22 The Origin of Species

That “Mystery of Mysteries” In the Galápagos Islands Darwin discovered plants and animals found nowhere else on Earth © 2017 Pearson Education, Ltd. 2

Figure 22.1 Figure 22.1 How did this flightless bird come to live on the isolated Galápagos Islands? © 2017 Pearson Education, Ltd.

Speciation is the process by which one species splits into two or more species Speciation explains the features shared between organisms due to inheritance from their recent common ancestor © 2017 Pearson Education, Ltd. 4

Speciation forms a conceptual bridge between microevolution and macroevolution Microevolution consists of changes in allele frequency in a population over time Macroevolution refers to broad patterns of evolutionary change above the species level © 2017 Pearson Education, Ltd. 5

Concept 22.1: 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 © 2017 Pearson Education, Ltd. 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 populations together genetically © 2017 Pearson Education, Ltd. 7

(a) Similarity between different species Figure 22.2 Figure 22.2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (a) Similarity between different species (b) Diversity within a species © 2017 Pearson Education, Ltd.

(a) Similarity between different species Figure 22.2-1 Figure 22.2-1 The biological species concept is based on the potential to interbreed rather than on physical similarity. (part 1: similarity) (a) Similarity between different species © 2017 Pearson Education, Ltd.

Figure 22.2-1a Figure 22.2-1a The biological species concept is based on the potential to interbreed rather than on physical similarity. (part 1a: Eastern meadowlark) © 2017 Pearson Education, Ltd.

Figure 22.2-1b Figure 22.2-1b The biological species concept is based on the potential to interbreed rather than on physical similarity. (part 1b: Western meadowlark) © 2017 Pearson Education, Ltd.

(b) Diversity within a species Figure 22.2-2 Figure 22.2-2 The biological species concept is based on the potential to interbreed rather than on physical similarity. (part 2: diversity) (b) Diversity within a species © 2017 Pearson Education, Ltd.

Reproductive Isolation Reproductive isolation is the existence of biological barriers that impede two species from producing viable, fertile offspring Sometimes hybrids, the offspring of crosses between different species, are successfully produced Reproductive barriers can be classified by whether they act before or after fertilization © 2017 Pearson Education, Ltd. 13

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 © 2017 Pearson Education, Ltd. 14

Habitat isolation Temporal isolation Behavioral isolation (a) (c) (e) Figure 22.3-1 Prezygotic barriers Habitat isolation Temporal isolation Behavioral isolation MATING ATTEMPT (a) (c) (e) Figure 22.3-1 Exploring reproductive barriers (part 1: prezygotic barriers) (d) (b) © 2017 Pearson Education, Ltd.

Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers © 2017 Pearson Education, Ltd. 16

Figure 22.3-1a (a) Figure 22.3-1a Exploring reproductive barriers (part 1a: habitat isolation, apple maggot fly) © 2017 Pearson Education, Ltd.

Figure 22.3-1b (b) Figure 22.3-1b Exploring reproductive barriers (part 1b: habitat isolation, blueberry maggot fly) © 2017 Pearson Education, Ltd.

Temporal isolation: Species that breed at different times of the day, different seasons, or different years cannot mix their gametes © 2017 Pearson Education, Ltd. 19

Figure 22.3-1c (c) Figure 22.3-1c Exploring reproductive barriers (part 1c: temporal isolation, summer) © 2017 Pearson Education, Ltd.

Figure 22.3-1d (d) Figure 22.3-1d Exploring reproductive barriers (part 1d: temporal isolation, winter) © 2017 Pearson Education, Ltd.

Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers © 2017 Pearson Education, Ltd. 22

Figure 22.3-1e (e) Figure 22.3-1e Exploring reproductive barriers (part 1e: behavioral isolation) © 2017 Pearson Education, Ltd.

Mechanical isolation Gametic isolation (f) (g) Figure 22.3-2 Prezygotic barriers Mechanical isolation Gametic isolation MATING ATTEMPT FERTILIZATION Figure 22.3-2 Exploring reproductive barriers (part 2: prezygotic barriers) (f) (g) © 2017 Pearson Education, Ltd.

Mechanical isolation: Morphological differences prevent successful mating © 2017 Pearson Education, Ltd. 25

Figure 22.3-2f (f) Figure 22.3-2f Exploring reproductive barriers (part 2f: mechanical isolation) © 2017 Pearson Education, Ltd.

Gametic isolation: Sperm of one species may not be able to fertilize eggs of another species © 2017 Pearson Education, Ltd. 27

Figure 22.3-2g (g) Figure 22.3-2g Exploring reproductive barriers (part 2g: gametic isolation) © 2017 Pearson Education, Ltd.

Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult by Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown © 2017 Pearson Education, Ltd. 29

Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown (h) Figure 22.3-3 Postzygotic barriers Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown VIABLE, FERTILE OFFSPRING FERTILIZATION (h) (i) (l) Figure 22.3-3 Exploring reproductive barriers (part 3: postzygotic barriers) (j) (k) © 2017 Pearson Education, Ltd.

Reduced hybrid viability: Genes of the different parent species may interact and impair the hybrid’s development or survival © 2017 Pearson Education, Ltd. 31

Figure 22.3-3h (h) Figure 22.3-3h Exploring reproductive barriers (part 3h: reduced hybrid viability) © 2017 Pearson Education, Ltd.

Reduced hybrid fertility: Even if hybrids are vigorous, they may be sterile © 2017 Pearson Education, Ltd. 33

Figure 22.3-3i (i) Figure 22.3-3i Exploring reproductive barriers (part 3i: reduced hybrid fertility, donkey) © 2017 Pearson Education, Ltd.

Figure 22.3-3j (j) Figure 22.3-3j Exploring reproductive barriers (part 3j: reduced hybrid fertility, horse) © 2017 Pearson Education, Ltd.

Figure 22.3-3k (k) Figure 22.3-3k Exploring reproductive barriers (part 3k: reduced hybrid fertility, mule) © 2017 Pearson Education, Ltd.

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 © 2017 Pearson Education, Ltd. 37

Figure 22.3-3l (l) Figure 22.3-3l Exploring reproductive barriers (part 3l: hybrid breakdown) © 2017 Pearson Education, Ltd.

Figure 22.3 Exploring reproductive barriers Prezygotic barriers Postzygotic barriers Habitat isolation Temporal isolation Behavioral isolation Mechanical isolation Gametic isolation Reduced hybrid viability Reduced hybrid fertility Hybrid breakdown VIABLE, FERTILE OFF- SPRING MATING ATTEMPT FERTILI- ZATION (a) (c) (e) (f) (g) (h) (i) (l) (d) (j) (b) Figure 22.3 Exploring reproductive barriers (k) © 2017 Pearson Education, Ltd.

Limitations of the Biological Species Concept The biological species concept cannot be applied to fossils or asexual organisms (including all prokaryotes) The biological species concept emphasizes absence of gene flow However, gene flow can occur between distinct species For example, grizzly bears and polar bears can mate to produce “grolar bears” © 2017 Pearson Education, Ltd. 40

Grizzly bear (U. arctos) Figure 22.4 Grizzly bear (U. arctos) Polar bear (U. maritimus) Figure 22.4 Hybridization between two species of bears in the genus Ursus Hybrid “grolar bear” © 2017 Pearson Education, Ltd.

Grizzly bear (U. arctos) Figure 22.4-1 Figure 22.4-1 Hybridization between two species of bears in the genus Ursus (part 1: grizzly bear) Grizzly bear (U. arctos) © 2017 Pearson Education, Ltd.

Polar bear (U. maritimus) Figure 22.4-2 Figure 22.4-2 Hybridization between two species of bears in the genus Ursus (part 2: polar bear) Polar bear (U. maritimus) © 2017 Pearson Education, Ltd.

Hybrid “grolar bear” Figure 22.4-3 Figure 22.4-3 Hybridization between two species of bears in the genus Ursus (part 3: “grolar” bear) Hybrid “grolar bear” © 2017 Pearson Education, Ltd.

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 and does not rely on information about gene flow but on subjective criteria © 2017 Pearson Education, Ltd. 45

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 © 2017 Pearson Education, Ltd. 46

The phylogenetic species concept defines a species as the smallest group of individuals that share a common ancestor, a single branch 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 © 2017 Pearson Education, Ltd. 47

Concept 22.2: Speciation can take place with or without geographic separation Speciation can occur in two ways Allopatric speciation Sympatric speciation © 2017 Pearson Education, Ltd. 48

(a) Allopatric speciation (b) Sympatric speciation Figure 22.5 Figure 22.5 The geography of speciation (a) Allopatric speciation (b) Sympatric speciation © 2017 Pearson Education, Ltd.

Allopatric (“Other Country”) Speciation In allopatric speciation, gene flow is interrupted when a population is divided into geographically isolated subpopulations For example, the flightless cormorant of the Galápagos likely originated from a flying species on the mainland © 2017 Pearson Education, Ltd. 50

The Process of Allopatric Speciation The definition of a geographic barrier depends on the ability of a population to disperse For example, a canyon may create a barrier for small rodents, but not birds, coyotes, or pollen © 2017 Pearson Education, Ltd. 51

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 For example, mosquitofish in the Bahamas comprise several isolated populations in different ponds Populations in ponds with high predation rates have evolved different body shapes from populations in “low-predation” ponds © 2017 Pearson Education, Ltd. 52

shape that enables rapid bursts of speed Without predators: body Figure 22.6 With predators: body shape that enables rapid bursts of speed Without predators: body shape that favors long, steady swimming (a) Differences in body shape Escape acceleration (m per s2) 110 Survival rate with predators 1.0 100 0.8 90 0.6 80 Figure 22.6 Evolution in mosquitofish populations 0.4 70 0.2 60 50 0.0 Source pond: predators present Source pond: no predators Source pond: predators present Source pond: no predators (b) Differences in escape acceleration and survival © 2017 Pearson Education, Ltd.

(a) Differences in body shape Figure 22.6-1 With predators: body shape that enables rapid bursts of speed Without predators: body shape that favors long, steady swimming Figure 22.6-1 Evolution in mosquitofish populations (part 1: body shape) (a) Differences in body shape © 2017 Pearson Education, Ltd.

shape that enables rapid bursts of speed Figure 22.6-1a With predators: body shape that enables rapid bursts of speed Figure 22.6-1a Evolution in mosquitofish populations (part 1a: body shape under high predation) © 2017 Pearson Education, Ltd.

Without predators: body shape that favors long, steady swimming Figure 22.6-1b Without predators: body shape that favors long, steady swimming Figure 22.6-1b Evolution in mosquitofish populations (part 1b: body shape under low predation) © 2017 Pearson Education, Ltd.

(b) Differences in escape acceleration and survival Figure 22.6-2 Escape acceleration (m per s2) Survival rate with predators 110 1.0 100 0.8 90 0.6 80 0.4 70 0.2 60 Figure 22.6-2 Evolution in mosquitofish populations (part 2: acceleration and survival) 50 0.0 Source pond: predators present Source pond: no predators Source pond: predators present Source pond: no predators (b) Differences in escape acceleration and survival © 2017 Pearson Education, Ltd.

Evidence of Allopatric Speciation Reproductive barriers can develop in lab populations that are experimentally isolated and subjected to different environmental conditions For example, isolated lab populations of fruit flies raised on different diets evolve to digest their food source more efficiently and prefer to mate with partners adapted to the same food source © 2017 Pearson Education, Ltd. 58

Experiment Initial population of fruit flies (Drosophila Figure 22.7 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 22.7 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? population 1 Starch 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 © 2017 Pearson Education, Ltd.

Experiment Initial population of fruit flies (Drosophila Figure 22.7-1 Experiment Initial population of fruit flies (Drosophila pseudoobscura) Some flies raised on starch medium Figure 22.7-1 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? (part 1: experiment) Some flies raised on maltose medium Mating experiments after 40 generations © 2017 Pearson Education, Ltd.

Results Female Female Starch Maltose 22 9 18 15 Male Male 8 20 12 15 Figure 22.7-2 Results Female Female Starch population 1 Starch population 2 Starch Maltose population 1 Starch Starch 22 9 18 15 Male Male population 2 Starch Maltose Figure 22.7-2 Inquiry: Can divergence of allopatric populations lead to reproductive isolation? (part 2: results) 8 20 12 15 Number of matings in experimental group Number of matings in control group © 2017 Pearson Education, Ltd.

Allopatric speciation has also been observed in nature For example, sister species of snapping shrimp (Alpheus) began to diverge 9 to 3 million years ago when they became isolated by the formation of the Isthmus of Panama © 2017 Pearson Education, Ltd. 62

A. formosus A. nuttingi ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN Figure 22.8 A. formosus A. nuttingi ATLANTIC OCEAN Isthmus of Panama PACIFIC OCEAN Figure 22.8 Allopatric speciation in snapping shrimp (Alpheus) A. panamensis A. millsae © 2017 Pearson Education, Ltd.

Figure 22.8-1 A. formosus Figure 22.8-1 Allopatric speciation in snapping shrimp (Alpheus) (part 1a: A. formosus) © 2017 Pearson Education, Ltd.

Figure 22.8-2 A. nuttingi Figure 22.8-2 Allopatric speciation in snapping shrimp (Alpheus) (part 1b: A. nuttingi) © 2017 Pearson Education, Ltd.

Figure 22.8-3 A. panamensis Figure 22.8-3 Allopatric speciation in snapping shrimp (Alpheus) (part 1c: A. panamensis) © 2017 Pearson Education, Ltd.

Figure 22.8-4 A. millsae Figure 22.8-4 Allopatric speciation in snapping shrimp (Alpheus) (part 1d: A. millsae) © 2017 Pearson Education, Ltd.

Regions with many geographic barriers typically have more species than do regions with fewer barriers Reproductive isolation between populations generally increases as the geographic distance between them increases Reproductive isolation is the result of intrinsic barriers; physical separation is not a biological barrier © 2017 Pearson Education, Ltd. 68

Sympatric (“Same Country”) Speciation In sympatric speciation, speciation takes place in populations that live in the same geographic area Sympatric speciation occurs when gene flow is reduced between groups that remain in contact through factors including Polyploidy Habitat differentiation Sexual selection © 2017 Pearson Education, Ltd. 69

Polyploidy Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division Polyploidy is much more common in plants than in animals An autopolyploid is an individual with more than two chromosome sets, derived from one species The offspring of mating between autopolyploids and diploids have reduced fertility © 2017 Pearson Education, Ltd. 70

An allopolyploid is a species with multiple sets of chromosomes derived from different species Allopolyploids can successfully mate with each other but cannot interbreed with either parent species © 2017 Pearson Education, Ltd. 71

Tetraploid cell 4n New species (4n) Figure 22.9 Cell division error 2n = 6 Tetraploid cell 4n Meiosis 2n Figure 22.9 Sympatric speciation by autopolyploidy 2n New species (4n) Gametes produced by tetraploids © 2017 Pearson Education, Ltd.

At least five new plant species have evolved by polyploidy speciation since 1850 For example, three diploid species of goatsbeard plant (Tragopogon) have interbred to produce two new tetraploid species © 2017 Pearson Education, Ltd. 73

Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Normal gamete Figure 22.10-s1 Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Normal gamete n = 2 Figure 22.10-s1 One mechanism for allopolyploid speciation in plants (step 1) © 2017 Pearson Education, Ltd.

Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Normal gamete Figure 22.10-s2 Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Normal gamete n = 2 Sterile hybrid with 5 chromosomes Figure 22.10-s2 One mechanism for allopolyploid speciation in plants (step 2) © 2017 Pearson Education, Ltd.

Mitotic or meiotic error doubles the chromosome number. Figure 22.10-s3 Species A 2n = 6 Species B 2n = 4 Normal gamete n = 3 Normal gamete n = 2 Sterile hybrid with 5 chromosomes Figure 22.10-s3 One mechanism for allopolyploid speciation in plants (step 3) Mitotic or meiotic error doubles the chromosome number. New species: viable, fertile hybrid (allopolyploid) 2n = 10 © 2017 Pearson Education, Ltd.

Many important crops (oats, cotton, potatoes, tobacco, and wheat) are polyploids Plant geneticists can produce new polyploid species using chemicals to induce errors in cell division © 2017 Pearson Education, Ltd. 77

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 Flies that use different host species are reproductively isolated by both habitat and temporal barriers © 2017 Pearson Education, Ltd. 78

Sexual Selection Sexual selection can drive sympatric speciation Sexual selection for mates of different colors has likely contributed to speciation in cichlid fish in Lake Victoria © 2017 Pearson Education, Ltd. 79

T. dubius (12) Hybrid species: T. miscellus (24) Hybrid species: Figure 22.11 T. dubius (12) Hybrid species: T. miscellus (24) Hybrid species: T. mirus (24) Figure 22.11 Allopolyploid speciation in Tragopogon T. pratensis (12) T. porrifolius (12) © 2017 Pearson Education, Ltd.

Allopatric and Sympatric Speciation: A Review In allopatric speciation, geographic isolation restricts gene flow between populations Intrinsic barriers to reproduction 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 by reproductive barriers © 2017 Pearson Education, Ltd. 81

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 © 2017 Pearson Education, Ltd. 82

Concept 22.3: Hybrid zones reveal factors that cause reproductive isolation A hybrid zone is a region in which members of different species mate and produce hybrids Hybrids are the result of mating between species with incomplete reproductive barriers © 2017 Pearson Education, Ltd. 83

Patterns Within Hybrid Zones A hybrid zone can occur in a single band where adjacent species meet For example, two species of toad in the genus Bombina interbreed in a long and narrow hybrid zone © 2017 Pearson Education, Ltd. 84

Monochromatic orange light Figure 22.12 Experiment Monochromatic orange light Normal light P. pundamilia Figure 22.12 Inquiry: Does sexual selection in cichlids result in reproductive isolation? P. nyererei © 2017 Pearson Education, Ltd.

Normal light P. pundamilia Figure 22.12-1 Figure 22.12-1 Inquiry: Does sexual selection in cichlids result in reproductive isolation? (part 1: P. pundamilia, normal) © 2017 Pearson Education, Ltd.

Monochromatic orange light Figure 22.12-2 Monochromatic orange light P. pundamilia Figure 22.12-2 Inquiry: Does sexual selection in cichlids result in reproductive isolation? (part 2: P. pundamilia, monochromatic) © 2017 Pearson Education, Ltd.

Normal light P. nyererei Figure 22.12-3 Figure 22.12-3 Inquiry: Does sexual selection in cichlids result in reproductive isolation? (part 3: P. nyererei, normal) © 2017 Pearson Education, Ltd.

Monochromatic orange light Figure 22.12-4 Monochromatic orange light P. nyererei Figure 22.12-4 Inquiry: Does sexual selection in cichlids result in reproductive isolation? (part 4: P. nyererei, monochromatic) © 2017 Pearson Education, Ltd.

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 © 2017 Pearson Education, Ltd. 90

Hybrid Zones over Time When closely related species meet in a hybrid zone, there are three possible outcomes Reinforcement Fusion Stability © 2017 Pearson Education, Ltd. 91

B. variegata-specific allele Figure 22.13 Fire-bellied toad range Fire-bellied toad, Bombina bombina Hybrid zone Yellow-bellied toad range B. variegata-specific allele Frequency of 0.99 Hybrid zone Figure 22.13 A hybrid zone for Bombina toads in Europe 0.9 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) © 2017 Pearson Education, Ltd.

Fire-bellied toad range Hybrid zone Yellow-bellied toad range Figure 22.13-1 Fire-bellied toad range Figure 22.13-1 A hybrid zone for Bombina toads in Europe (part 1: map) Hybrid zone Yellow-bellied toad range © 2017 Pearson Education, Ltd.

B. variegata-specific allele Figure 22.13-2 0.99 B. variegata-specific allele Frequency of Hybrid zone 0.9 0.5 Yellow-bellied toad range Fire-bellied toad range 0.1 Figure 22.13-2 A hybrid zone for Bombina toads in Europe (part 2: graph) 0.01 40 30 20 10 10 20 Distance from hybrid zone center (km) © 2017 Pearson Education, Ltd.

Yellow-bellied toad, Bombina variegata Figure 22.13-3 Figure 22.13-3 A hybrid zone for Bombina toads in Europe (part 3: B. variegata) Yellow-bellied toad, Bombina variegata © 2017 Pearson Education, Ltd.

Fire-bellied toad, Bombina bombina Figure 22.13-4 Figure 22.13-4 A hybrid zone for Bombina toads in Europe (part 4: B. bombina) Fire-bellied toad, Bombina bombina © 2017 Pearson Education, Ltd.

Reinforcement occurs when hybrids are less fit than the parent species Natural selection strengthens (reinforces) reproductive barriers, and over time the rate of hybridization decreases Where reinforcement occurs, reproductive barriers should be stronger for sympatric than for allopatric species © 2017 Pearson Education, Ltd. 97

Fusion of the parent species into a single species may occur if hybrids are as fit as parents, allowing substantial gene flow between species For example, fusion of cichlid species in Lake Victoria may be occurring because water pollution has reduced the ability of female cichlids to distinguish males of different species by color © 2017 Pearson Education, Ltd. 98

Gene flow Population Barrier to gene flow Figure 22.14-s1 Figure 22.14-s1 Formation of a hybrid zone and possible outcomes for hybrids over time (step 1) Barrier to gene flow © 2017 Pearson Education, Ltd.

Isolated population diverges. Gene flow Population Barrier to Figure 22.14-s2 Isolated population diverges. Gene flow Population Figure 22.14-s2 Formation of a hybrid zone and possible outcomes for hybrids over time (step 2) Barrier to gene flow © 2017 Pearson Education, Ltd.

Isolated population diverges. Hybrid zone Gene flow Population Figure 22.14-s3 Isolated population diverges. Hybrid zone Gene flow Population Figure 22.14-s3 Formation of a hybrid zone and possible outcomes for hybrids over time (step 3) Barrier to gene flow Hybrid individual © 2017 Pearson Education, Ltd.

Isolated population diverges. Possible outcomes: Hybrid zone Figure 22.14-s4 Isolated population diverges. Possible outcomes: Hybrid zone Reinforcement Gene flow Fusion Population Figure 22.14-s4 Formation of a hybrid zone and possible outcomes for hybrids over time (step 4) Barrier to gene flow Hybrid individual Stability © 2017 Pearson Education, Ltd.

Stability of the hybrid zone will result if hybrids continue to be produced over time This may occur if hybrids are at least as likely to survive and reproduce as their parent species If they are not, gene flow from outside may overwhelm selection for increased reproductive isolation inside the hybrid zone © 2017 Pearson Education, Ltd. 103

Concept 22.4: 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 © 2017 Pearson Education, Ltd. 104

The Time Course of Speciation Broad patterns in speciation can be studied using the fossil record, morphological data, or molecular data © 2017 Pearson Education, Ltd. 105

Patterns in the Fossil Record The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then disappear These periods of apparent stasis punctuated by sudden change are called punctuated equilibria The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence © 2017 Pearson Education, Ltd. 106

Pundamilia pundamilia Figure 22.15 Pundamilia nyererei Pundamilia pundamilia Figure 22.15 Fusion: the breakdown of reproductive barriers Pundamilia “turbid water,” hybrid offspring from a location with turbid water © 2017 Pearson Education, Ltd.

Pundamilia nyererei Figure 22.15-1 Figure 22.15-1 Fusion: the breakdown of reproductive barriers (part 1: P. nyererei) Pundamilia nyererei © 2017 Pearson Education, Ltd.

Pundamilia pundamilia Figure 22.15-2 Figure 22.15-2 Fusion: the breakdown of reproductive barriers (part 2: P. pundamilia) Pundamilia pundamilia © 2017 Pearson Education, Ltd.

Pundamilia “turbid water,” hybrid offspring from a location Figure 22.15-3 Figure 22.15-3 Fusion: the breakdown of reproductive barriers (part 3: P. “turbid water”) Pundamilia “turbid water,” hybrid offspring from a location with turbid water © 2017 Pearson Education, Ltd.

Speciation Rates The punctuated pattern in the fossil record and evidence from lab studies suggest that speciation can be rapid For example, the sunflower Helianthus anomalus originated from the hybridization of two other sunflower species and quickly diverged into a new species © 2017 Pearson Education, Ltd. 111

(a) Punctuated model Time (b) Gradual model Figure 22.16 Figure 22.16 Two models for the tempo of speciation (b) Gradual model © 2017 Pearson Education, Ltd.

Figure 22.17 A hybrid sunflower species and its dry sand dune habitat © 2017 Pearson Education, Ltd.

The interval between documented speciation events ranges from 4,000 years (some cichlids) to 40 million years (some beetles), with an average of 6.5 million years © 2017 Pearson Education, Ltd. 114

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 in question, speciation might require the change of only a single gene or many genes For example, in Japanese Euhadra snails, the direction of shell spiral affects mating and is controlled by a single gene © 2017 Pearson Education, Ltd. 115

In monkey flowers (Mimulus), two genes affect flower color, which influences pollinator attraction Monkey flowers with different alleles at the flower color loci are less likely to interbreed because they attract different species of pollinators © 2017 Pearson Education, Ltd. 116

chromosomes are shown) Figure 22.18 Experiment H. annuus gamete H. petiolarus gamete F1 experimental hybrid (4 of the 2n = 34 chromosomes are shown) Results H. anomalus Chromosome 1 Figure 22.18 Inquiry: How does hybridization lead to speciation in sunflowers? Experimental hybrid H. anomalus Chromosome 2 Experimental hybrid H. annuus–specific marker H. petiolarus–specific marker © 2017 Pearson Education, Ltd.

In other organisms, speciation can be influenced by larger numbers of genes and gene interactions For example, isolation between species of sunflowers in the genus Helianthus is influenced by at least 26 chromosome sections (number of genes unknown) © 2017 Pearson Education, Ltd. 118

From Speciation to Macroevolution Macroevolution is large-scale evolutionary change resulting from the cumulative effect of many speciation and extinction events © 2017 Pearson Education, Ltd. 119

(b) M. lewisii with M. cardinalis allele Figure 22.19 (a) Mimulus lewisii (b) M. lewisii with M. cardinalis allele Figure 22.19 A locus that influences pollinator choice (c) Mimulus cardinalis (d) M. cardinalis with M. lewisii allele © 2017 Pearson Education, Ltd.

(a) Mimulus lewisii Figure 22.19-1 Figure 22.19-1 A locus that influences pollinator choice (part 1: M. lewisii) (a) Mimulus lewisii © 2017 Pearson Education, Ltd.

(b) M. lewisii with M. cardinalis allele Figure 22.19-2 Figure 22.19-2 A locus that influences pollinator choice (part 2: M. lewisii with M. cardinalis allele) (b) M. lewisii with M. cardinalis allele © 2017 Pearson Education, Ltd.

(c) Mimulus cardinalis Figure 22.19-3 Figure 22.19-3 A locus that influences pollinator choice (part 3: M. cardinalis) (c) Mimulus cardinalis © 2017 Pearson Education, Ltd.

(d) M. cardinalis with M. lewisii allele Figure 22.19-4 Figure 22.19-4 A locus that influences pollinator choice (part 4: M. cardinalis with M. lewisii allele) (d) M. cardinalis with M. lewisii allele © 2017 Pearson Education, Ltd.

Figure 22.UN01-1 Figure 22.UN01-1 Skills exercise: identifying independent and dependent variables, making a scatter plot, and interpreting data (part 1) © 2017 Pearson Education, Ltd.

Figure 22.UN01-2 Figure 22.UN01-2 Skills exercise: identifying independent and dependent variables, making a scatter plot, and interpreting data (part 2) © 2017 Pearson Education, Ltd.

Allopatric speciation Sympatric speciation Figure 22.UN02 Original population Figure 22.UN02 Summary of key concepts: speciation and geographic separation Allopatric speciation Sympatric speciation © 2017 Pearson Education, Ltd.

Ancestral species: AA BB DD Triticum monococcum (14) Wild Triticum Figure 22.UN03 Ancestral species: AA BB DD Triticum monococcum (14) Wild Triticum (14) Wild T. tauschii (14) Product: Figure 22.UN03 Test your understanding, question 7 (polyploidy) AA BB DD T. aestivum (bread wheat) (42) © 2017 Pearson Education, Ltd.

Figure 22.UN04 Figure 22.UN04 Test your understanding, question 11 (strawberry poison dart frog, Dendrobates pumilio) © 2017 Pearson Education, Ltd.