Chapter 17: Speciation - Checking Understanding Assignment

Chapter 17: Speciation - Checking Understanding Assignment
Directions: This assignment must be completed and turned in at the end of class today.
1. You may only use your chapter 17 notes and lab partners as resources.
2. As always, use of proper terminology and original phrasing in your answers is expected.
3. Examples from the book may be used to supplement an explanation, but not to replace one.
4. One assignment per group will be collected, but all individuals record their answer for their notes.
A ring species is 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.
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.
The geographical variation in songs of greenish warblers provides a rare illustration of gradual change in a trait.
On the next page 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.
1. What is the significance of the changing song in the evolution of the greenish warblers?
2. Does this example of ring speciation represent allopatric or sympatric speciation?
3. When comparing the “head” species to the “tail” species in the ring, has a prezygotic or postzygotic barrier been
established? Explain.
4. Which specific mode of reproductive isolation has led to the separation of head and tail species? (temporal,
mechanical, etc)
5. Explain how it came to be that the head and tail species can no longer reproduce, yet many variations along the
ring’s path are reproductively compatible with one another.
6. Which concept of species is being used to define the “head” and “tail” of the ring as separate species? Explain.
7. Briefly discuss the three different concepts of a species that are discussed in the book and address any
advantages/disadvantages each concept might have.
8. In what way is the biological concept of a species, implied or at least relevant to the other two?
9. Explain, to the extent that was discussed in the book, the molecular basis for hybrid sterility. In other words,
why causes hybrids of some species to be reproductively sterile?
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