotic barriers
Postzygotic barriers
Gametic Isolation
Reduced Hybrid Viability
Reduced Hybrid Fertility
Hybrid Breakdown
Viable,
fertile
offspring
Fertilization
(g)
(h)
(i)
(l)
(j)
The book is WAY oversimplifying. Hybrid fertility is complicated
Figure 24.4 Reproductive barriers
Haldane’s Rule
(k)

15. Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes

16. (c) Eastern spotted skunk (Spilogale putorius) Fig. 24-4e
Figure 24.4 Reproductive barriers
Eastern spotted skunk
(Spilogale putorius)

17. (d) Western spotted skunk (Spilogale gracilis) Fig. 24-4f
Figure 24.4 Reproductive barriers
Western spotted skunk
(Spilogale gracilis)

18. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers
Video: Albatross Courtship Ritual
Video: Giraffe Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual

19. Courtship ritual of blue- footed boobies
Fig. 24-4g
(e)
Figure 24.4 Reproductive barriers
Courtship ritual of blue-
footed boobies

20. Mechanical isolation: Morphological differences can prevent successful mating

21. Bradybaena with shells spiraling in opposite directions
Fig. 24-4h
(f)
Figure 24.4 Reproductive barriers
Bradybaena with shells
spiraling in opposite
directions

22. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species

23. Fig. 24-4k
(g)
Figure 24.4 Reproductive barriers
Sea urchins

24. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:
Reduced hybrid viability
Reduced hybrid fertility
Not always sterility
Haldane’s rule
Hybrid breakdown

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

26. Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile

27. Tursiops x Pseudorca = fertile female offspring

28. Haldane’s Rule
"When in the F1 offspring of two different animal races one sex is absent, rare, or sterile, that sex is the heterozygous sex [heterogametic sex]."

29. 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

30. Limitations of the Biological Species Concept
The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes)

31. 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

32. 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

33. 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

34. 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

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

36. 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

37. Reproductive isolation between populations generally increases as the distance between them increases

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

39. 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
Abnormal # of chromosomes USUALLY interferes with breeding ability

40. 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

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

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

43. 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

44. 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

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

46. 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

47. 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

48. 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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

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

57. 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

58. 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

59. 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

60. (a) Typical Mimulus lewisii
Fig
(a) 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

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

62. 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

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Not happy with your grade? Need help understanding the material?
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