Chapter 17 Opener

Figure 24.5 A summary of reproductive barriers between closely related species

Figure 24.6 Two modes of speciation

Figure 24.7 Allopatric speciation of squirrels in the Grand Canyon

Figure 24.8 Has speciation occurred during geographic isolation?

Figure 24.9 Ensatina eschscholtzii, a ring species

Figure 17.1 Members of the Same Species Look Alike—or Not

Figure 17.1 Members of the Same Species Look Alike—or Not (Part 1)

Figure 17.1 Members of the Same Species Look Alike—or Not (Part 2)

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed (Part 1)

Figure 17.2 Cryptic Species Look Alike but Do Not Interbreed (Part 2)

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life A ring species the name given to a ring of populations that encircles an area of unsuitable habitat. Ring species provide a unique glimpse into one way in which new species arise. Ring species acquire new traits as the distance from their ancestral home increases; when the “head” of the ring meets the “tail,” they have become two distinct species. A ring species is therefore a ring of populations in which there is only one place where two distinct species meet. At one location within the ring, two distinct forms coexist but do not interbreed. Around the rest of the ring, the traits of one species change gradually through intermediate populations, eventually changing so much that it they are considered a separate (second) species. Ring species allow us to examine variation in space and use it to infer how changes occurred over time. INSTRUCTOR NOTES: This and the following two slides could alternatively be handed out as a class discussion exercise prior to the final slide assessing the understanding of the concept. In the figure, interbreeding populations are represented by colored blocks. Variation along a cline may bend all the way around forming a ring. IMAGE SOURCE: Wikimedia Commons

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life One example of a ring species is the Greenish warbler (Phylloscopus trochiloides), a bird native to parts of Asia and eastern Europe. These songbirds breed in the forests of central and northern Asia and eastern Europe. In the center of Asia, the large region of desert, including the Tibetan Plateau and the Taklamakan and Gobi Deserts, are inhospitable habitats for these birds. As a result, their distribution follows a ring of mountains that surrounds the desert, including the forests of Siberia to the north. Greenish warblers have remarkable geographic variation in plumage patterns, songs, and genetics. Male warbler song is crucial to their successful reproduction. In birds, males usually react aggressively to other males singing the same type of song, since these are potential competitors for female mates. IMAGE SOURCE: Wikimedia Commons

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life The geographical variation in songs of greenish warblers provides a rare illustration of gradual change in a trait. To the right are examples of song spectrograms (a pictorial representation of their song pattern) from 12 populations of greenish warblers. Spectrograms are arranged in geographic order according to the ring species configuration, starting in western Siberia and moving south, then east, then north around the ring. Note the gradual change in song length and syntax (wave shape) around the ring. REFERENCE: Irwin, D. E. 2000. Song variation in an avian ring species. Evolution 54 (3): 998–1010.

Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision Referring to the figure, if female warblers from populations JL do not recognize the songs of males in population XN, they are unable to pair with them. This is an example of the evolution of reproductive isolation. a. True b. False c. I don’t understand this question. Answer: a (The song differences have caused reproductive isolation.) [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don’t know” or “I don’t understand this question” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] REFERENCE: Irwin, D. E. 2000. Song variation in an avian ring species. Evolution 54 (3): 998–1010. 18

Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision When the female warblers from population JL do not recognize the songs of males in population XN, and therefore do not mate with them, what processes could have occurred? Discuss. Answer: Speciation, reproductive isolation, evolution INSTRUCTOR NOTES: Speciation is essentially the evolution of reproductive isolation between two populations, and song differences can cause reproductive isolation. Hence, the geographical variation in songs of Greenish warblers provides a rare illustration of how gradual change in a trait can cause speciation. Speciation is a mechanism of evolution. Ernst Mayr called ring species “the perfect demonstration of speciation” because they show a range of intermediate forms between two species. Ernst Mayr proposed the biological species concept (see page 333 of the textbook). Greenish warblers illustrate three fundamental ways that ring species can teach us about evolution: Ring species provide strong evidence for evolution causing the appearance of new species, demonstrating that many small changes can eventually accumulate into large differences between distinct species. Some critics of evolutionary theory think that evolution can only cause limited change within a species and cannot lead to the evolution of new species. Ring species provide evidence that evolution can indeed to the formation of new species. Variation between species is qualitatively similar, though different in degree, to variation within a species. Ring species allow a reconstruction of the history and causes of divergence during speciation, since spatial variation may illustrate change through time. Without the rings of populations connecting the terminal forms, we would have little understanding of the history of divergence of greenish warbler songs. Ring species provide evidence that speciation can occur without complete geographic isolation. As discussed at the beginning of this article, the prevailing view of speciation has been that two populations must become geographically isolated, such that they do not exchange genes, before speciation can occur (this process is called ‘allopatric speciation’). Ring species, however, show that the ends of a long chain of interbreeding populations can diverge to the point that they do not directly interbreed, even though genes can travel between them through the intermediate populations (in other words, they are connected by ‘gene flow’). REFERENCE: Irwin, D. E. 2000. Song variation in an avian ring species. Evolution 54 (3): 998–1010. 19

Concept 17.2 Speciation Is a Natural Consequence of Population Subdivision When the female warblers from population JL do not recognize the songs of males in population XN, and therefore do not mate with them, what processes could have occurred? a. Speciation b. Reproductive isolation c. Evolution d. All of the above e. None of the above Answer: d INSTRUCTOR NOTES: Speciation is essentially the evolution of reproductive isolation between two populations, and song differences can cause reproductive isolation. Hence, the geographical variation in songs of Greenish warblers provides a rare illustration of how gradual change in a trait can cause speciation. Speciation is a mechanism of evolution. Ernst Mayr called ring species “the perfect demonstration of speciation” because they show a range of intermediate forms between two species. Ernst Mayr proposed the biological species concept (see page 333 of the textbook). Greenish warblers illustrate three fundamental ways that ring species can teach us about evolution: Ring species provide strong evidence for evolution causing the appearance of new species, demonstrating that many small changes can eventually accumulate into large differences between distinct species. Some critics of evolutionary theory think that evolution can only cause limited change within a species and cannot lead to the evolution of new species. Ring species provide evidence that evolution can indeed to the formation of new species. Variation between species is qualitatively similar, though different in degree, to variation within a species. Ring species allow a reconstruction of the history and causes of divergence during speciation, since spatial variation may illustrate change through time. Without the rings of populations connecting the terminal forms, we would have little understanding of the history of divergence of greenish warbler songs. Ring species provide evidence that speciation can occur without complete geographic isolation. As discussed at the beginning of this article, the prevailing view of speciation has been that two populations must become geographically isolated, such that they do not exchange genes, before speciation can occur (this process is called ‘allopatric speciation’). Ring species, however, show that the ends of a long chain of interbreeding populations can diverge to the point that they do not directly interbreed, even though genes can travel between them through the intermediate populations (in other words, they are connected by ‘gene flow’). REFERENCE: Irwin, D. E. 2000. Song variation in an avian ring species. Evolution 54 (3): 998–1010. 20

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life, and 17.2 Speciation Is a Natural Consequence of Population Subdivision INSTRUCTOR NOTES: This is a summary slide of the case study as presented thus far, for optional use as a slide to summarize Concepts 17.1 and 17.2 questions and answers. In Siberia, two distinct forms of Greenish warblers coexist, one in the west and one in the east, their distributions narrowly overlapping in central Siberia, where they do not interbreed. These forms differ in color patterns, the songs that males sing to attract mates, and genetic characteristics. Also, males of each form usually do not recognize the song of the other form, but respond strongly to their own. The traits that differ between the two Siberian forms change gradually through the chain of populations encircling the Tibetan Plateau to the south. Thus two distinct species are connected by gradual variation in morphological, behavioral, and genetic traits. DNA evidence points to an ancestor somewhere in the Himalayas. The pattern of song variation is particularly interesting: Songs are short and simple in the south, but to the north songs become gradually longer and more complex along both pathways into Siberia. However, songs have also become different in structure, resulting in distinct differences in songs between the Siberian forms. Song patterns changed as new species emerged. The birds distinguish between these differences; males respond aggressively to tape recordings of their own songs, thinking that another male has invaded their territory, but they do not respond to songs of the other form. In most species of songbirds, songs play an important role in mate choice; usually, only males sing, and females listen to songs when deciding which male to choose as a mate. IMAGE SOURCE: The San Francisco Chronicle

Figure 17.3 The Dobzhansky–Muller Model

Figure 17.3 The Dobzhansky–Muller Model

Figure 17.4 Speciation by Centric Fusion

Figure 17.4 Speciation by Centric Fusion

Figure 17.4 Speciation by Centric Fusion (Part 1)

Figure 17.4 Speciation by Centric Fusion (Part 2)

Figure 17.5 Reproductive Isolation Increases with Genetic Divergence

Figure 17.5 Reproductive Isolation Increases with Genetic Divergence

Figure 17.6 Allopatric Speciation

Figure 17.6 Allopatric Speciation

Figure 17.6 Allopatric Speciation (Part 1)

Figure 17.6 Allopatric Speciation (Part 2)

Concept 17.1 Species Are Reproductively Isolated Lineages on the Tree of Life, and Speciation Is a Natural Consequence of Population Subdivision INSTRUCTOR NOTES: This is a summary slide of the case study as presented thus far, for optional use as a slide to summarize Concepts 17.1 and 17.2 questions and answers. In Siberia, two distinct forms of Greenish warblers coexist, one in the west and one in the east, their distributions narrowly overlapping in central Siberia, where they do not interbreed. These forms differ in color patterns, the songs that males sing to attract mates, and genetic characteristics. Also, males of each form usually do not recognize the song of the other form, but respond strongly to their own. The traits that differ between the two Siberian forms change gradually through the chain of populations encircling the Tibetan Plateau to the south. Thus two distinct species are connected by gradual variation in morphological, behavioral, and genetic traits. DNA evidence points to an ancestor somewhere in the Himalayas. The pattern of song variation is particularly interesting: Songs are short and simple in the south, but to the north songs become gradually longer and more complex along both pathways into Siberia. However, songs have also become different in structure, resulting in distinct differences in songs between the Siberian forms. Song patterns changed as new species emerged. The birds distinguish between these differences; males respond aggressively to tape recordings of their own songs, thinking that another male has invaded their territory, but they do not respond to songs of the other form. In most species of songbirds, songs play an important role in mate choice; usually, only males sing, and females listen to songs when deciding which male to choose as a mate. IMAGE SOURCE: The San Francisco Chronicle

Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry Refer to the case study just discussed about Greenish warblers. Does this example of ring speciation represent allopatric or sympatric speciation? Recall that allopatric speciation requires a geographic barrier, while sympatric does not.

Concept 17.3 Speciation May Occur through Geographic Isolation or in Sympatry The ring speciation of Greenish warblers is a variation of allopatric speciation (requiring geographic isolation). a. True b. False c. I don’t know. Answer: a This is tricky but ring speciation is generally considered to be an example of geographic isolation. The geographic barrier is in this case the centre of the ring, which prevents the populations at the “head” of the ring from breeding with those at the “tail” of the ring since by the time they meet again, they have become reproductively isolated. Nevertheless, there is not complete geographic isolation between the head and tail of the ring. The separate species are really only sympatric after they speciate—the biogeographic ring occurs as a chronological and spatial process. INSTRUCTOR NOTES: Students could make the case for either a or b being correct, depending upon how they articulate their arguments. The point is to generate discussion and see if they are understanding the process of speciation and the difference between allopatric and sympatric speciation, as well as whether they understand how ring speciation is a special case and how it works. So it is recommended that students get participation points only for this question, and are not marked as wrong if answer B is chosen. [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don’t know” or “I don’t understand this question” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.]

Apply the concept page 338 Speciation may occur through geographic isolation The different species of Darwin’s finches shown in the phylogeny in figure 17.7 have all evolved on islands of the Galapagos archipelago within the past 3 million years. Molecular clock analysis has been used to determine the dates of the various speciation events in that phylogeny. Geological techniques for dating rock sample have been used to determine the ages of the various Galapagos islands. The table show the number of species of Darwin’s finches and the number of islands that have existed in the archipelago at several times during the past 4 million years.

Plot the number of species of Darwin’s finches and the number of islands in the Galapagos archipelago (dependent variables) as a function of time (independent variable). Are the data consistent with the hypothesis that isolation of populations on newly formed islands is related to speciation in this group of birds? Why or why not? If no more islands form in the Galapagos archipelago, do you think that speciation by geographic isolation will continue to occur among Darwin’s finches? Why or why not? What additional data could you collect to test your hypothesis (without waiting to see if speciation occurs)?

Apply the Concept, Ch. 17, p. 338

Figure 17.7 Allopatric Speciation among Darwin’s Finches

Figure 17.7 Allopatric Speciation among Darwin’s Finches

Figure 17.8 Mechanical Isolation through Mimicry

Figure 17.9 Temporal Isolation of Breeding Seasons

Figure 17.9 Temporal Isolation of Breeding Seasons (Part 1)

Figure 17.9 Temporal Isolation of Breeding Seasons (Part 2)

Figure 17.10 Behavioral Isolation in Mating Calls

Figure 17.10 Behavioral Isolation in Mating Calls

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 1)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 2)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 3)

Figure 17.11 Floral Morphology Is Associated with Pollinator Morphology (Part 4)

Figure 17.12 Flower Color and Reproductive Isolation

Figure 17.12 Flower Color and Reproductive Isolation (Part 1)

Figure 17.12 Flower Color and Reproductive Isolation (Part 2)

Apply the concept page 344 Reproductive isolation is reinforced when diverging species come into contact As shown in Fig. 17.9, the leopard frogs Rana berlandieri and R. sphenocephala usually have non-overlapping breeding seasons in areas of sympatry, but where the species are allopatric, both species breed in both spring and fall. But when new ponds are created where the ranges of the two species come together, frogs from previously allopatric populations may colonize the new ponds and hybridize during their overlapping breeding seasons. Imagine you have collected data on hybridization between these two frog species. You have sampled various life stages of frogs and their tadpoles for two years after an initial spring breeding season at a newly established pond.

Create four pie charts (one for each life stage) showing the percentage of each species and the percentage of hybrids at each stage. What are some possible reasons for the differences in the percentage of hybrids found at each life stage? Suggest some postzygotic isolating mechanisms that are consistent with your data. Over time, what changes might you expect in the breeding seasons of the two species at this particular pond, and why? How would future pie charts change if your predictions about breeding seasons are correct?

Apply the Concept, Ch. 17, p. 344

Figure 17.13 A Hybrid Zone

Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact If lack of recognition of male song by the female Greenish warblers is the main mode by which reproductive isolation occurs, is this an example of a prezygotic or postzygotic isolating mechanism? 60

Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact If lack of recognition of male song by the female Greenish warblers is the main mode by which reproductive isolation occurs, this is an example of a postzygotic isolating mechanism. a. True b. False c. I don’t know. Answer: b (This is a prezygotic isolating mechanism since it prevents the males and females from mating with one another—females don’t recognize the male song of the new species and so don’t mate with these males.) [NOTE TO THE INSTRUCTOR: It can be useful to include an “I don’t know” or “I don’t understand this question” choice with clickers, because it can help you discover how many students really haven’t understood the concept at all. Use of this option may depend on whether you assign participation-only points or performance points (or some combination) to clicker questions in your course. If you only assign participation points, it may be useful to leave the “I don't know” choice in the question, as it gives students a penalty-free way of indicating that more time may be needed on this concept.] 61

Concept 17.4 Reproductive Isolation Is Reinforced When Diverging Species Come into Contact If lack of recognition of male song by the female Greenish warblers is the main mode by which reproductive isolation occurs, this is an example of what type of prezygotic isolating mechanism? a. Mechanical b. Temporal c. Behavioral d. Habitat isolation e. Gametic isolation Answer: c (Songs are a behavioral mechanism by which reproduction is facilitated.) 62

Figure 17.14 Evolution in the Laboratory