Chapter 14 The Origin of Species

Transcription

1 Chapter 14 The Origin of Species PowerPoint Lectures Campbell Biology: Concepts & Connections, Eighth Edition REECE TAYLOR SIMON DICKEY HOGAN Lecture by Edward J. Zalisko

2 Introduction Bowerbirds, native to New Guinea and Australia, are named for the structure, called a bower, that the male weaves from twigs and grasses to attract females. After building his bower, the male collects objects such as fruits, seeds, insect parts, rocks, flowers, and leaves and arranges them artfully by color and type.

3 Figure

4 Figure Chapter 14: Big Ideas Defining Species Mechanisms of Speciation

5 Introduction Females tour the bowers of local males, inspecting each bower carefully while its owner courts her with a song and dance, and may visit promising candidates multiple times before finally mating with one.

6 Introduction Not all Vogelkop bowerbirds construct displays like the one in the photo. The males of another population build a simpler structure consisting of sticks loosely woven around a central sapling. Researchers hypothesize that this divergence in display preferences has started the two populations on separate evolutionary paths, with each path leading to a new species, or speciation.

7 Introduction An ancestral cormorant species is thought to have flown from the Americas to the Galápagos Islands more than 3 million years ago. Terrestrial mammals could not make the trip over the wide distance, and no predatory mammals naturally occur on these islands today. Without predators, the environment of these cormorants favored birds with smaller wings, perhaps channeling resources to the production of offspring.

8 DEFINING SPECIES

9 14.1 The origin of species is the source of biological diversity Darwin was eager to explore landforms newly emerged from the sea when he came to the Galápagos Islands. He noted that these volcanic islands, despite their geologic youth, were teeming with plants and animals found nowhere else in the world. He realized that these species, like the islands, were relatively new.

10 14.1 The origin of species is the source of biological diversity Microevolution is the change in the gene pool of a population from one generation to the next. Speciation is the process by which one species splits into two or more species. Each time speciation occurs, the diversity of life increases.

11 Figure 14.1

12 14.1 The origin of species is the source of biological diversity Over the course of 3.5 billion years, an ancestral species first gave rise to two or more different species, which then branched to new lineages, which branched again, until we arrive at the millions of species that live, or once lived, on Earth.

13 14.2 There are several ways to define a species The word species is from the Latin for kind or appearance. Although the basic idea of species as distinct lifeforms seems intuitive, devising a more formal definition is not easy and raises questions. In many cases, the differences between two species are obvious. In other cases, the differences between two species are not so obvious.

14 Figure 14.2a-0

15 Figure 14.2a-1

16 Figure 14.2a-2

17 14.2 There are several ways to define a species How similar are members of the same species? Whereas the individuals of many species exhibit fairly limited variation in physical appearance, certain other species our own, for example seem extremely varied.

18 Figure 14.2b

19 14.2 There are several ways to define a species The biological species concept defines a species as a group of populations whose members have the potential to interbreed in nature and produce fertile offspring (offspring that themselves can reproduce). Thus, members of a biological species are united by being reproductively compatible, at least potentially.

20 14.2 There are several ways to define a species Reproductive isolation prevents genetic exchange (gene flow) and maintains a boundary between species. But there are some pairs of clearly distinct species that do occasionally interbreed. The resulting offspring are called hybrids. An example is the grizzly bear (Ursus arctos) and the polar bear (Ursus maritimus), whose hybrid offspring have been called grolar bears.

21 Figure 14.2c-0 Grizzly bear Polar bear Hybrid grolar bear

22 Figure 14.2c-1 Grizzly bear

23 Figure 14.2c-2 Polar bear

24 Figure 14.2c-3 Hybrid grolar bear

25 14.2 There are several ways to define a species There are other instances in which applying the biological species concept is problematic. There is no way to determine whether organisms that are now known only through fossils were once able to interbreed. Reproductive isolation does not apply to prokaryotes or other organisms that reproduce only asexually. Therefore, alternate species concepts can be useful.

26 14.2 There are several ways to define a species The morphological species concept classifies organisms based on observable physical traits and can be applied to asexual organisms and fossils. However, there is some subjectivity in deciding which traits to use.

27 14.2 There are several ways to define a species The ecological species concept defines a species by its ecological niche and focuses on unique adaptations to particular roles in a biological community. For example, two species may be similar in appearance but distinguishable based on what they eat or the depth of water in which they are usually found.

28 14.2 There are several ways to define a species The phylogenetic species concept defines a species as the smallest group of individuals that share a common ancestor and thus form one branch of the tree of life. Biologists trace the phylogenetic history of a species by comparing its morphology, DNA sequences, or biochemical pathways. However, agreeing on the amount of difference required to establish separate species remains a challenge.

29 14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate Reproductive barriers serve to isolate the gene pools of species and prevent interbreeding. Depending on whether they function before or after zygotes form, reproductive barriers are categorized as prezygotic or postzygotic.

30 14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate Five types of prezygotic barriers prevent mating or fertilization between species. 1. In habitat isolation, there is a lack of opportunity for mates to encounter each other. 2. In temporal isolation, there is breeding at different times or seasons.

31 Figure Habitat isolation (different habitats) PREZYGOTIC BARRIERS Temporal isolation (breeding at different times) Behavioral isolation (different courtship rituals) Mechanical isolation (incompatible reproductive parts) Gametic isolation (incompatible gametes) POSTZYGOTIC BARRIERS Reduced hybrid vitality (short-lived hybrids) Reduced hybrid fertility (sterile hybrids) Hybrid breakdown (fertile hybrids with sterile offspring)

32 Figure Habitat isolation (lack of opportunities to encounter each other) The garter snake Thamnophis atratus lives mainly in water. The garter snake Thamnophis sirtalis lives on land.

33 Figure

34 Figure

35 Figure Temporal isolation (breeding at different times or seasons) The eastern spotted skunk (Spilogale putorius) breeds in late winter. The western spotted skunk (Spilogale gracilis) breeds in the fall.

36 Figure

37 Figure

38 14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate 3. In behavioral isolation, there is failure to send or receive appropriate signals. 4. In mechanical isolation, there is physical incompatibility of reproductive parts. 5. In gametic isolation, there is molecular incompatibility of eggs and sperm or pollen and stigma.

39 Video: Blue-Footed Boobies Courtship Ritual

40 Video: Albatross Courtship Ritual

41 Video: Giraffe Courtship Ritual

42 Figure Behavioral isolation (different courtship rituals) The blue-footed booby (Sula nebouxii) performs an elaborate courtship dance. The masked booby (Sula dactylatra) performs a different courtship ritual.

43 Figure

44 Figure

45 Figure Mechanical isolation (physical incompatibility of reproductive parts) Heliconia latispatha is pollinated by hummingbirds with short, straight bills. Heliconia pogonantha is pollinated by hummingbirds with long, curved bills.

46 Figure

47 Figure

48 Figure Gametic isolation (molecular incompatibility of eggs and sperm or pollen and stigma) Purple sea urchin (Strongylocentrotus purpuratus) Red sea urchin (Strongylocentrotus franciscanus)

49 Figure

50 Figure

51 14.3 VISUALIZING THE CONCEPT: Reproductive barriers keep species separate Three types of postzygotic barriers operate after hybrid zygotes have formed. 1. In reduced hybrid viability, interaction of parental genes impairs the hybrid s development or survival. 2. In reduced hybrid fertility, hybrids are vigorous but cannot produce viable offspring. 3. In hybrid breakdown, hybrids are viable and fertile, but their offspring are feeble or sterile.

52 Figure Reduced hybrid viability (hybrid development or survival impaired by interaction of parental genes) Some salamander species can hybridize, but their offspring do not develop fully or are frail and will not survive long enough to reproduce.

53 Figure

54 Figure Reduced hybrid fertility (vigorous hybrids that cannot produce viable offspring) A mule is the sterile hybrid offspring of a horse and a donkey.

55 Figure

56 Figure Hybrid breakdown (viable and fertile hybrids with feeble or sterile offspring) The rice hybrids on the left and right are fertile, but plants of the next generation (middle) are sterile.

57 Figure

58 MECHANISMS OF SPECIATION

59 14.4 In allopatric speciation, geographic isolation leads to speciation A key event in the origin of a new species is the separation of a population from other populations of the same species. With its gene pool isolated, the splinter population can follow its own evolutionary course. Changes in allele frequencies caused by natural selection, genetic drift, and mutation will not be diluted by alleles entering from other populations (gene flow).

60 14.4 In allopatric speciation, geographic isolation leads to speciation In allopatric speciation, the initial block to gene flow may come from a geographic barrier that isolates a population.

61 14.4 In allopatric speciation, geographic isolation leads to speciation Several geologic processes can isolate populations. A mountain range may emerge and gradually split a population of organisms that can inhabit only lowlands. A large lake may subside until there are several smaller lakes, isolating certain fish populations. Continents themselves can split and move apart. Allopatric speciation can also occur when individuals colonize a remote area and become geographically isolated from the parent population.

62 14.4 In allopatric speciation, geographic isolation leads to speciation How large must a geographic barrier be to keep allopatric populations apart? The answer depends on the ability of the organisms to move. Birds, mountain lions, and coyotes can easily cross mountain ranges. In contrast, small rodents may find a canyon or a wide river a formidable barrier. The Grand Canyon and Colorado River separate two species of antelope squirrels.

63 Figure 14.4a-0 South rim North rim A. harrisii A. leucurus

64 Figure 14.4a-1 A. harrisii

65 Figure 14.4a-2 A. leucurus

66 14.4 In allopatric speciation, geographic isolation leads to speciation Thirty species of snapping shrimp in the genus Alpheus live off the Isthmus of Panama, the land bridge that connects South and North America. Morphological and genetic data group these shrimp into 15 pairs of species, with the members of each pair being each other s closest relative. In each case, one member of the pair lives on the Atlantic side of the isthmus, while the other lives on the Pacific side. This strongly suggests that geographic separation of the ancestral species of these snapping shrimp led to allopatric speciation.

67 Figure 14.4b A. formosus A. nuttingi ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN A. panamensis A. millsae

68 14.5 Reproductive barriers can evolve as populations diverge How do reproductive barriers arise? The environment of an isolated population may include different food sources, different types of pollinators, and different predators. As a result of natural selection acting on preexisting variations (or as a result of genetic drift or mutation), a population s traits may change in ways that also establish reproductive barriers.

69 14.5 Reproductive barriers can evolve as populations diverge Researchers have successfully documented the evolution of reproductive isolation with laboratory experiments. These studies have included laboratory studies of fruit flies and field studies of monkey flowers and their pollinators.

70 Figure 14.5a Starch medium Initial sample of fruit flies Maltose medium Male Starch Maltose Female Starch Maltose 22 8 Number of matings in experimental groups Mating experiments 9 20 Results Male Pop#2 Pop#1 Population # Female Population # Number of matings In starch control groups

71 Figure 14.5b-0 Pollinator choice in typical monkey flowers Pollinator choice after color allele transfer Typical M. lewisii (pink) M. lewisii with red-color allele Typical M. cardinalis (red) M. cardinalis with pink-color allele

72 Figure 14.5b-1 Typical M. lewisii (pink)

73 Figure 14.5b-2 M. lewisii with red-color allele

74 Figure 14.5b-3 Typical M. cardinalis (red)

75 Figure 14.5b-4 M. cardinalis with pink-color allele

76 14.6 Sympatric speciation takes place without geographic isolation Sympatric speciation occurs when a new species arises within the same geographic area as its parent species. How can reproductive isolation develop when members of sympatric populations remain in contact with each other? Gene flow between populations may be reduced by polyploidy, habitat differentiation, or sexual selection.

77 14.6 Sympatric speciation takes place without geographic isolation Many plant species have originated from sympatric speciation that occurs when accidents during cell division result in extra sets of chromosomes. New species formed in this way are polyploid, in that their cells have more than two complete sets of chromosomes.

78 14.6 Sympatric speciation takes place without geographic isolation Sympatric speciation can result from polyploidy within a species (by self-fertilization) or between two species (by hybridization).

79 Figure 14.6a-1 1 Parent species 2n = 6 Tetraploid cells 4n = 12

80 Figure 14.6a Parent species 2n = 6 Tetraploid cells 4n = 12 Diploid gametes 2n = 6

81 Figure 14.6a Selffertilization Parent species 2n = 6 Tetraploid cells 4n = 12 Diploid gametes 2n = 6 Viable, fertile tetraploid species 4n = 12

82 Figure 14.6b-1 Species A 2n = 4 Gamete n = 2 Species B 2n = 6 Gamete n = 3

83 Figure 14.6b-2 Chromosomes cannot pair Species A Gamete 2n = 4 n = 2 1 Species B 2n = 6 Gamete n = 3 Sterile hybrid n = 5 Can reproduce asexually 2

84 Figure 14.6b-3 Chromosomes cannot pair Species A Gamete 2n = 4 n = Species B 2n = 6 Gamete n = 3 Sterile hybrid n = 5 Can reproduce asexually 2 Viable, fertile hybrid species 2n = 10

85 14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation Plant biologists estimate that 80% of all living plant species are descendants of ancestors that formed by polyploid speciation. Hybridization between two species accounts for most of these species, perhaps because of the adaptive advantage of the diverse genes a hybrid inherits from different parental species.

86 14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation Polyploid plants include cotton, oats, potatoes, bananas, peanuts, barley, plums, apples, sugarcane, coffee, and wheat.

87 Figure

88 14.7 EVOLUTION CONNECTION: The origin of most plant species can be traced to polyploid speciation Wheat has been domesticated for at least 10,000 years and is the most widely cultivated plant in the world. Bread wheat, Triticum aestivum, is a polyploid with 42 chromosomes and the result of hybridization and polyploidy.

89 Figure AA BB Domesticated Triticum monococcum (14 chromosomes) 1 Hybridization Wild Triticum (14 chromosomes) AB Sterile hybrid (14 chromosomes) 2 Cell division error and self-fertilization AABB T. turgidum Emmer wheat (28 chromosomes) 3 DD Hybridization Wild T. tauschii (14 chromosomes) ABD Sterile hybrid (21 chromosomes) 4 Cell division error and self-fertilization AABBDD T. aestivum Bread wheat (42 chromosomes)

90 Figure AA BB Domesticated Triticum monococcum (14 chromosomes) 1 Hybridization Wild Triticum (14 chromosomes) AB Sterile hybrid (14 chromosomes) 2 AABB Cell division error and self-fertilization DD T. turgidum Emmer wheat (28 chromosomes) Wild T. tauschii (14 chromosomes)

91 Figure AABB T. turgidum Emmer wheat (28 chromosomes) 3 DD Hybridization Wild T. tauschii (14 chromosomes) ABD Sterile hybrid (21 chromosomes) 4 Cell division error and self-fertilization AABBDD T. aestivum Bread wheat (42 chromosomes)

92 14.8 Isolated islands are often showcases of speciation Isolated island chains are often inhabited by unique collections of species. Islands that have physically diverse habitats and that are far enough apart to permit populations to evolve in isolation but close enough to allow occasional dispersions to occur are often the sites of multiple speciation events. The evolution of many diverse species from a common ancestor is known as adaptive radiation.

93 14.8 Isolated islands are often showcases of speciation The Galápagos Archipelago is located about 900 km (560 miles) west of Ecuador, is one of the world s great showcases of adaptive radiation, was formed naked from underwater volcanoes from 5 million to 1 million years ago, was colonized gradually from other islands and the South America mainland, and has many species of plants and animals found nowhere else in the world.

94 14.8 Isolated islands are often showcases of speciation The Galápagos Islands currently have 14 species of closely related finches, called Darwin s finches, because Darwin collected them during his aroundthe-world voyage on the Beagle. These birds share many finchlike traits, differ in their feeding habits and their beaks, specialized for what they eat, and arose through adaptive radiation.

95 Figure Cactus-seed-eater (cactus finch) Tool-using insect-eater (woodpecker finch) Seed-eater (large ground finch)

96 Figure Cactus-seed-eater (cactus finch)

97 Figure Tool-using insect-eater (woodpecker finch)

98 Figure Seed-eater (large ground finch)

99 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation We can see speciation occurring. The species living today represent a snapshot, a brief instant in this vast span of time. The environment continues to change, sometimes rapidly due to human impact, and natural selection continues to act on affected populations. Researchers have documented at least two dozen cases in which populations are diverging as they exploit different food resources or breed in different habitats.

100 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation Sexual selection is a form of natural selection in which individuals with certain traits are more likely to obtain mates. In addition to the bowerbirds already discussed, biologists have identified several other animal populations that are diverging as a result of differences in how males attract females or how females choose mates.

101 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation Biologists can also test hypotheses about the process of speciation by studying species that arose recently. Cichlids are a family of fishes that live in tropical lakes and rivers. They come in all colors of the rainbow. They are renowned for the spectacular adaptive radiations that stocked the large lakes of East Africa with more than a thousand species of cichlids in less than 100,000 years. In the largest of these lakes, Lake Victoria, roughly 500 species evolved in about 15,000 years.

102 Figure 14.9a Uganda Kenya Lake Victoria Tanzania Indian Ocean

103 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation In Lake Victoria, there are pairs of closely related cichlid species that differ in color but nothing else. Breeding males of Pundamilia nyererei have a bright red back and dorsal fin. Breeding males of Pundamilia pundamilia males are metallic blue-gray.

104 Figure 14.9b Pundamilia nyererei Pundamilia pundamilia

105 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation Pundamilia females prefer brightly colored males. Mate-choice experiments performed in the laboratory showed that P. nyererei females prefer red males over blue males, P. pundamilia females prefer blue males over red males, the vision of P. nyererei females is more sensitive to red light than blue light, and the vision of P. pundamilia females is more sensitive to blue light than red light. Researchers also demonstrated that this color sensitivity is heritable.

106 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation As light travels through water, suspended particles selectively absorb and scatter the shorter (blue) wavelengths, so light becomes increasingly red with increasing depth. Thus, in deeper waters, P. nyererei males are pleasingly apparent to females with red-sensitive vision but virtually invisible to P. pundamilia females.

107 14.9 SCIENTIFIC THINKING: Lake Victoria is a living laboratory for studying speciation When biologists sampled cichlid populations in Lake Victoria, they found that P. nyererei breeds in deep water, while P. pundamilia inhabits shallower habitats where the blue males shine brightly. As a consequence of their mating behavior, the two species encounter different environments that may result in further divergence.

108 14.10 Hybrid zones provide opportunities to study reproductive isolation What happens when separated populations of closely related species come back into contact with each other? Biologists try to answer such questions by studying hybrid zones, regions in which members of different species meet and mate to produce at least some hybrid offspring. Figure 14.10A illustrates the formation of a hybrid zone, starting with the ancestral species.

109 Figure 14.10a Three populations of a species Newly formed species Hybrid zone Gene flow Population Barrier to gene flow 4 Gene flow Hybrid individual

110 14.10 Hybrid zones provide opportunities to study reproductive isolation Reinforcement When hybrid offspring are less fit than members of both parent species, we might expect natural selection to strengthen, or reinforce, reproductive barriers, thus reducing the formation of unfit hybrids, and that barriers between species should be stronger where the species overlap (that is, where the species are sympatric). The closely related collared flycatcher and pied flycatcher are an example of reinforcement.

111 Figure 14.10b-0 Allopatric populations Sympatric populations Male collared flycatcher Male pied flycatcher Pied flycatcher from allopatric population Pied flycatcher from sympatric population

112 Figure 14.10b-1 Allopatric populations Sympatric populations Male collared flycatcher Male pied flycatcher

113 Figure 14.10b-2 Pied flycatcher from allopatric population

114 Figure 14.10b-3 Pied flycatcher from sympatric population

115 14.10 Hybrid zones provide opportunities to study reproductive isolation Fusion What happens when the reproductive barriers between species are not strong and the species come into contact in a hybrid zone? So much gene flow may occur that the speciation process reverses, causing the two hybridizing species to fuse into one. Such a situation has been occurring among the cichlid species in Lake Victoria.

116 14.10 Hybrid zones provide opportunities to study reproductive isolation Pollution caused by development along the shores of Lake Victoria has turned the water murky. What happens when P. nyererei or P. pundamilia females can t tell red males from blue males? The behavioral barrier crumbles. Many viable hybrid offspring are produced by interbreeding. The once isolated gene pools of the parent species are combining, with two species fusing into a single hybrid species.

117 Figure 14.10c Hybrid: Pundamilia turbid water

118 14.11 Speciation can occur rapidly or slowly There are two models for the tempo of speciation. 1. The punctuated equilibria model draws on the fossil record, where species change most as they arise from an ancestral species and then change relatively little for the rest of their existence. 2. Other species appear to have evolved more gradually. The time interval between speciation events varies from a few thousand years to tens of millions of years.

119 Animation: Macroevolution

120 Figure Punctuated pattern Gradual pattern Time

121 You should now be able to 1. Distinguish between microevolution and speciation. 2. Compare the definitions, advantages, and disadvantages of the different species concepts. 3. Describe five types of prezygotic barriers and three types of postzygotic barriers that prevent populations of closely related species from interbreeding. 4. Explain how geologic processes can fragment populations and lead to speciation.

122 You should now be able to 5. Explain how reproductive barriers might evolve in isolated populations of organisms. 6. Explain how sympatric speciation can occur, noting examples in plants and animals. 7. Explain why polyploidy is important to modern agriculture. 8. Explain how modern wheat evolved. 9. Describe the circumstances that led to the adaptive radiation of the Galápagos finches.

123 You should now be able to 10. Explain how new species of fish have evolved in Lake Victoria. 11. Explain how hybrid zones are useful in the study of reproductive isolation. 12. Compare the gradual model and the punctuated equilibrium model of evolution.

124 Figure 14.UN01 Reinforcement

125 Figure 14.UN02 Fusion

126 Figure 14.UN03 Stability

127 Figure 14.UN04 Reinforcement Fusion Stability

128 Figure 14.UN05 Gametes Prezygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Zygote Postzygotic barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown Viable, fertile offspring

129 Figure 14.UN06 Original population a. b.

130 Figure 14.UN07 Species may interbreed in a a. outcome may be b. c. d. when when when are reproductive barriers e. f. keeps are and a few hybrids continue to be produced species seperate speciation is reversed