1. Chapter 24
The Origin of Species

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

4. 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

5. Animation: Macroevolution
Two basic patterns of evolutionary change:
Anagenesis (phyletic evolution) transforms one species into another
Cladogenesis (branching evolution) is the splitting of a gene pool, giving rise to one or more new species
Animation: Macroevolution

6. LE 24-2
Anagenesis
Cladogenesis

7. Concept 24.1: The biological species concept emphasizes reproductive isolation
Species is a Latin word meaning “kind” or “appearance”

8. The Biological Species Concept
Members of a biological species are reproductively compatible, at least potentially; they cannot interbreed with other populations.

9. LE 24-3 Similarity between different species.
Diversity within a species.

10. Reproductive Isolation
Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile hybrids
Two types of barriers: prezygotic and postzygotic

11. Prezygotic barriers impede mating or hinder fertilization if mating does occur:
Habitat isolation
Temporal isolation
Behavioral isolation
Mechanical isolation
Gametic isolation

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

13. LE 24-4a
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Gametic
isolation
Individuals of
different
species
Mating
attempt
Fertilization
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION
GAMETIC ISOLATION
Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN

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

17. Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers

19. Mechanical isolation: Morphological differences can prevent successful mating

21. Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species
Video: Albatross Courtship Ritual
Video: Blue-footed Boobies Courtship Ritual
Video: Giraffe Courtship Ritual

23. LE 24-4aa
Prezygotic barriers impede mating or hinder fertilization if mating does occur
Habitat
isolation
Temporal
isolation
Behavioral
isolation
Mechanical
isolation
Gametic
isolation
Individuals of
different
species
Mating
attempt
Fertilization
HABITAT ISOLATION
TEMPORAL ISOLATION
BEHAVIORAL ISOLATION
MECHANICAL ISOLATION
GAMETIC ISOLATION

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

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

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

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

31. LE 24-4ab Postzygotic barriers prevent a hybrid zygote from
developing into a viable, fertile adult
Reduced
hybrid
viability
Reduced
hybrid
fertility
Hybrid
breakdown
Viable,
fertile
offspring
Fertilization
REDUCED HYBRID
VIABILITY
REDUCED HYBRID
FERTILITY
HYBRID BREAKDOWN

32. Limitations of the Biological Species Concept
The biological species concept does not apply to
Asexual organisms
Fossils
Organisms about which little is known regarding their reproduction

33. Other Definitions of Species
Morphological: defines a species by structural features
Paleontological: focuses on morphologically discrete species known only from the fossil record
Ecological: views a species in terms of its ecological niche
Phylogenetic: defines a species as a set of organisms with a unique genetic history

34. Concept 24.2: Speciation can take place with or without geographic separation
Speciation can occur in two ways:
Allopatric speciation
Sympatric speciation

35. LE 24-5
Allopatric speciation
Sympatric speciation

36. Allopatric (“Other Country”) Speciation
In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations
One or both populations may undergo evolutionary change during the period of separation

37. LE 24-6
A. harrisi
A. leucurus

38. To determine if allopatric speciation has occurred, reproductive isolation must have been established

39. after several generations
LE 24-7a
Initial population
of fruit flies
(Drosophila
pseudoobscura)
Some flies
raised on
starch medium
Some flies
raised on
maltose medium
Mating experiments
after several generations

40. Female Female Same population Different populations Starch Maltose
LE 24-7b
Female
Female
Same
population
Different
populations
Starch
Maltose
population
Same
Starch 22 9 18 15
Male
Male
Maltose 8 20
populations
Different 12 15
Mating frequencies
in experimental group
Mating frequencies
in control group

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

42. Polyploidy
Polyploidy is presence of extra sets of chromosomes due to accidents during cell division
It has caused the evolution of some plant species
An autopolyploid is an individual with more than two chromosome sets, derived from one species

43. Failure of cell division in a cell of a growing
LE 24-8
Failure of cell division
in a cell of a growing
diploid plant after chromosome duplication gives rise to a tetraploid branch or other tissue.
Offspring with tetraploid karyo-
types may be viable and fertile—a new biological species.
Gametes produced by flowers on this tetraploid branch are diploid. 2n
2n = 6
4n = 12 4n

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

45. LE 24-9 Unreduced gamete with 4 chromosomes Unreduced gamete
Hybrid with
7 chromosomes
Viable fertile hybrid
(allopolyploid)
Meiotic error;
chromosome
number not
reduced from
2n to n
Species A
2n = 4
2n = 10
Normal gamete
n = 3
Normal gamete
n = 3
Species B
2n = 6

46. Habitat Differentiation and Sexual Selection
Sympatric speciation can also result from the appearance of new ecological niches
In cichlid fish, sympatric speciation has resulted from nonrandom mating due to sexual selection

47. Monochromatic orange light
LE 24-10
P. pundamilia
P. nyererei
Normal light
Monochromatic orange light

48. Allopatric and Sympatric Speciation: A Summary
In allopatric speciation, a new species forms while geographically isolated from its parent population
In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species

49. Adaptive Radiation
Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities

51. The Hawaiian archipelago is one of the world’s great showcases of adaptive radiation

52. LE 24-12 N Dubautia laxa 1.3 million years KAUAI 5.1 million years
MOLOKAI
MAUI
OAHU
3.7
million
years
LANAI
Argyroxiphium sandwicense
HAWAII
0.4
million
years
Dubautia waialealae
Dubautia scabra
Dubautia linearis

53. Studying the Genetics of Speciation
The explosion of genomics is enabling researchers to identify specific genes involved in some cases of speciation

54. The Tempo of Speciation
The fossil record includes many episodes in which new species appear suddenly in a geologic stratum, persist essentially unchanged through several strata, 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

55. LE 24-13
Time
Gradualism model
Punctuated equilibrium model

56. Concept 24.3: Macroevolutionary changes can accumulate through many speciation events
Macroevolutionary change is cumulative change during thousands of small speciation episodes

57. Evolutionary Novelties
Most novel biological structures evolve in many stages from previously existing structures

58. Some complex structures, such as the eye, have had similar functions during all stages of their evolution

59. LE 24-14 Pigmented cells (photoreceptors) Pigmented cells Epithelium
Nerve fibers
Nerve fibers
Patch of pigmented cells
Eyecup
Fluid-filled cavity
Cellular
fluid
(lens)
Cornea
Epithelium
Optic
nerve
Pigmented
layer (retina)
Optic nerve
Pinhole camera-type eye
Eye with primitive lens
Cornea
Lens
Retina
Optic nerve
Complex camera-type eye

60. Evolution of the Genes That Control Development
Genes that program development control the rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult

61. Changes in Rate and Timing
Heterochrony is an evolutionary change in the rate or timing of developmental events
It can have a significant impact on body shape
Allometric growth is the proportioning that helps give a body its specific form
Animation: Allometric Growth

62. Newborn 2 5 15 Adult Age (years)
LE 24-15a
Newborn 2 5 15
Adult
Age (years)
Differential growth rates in a human

63. Different allometric patterns contribute to the contrasting shapes of human and chimpanzee skulls

64. Chimpanzee fetus Chimpanzee adult Human fetus Human adult
LE 24-15b
Chimpanzee fetus
Chimpanzee adult
Human fetus
Human adult
Comparison of chimpanzee and human skull growth

65. Heterochrony has also played a part in the evolution of salamander feet

66. LE 24-16
Ground-dwelling salamander
Tree-dwelling salamander

67. In paedomorphosis, the rate of reproductive development accelerates compared with somatic development
The sexually mature species may retain body features that were juvenile structures in an ancestral species

69. Changes in Spatial Pattern
Substantial evolutionary change can also result from alterations in genes that control the placement and organization of body parts
Homeotic genes determine such basic features as where wings and legs will develop on a bird or how a flower’s parts are arranged

70. The products of one class of homeotic genes called Hox genes
Hox genes provide positional information in the development of fins in fish and limbs in tetrapods

71. LE 24-18 Chicken leg bud Region of Hox gene expression
Zebrafish fin bud

72. Evolution of vertebrates from invertebrate animals was associated with alterations in Hox genes

73. Hypothetical vertebrate ancestor (invertebrate)
LE 24-19
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
Hypothetical early
vertebrates (jawless)
with two Hox clusters
Second Hox
duplication
Vertebrates (with jaws)
with four Hox clusters

74. Evolution Is Not Goal Oriented
The fossil record often shows apparent trends in evolution that may arise because of adaptation to a changing environment

75. Hippidion and other genera
LE 24-20
Recent
Equus
Hippidion and other genera
Pleistocene
Nannippus
Pliohippus
Pliocene
Hipparion
Neohipparion
Sinohippus
Megahippus
Callippus
Archaeohippus
Merychippus
Miocene
Anchitherium
Hypohippus
Parahippus
Miohippus
Oligocene
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
Pachynolophus
Orohippus
Key
Grazers
Hyracotherium
Browsers

76. According to the species selection model, trends may result when species with certain characteristics endure longer and speciate more often than those with other characteristics
The appearance of an evolutionary trend does not imply that there is some intrinsic drive toward a particular phenotype