Allopatric Speciation

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[h] Allopatric Speciation Interactive Reading

[q] Until about 2.8 million years ago, there was a gap between the southern tip of North America and the northern tip of South America. This gap allowed water to flow from the Pacific ocean to the Atlantic ocean, and vice versa. But then, movement of the continents created a land bridge between the two continents: the Isthmus of Panama (shown below).

[q] Today, two closely related species of shrimp are found on each side of the Isthmus. Under natural conditions, they have no contact. But when they’re experimentally brought together, these two species don’t interbreed. Instead, they aggressively snap their claws at one another. (source: Biology, Starr, Taggart, Evers, Starr, 14th ed.)

In your student learning guide, use what you’ve learned about species, reproductive barriers, natural selection, and population genetics, to write a short explanation of what’s going on. This is just a preliminary idea, so speculate!

Click “continue” when you’re ready.

[q]

The image at left shows a model for explaining what you encountered on the previous card. It’s called allopatric speciation.  The “allo” prefix means “other” or “another.” The “patric” stem means “country” (this stem also appears in words like patriotic).

Take a moment and see if you can think through an explanation of what happened in the isthmus of Panama, using the the model below. Go ahead and give it a try, preferably by talking it through with a partner (or even saying it out loud to yourself). Click “continue” when you’re ready.

 

[fx] No, that’s not correct. Please try again.

[q] Here’s the same image that started this interactive reading. If you’re understanding allopatric speciation, you should be  be able to write a short paragraph explaining the speciation of shrimp that followed the formation of the isthmus of Panama. Give it a try, and then compare your answer to mine.

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[qwiz qrecord_id=”sciencemusicvideosMeister1961-Peripatric Speciation”]

[h]Peripatric Speciation

[i]

[q]The small sub-population on the periphery is probably already going to be somewhat genetically and phenotypically  [hangman] from the main population at “1.”

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[q]Because the sub-population on the periphery is small, it’s going to experience a lot more [hangman] drift than a larger population would.

 

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[q]Because the sub-population is on the periphery of its parent population’s range, it’s going to experience the most different conditions, and therefore be subject to different types of natural [hangman], further changing its gene pool.

 

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[q]As different as the peripheral sub-population might become, speciation will only proceed if the sub-population at 2 becomes geographically [hangman] from its parent population.

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[q]If step 3 has resulted in speciation, then you could predict that if the members of the blue population and green population came back into contact with one another, they’d no longer be able to [hangman]

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[x][restart]

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[h]Variation in Yarrow in a Sierra Transect

[q]To continue our study of speciation, let’s do some (imaginary) plant collecting.

We’re going to walk in a line across a California mountain range.  As we go, we’ll measure the height of a wildflower called Yarrow (Achillea millefolium). Yarrow ranges from 0.2 to 1.0 meters in height.

[q]

Our transect will start in Mather, California, in the foothills on the west side of the Sierra Nevada mountain range. Heading east, we’ll gain elevation as we pass through Aspen Valley, Yosemite Creek, and so on. On the East side of the Sierra summit, we’ll pass through through Bighorn Lake, Conway Summit, and Leevining. 

Note the changes in elevation along our route. Predict how you think the height of Yarrow is going to vary along this West-East transect, and list a few reasons why.

[q]Here’s how the height of yarrow varies.

Why? Environmental conditions vary as you move up and down the slopes of the Sierra. Two major variables are snowfall, which increases with elevation, and temperature, with much lower winter temperatures at higher elevations. Put these two together, and plants at higher elevation have a much shorter growing season.

But what you’re seeing above isn’t just the environmental response. The heights recorded above each study area are the mean heights when seeds from plants in each study area were grown at sea level (at Stanford University). It’s not as if the plants in each study area are genetically identical, and have different heights as a result of different environmental conditions. Rather, what you’re looking at is a gradual shift in genetic variation that changes along a geographic axis. What you’re looking at, in other words, is called an ecocline (or just cline):  (Sources:ScienceBlogs; Ecotypes ; Biology, Solomon, Martin, Martin, and Berg)

[q]True or false: The yarrow collected at Tuolumne meadows is shorter than the yarrow from Mather only because it has a shorter growing season. Genetically, the two populations have similar genes for height.

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[q]The idea of a cline is that each subpopulation along the transect differs in two ways. 1) In terms of appearance (or [hangman], and 2) in terms of underlying genetic structure. In other words, you could expect [hangman] frequencies to differ in each subpopulation.

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[h]Hybrid Zones

[q]Hybrid Zone: North American Orioles

 

[q]Hybrid Zone: Swallowtail Butterflies

[q]Hybrid Zone: Redstarts, South America

[q]Hybrid Zone: European Crows and Magpies

[x]Continue reading below to learn about another phenomenon associated with allopatric speciation.

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[qwiz qrecord_id=”sciencemusicvideosMeister1961-Ring Species, Interactive Reading”]

[h]Ring Species

[i]

[q]Ring species also involve clinal variation, gene exchange, and reproductive isolation.

Ensatina eschscholtzii is species of salamander whose range extends from Baja California (in Mexico) up to British Columbia (in Canada). Across its geographic range, E. eschscholtzii  has diversified into several subspecies (numbered 1 through 7). While each subspecies can interbreed with the neighboring population (subspecies 1 can breed with 2, 2 with 3, and so on, no interbreeding is possible between subspecies 1 and subspecies 7.

Note that the one place where Ensatina salamanders can’t live is California’s Central Valley, which is much hotter and drier than either the California coast ranges, the Sierra Nevada mountains, or the mountains of Oregon, Washington, and British Columbia.

Based on what you know about speciation, see if you can explain what’s going on. Click “continue” when you’re ready.

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[qwiz random = “true” qrecord_id=”sciencemusicvideosMeister1961-Allopatric Speciation CFU”] [h]

Allopatric Speciation (and its variations): Checking Understanding

[i]

 

[q] Speciation that involves a geographical barrier is

[hangman]

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[q] In the diagram below, which phase shows when a geographic barrier first subdivides a population into two isolated gene pools. 

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[q]In the diagram below, which letter or number represents gene flow?

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[q]By stage 4, there are two species that can no longer___________ .

[hangman ]

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[q]The basic idea of allopatric speciation is that ______________ isolation leads to genetic differentiation, which leads to reproductive isolation.

[hangman ]

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[q]The basic idea of allopatric speciation is that geographical isolation leads to genetic differentiation, which leads to ___________ isolation.

[hangman ]

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[q]The diagram below represents _______________ speciation.

[hangman]

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[q]A ______ is a gradual shift in genetic variation that occurs along a geographical axis.

[hangman ]

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[q]A cline is a gradual shift in genetic ____________ that occurs along a geographical axis.

[hangman ]

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[q]Number 3 below represents a _________ zone.

[hangman]

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[q]The model below is a representation of ____________ speciation.

[hangman]

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[q]In the speciation model below, the gradual shift between green and blue in diagram 2 is called a ________.

[hangman]

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[q]The complex of E. eschscholtzii subspecies shown below  is called a ________ species.

[hangman]

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[q]A key characteristic of the E. eschscholtzii subspecies complex shown below is the inability of subspecies 1 and subspecies 7 to __________.

[hangman]

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[q]In the E. eschscholtzii ring species complex shown below, if it weren’t for the gene flow between all of the adjacent subspecies, you could almost think of subspecies 1 and 7 as separate  __________.

[hangman]

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[x][restart]

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