Link to Species and Speciation Student Learning Guide handout.

1. Introduction

In the previous tutorial, we explored the biological species concept, and the barriers that keep closely related species apart. Now we’ll turn our focus to speciation: the process by which new species arise.

2. Interactive Reading: Allopatric Speciation

To get you thinking about how speciation occurs, let’s start with a case study.

[qwiz qrecord_id=”sciencemusicvideosMeister1961-Allopatric Speciation: Interactive Reading”]

[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 that allowed water to flow from the Pacific to the Atlantic ocean, and vice versa. But then, movement of tectonic plates created a land bridge between the two continents: the Isthmus of Panama (shown below).

Today, two closely related species of shrimp are found on each side of the Isthmus. When experimentally brought together, these two species don’t interbreed, but, rather, aggressively snap their claws at one another. (source: Biology, Starr, Taggart, Evers, Starr, 14th ed.)

Using what you’ve learned about species, reproductive barriers, natural selection, and population genetics, write a short paragraph explaining what’s going on.

Click “continue” when you’re ready.

[q]The image below shows a model for explaining what you encountered on the previous card. It’s called allopatric speciation. Let’s take the word Allopatric apart. The “allo” prefix means “other” or “another.” The “patric” stem means “country” (this stem also appears in words like patriotic).

Before seeing my explanation, you should 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.

[q labels = “top”]Now let’s look at this model, step by step.

In stage one there’s only one __________  with two sub populations. What maintains the sub- populations as part of a single species is the free flow of _________ from one population to the other.

In stage two, a _________ disrupts gene flow. In each sub-population, processes like ___________ lead distinct variants to arise.

Thinking about this in terms of population genetics, you might say that ________ frequencies are starting to differ between these two sub-populations.

[l]allele

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

[f*] Great!

[l]barrier

[fx] No. Please try again.

[f*] Good!

[l]genes

[fx] No. Please try again.

[f*] Excellent!

[l]mutation

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

[f*] Correct!

[l]species

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

[f*] Excellent!

[q labels = “top”]

Note that in stage three, the environment has changed. Now, each population will be subject to different _________ pressures. This can lead to further _____________ between each sub-population’s ________ pool

In stage four, the geographic barrier has been _________. This can involve a  _______ drying up, or a mountain range _______ away. When the two populations encounter each other again, they’ve evolved into two distinct _______ that are ______________ isolated from one another, and which can no longer ____________.

[l]differentiation

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

[f*] Great!

[l]eroding

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

[f*] Great!

[l]gene

[fx] No. Please try again.

[f*] Correct!

[l]interbreed

[fx] No. Please try again.

[f*] Good!

[l]removed

[fx] No. Please try again.

[f*] Correct!

[l]reproductively

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

[f*] Correct!

[l]river

[fx] No. Please try again.

[f*] Good!

[l]selective

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

[f*] Excellent!

[l]species

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

[f*] Good!

[q labels = “top”]

Here’s a quick summary of the allopatric model of speciation: _____________ isolation leads to ___________ differentiation, which results in ____________ isolation.

[l]genetic

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

[f*] Good!

[l]geographical

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

[f*] Good!

[l]reproductive

[fx] No. Please try again.

[f*] Great!

[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.

[c*]Show the answer

[f]Originally, the two species of shrimp were one species, connected by gene flow. When the isthmus formed, gene flow was no longer possible. Random mutation, along with different selective pressures on each side of the isthmus, led the Atlantic and Pacific populations to differentiate. Now, when they’re placed together, they no longer interbreed. In other words, the initial geographic isolation led to genetic differentiation that has resulted in reproductive isolation. One species has evolved into two.

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3. Peripatric Speciation

Two of the great biologists of the last century, Ernst Mayr and Stephen Jay Gould, suggested that allopatric speciation would be accelerated in small, isolated populations found on the periphery of the parent species’ geographical range. This mode of speciation is called peripatric speciation. That should be easy to remember: the “peri” refers to periphery: an area on the side.

The quiz below is based on a visual model of peripatric speciation.

[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 morphologically ________ from the main population at “1.”

[hangman]

[c]different

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

[hangman]

[c]genetic

[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 ______________, further changing its gene pool.

[hangman]

[c]selection

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

[hangman]

[c]isolated

[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]

[c]interbreed

[/qwiz]

4. Clines, Hybrid Zones, and Ring Species

4a.Clines

[qwiz]

[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 a transect along which we’ll measure the height of a wildflower called Yarrow (Achillea millefolium). Yarrow ranges from 0.2 to 1.0 meters in height.

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.

[c*]Show the answer

[f]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)

[/qwiz]

4b. Parapatric Speciation, and Hybrid Zones

So, what’s the connection between clines and speciation?

Imagine that the low elevation and high elevation populations of yarrow, over time, became quite different. You’d have a situation that could be represented by the graphic on the right. The green circle (1) represents a population that’s fairly genetically uniform. Circle 2 represents a population with growing clinal variation, spread over a geographic axis. Connecting this to the population genetics learned in previous modules, that means that allele frequencies in the population represented by the green area of circle 2 are becoming different from the population represented by the blue area.  That becomes more extreme in circle 3, where you can imagine a population that now consists of two subspecies (one represented by green, one by blue). However, there’s still a hybrid zone in between that allows for gene flow. By step 4, genetic differentiation has proceeded to the the point where hybridization is no longer possible. Instead of one species, we now have two.

This kind of speciation pattern, which involves subpopulations of a species that gradually evolve reproductive isolation from one another while continuing to exchange genes, is called parapatric speciation.

In nature, it’s not uncommon to find hybrid zones between closely related species. That might have resulted from a parapatric speciation process. Or, it could have resulted from an allopatric or peripatric process where the geographical barrier disappeared before the two daughter species had completely diverged.

Click through the three examples below:

[qwiz summary=”false”]

[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.

[/qwiz]

4c. Ring Species

[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.

[q labels = “top”]

Ring species can be thought of a species with_________ variation that, for geographical reasons, has curled back upon itself. Diagram A at left represents a species that’s spread over a geographic range. While ________ are flowing between each adjacent population, there’d be significant differences in phenotypes and allele_____________ between populations 1 and 5. In fact, it’s possible that if individuals from populations 1 and 5 were experimentally removed from wherever they lived and placed in contact, they might have difficulty ____________.

 

 

[l]interbreeding

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

[f*] Excellent!

[l]clinal

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

[f*] Excellent!

[l]frequencies

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

[f*] Great!

[l]genes

[fx] No. Please try again.

[f*] Good!

[q]

A ring species is nature’s version of that experiment. As Ensatina salamanders evolved, they spread from one area to the next, perhaps moving up the California coast, then across the mountains of northern California, then down the Sierras. But where the ring curled back upon itself, there was so much _____________ that _______________ between subspecies 1 (E. eschscholtzii eschscholtzii) and subspecies 7 (E. eschscholtzii klauberi) had become _____________. In fact, it’s almost as if these two subspecies could be almost be considered to be different _____________…if not for the _____________ between all the intermediate _____________.

 

[l]differentiation

[fx] No. Please try again.

[f*] Correct!

[l]gene flow

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

[f*] Good!

[l]impossible

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

[f*] Great!

[l]interbreeding

[fx] No. Please try again.

[f*] Great!

[l]subspecies

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

[f*] Excellent!

[l]species

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

[f*] Correct!

[/qwiz]

5. Allopatric Speciation (and its variations): Checking Understanding

[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]

[c] allopatric

[f] Excellent!

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

[textentry single_char=”true”]

[c*] 2

[f] Excellent. “2” shows a geographic barrier subdividing this population into two gene pools.

[c] Enter word

[c] *

[f] No. Just look for the earliest moment in this sequence when alleles would no longer be able to flow from the left side of the circle to the right side.

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

[textentry single_char=”true”]

[c*] a

[f] NIce job. Letter “a” represents gene flow.

[c] Enter word

[c] *

[f] No. Gene flow is a process that moves alleles between populations, preventing differentiation.

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

[hangman ]

[c*] Interbreed

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

[hangman ]

[c*] geographical

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

[hangman ]

[c*] reproductive

[q]The diagram below represents _______________ speciation.

[hangman]

[c*] peripatric

[q labels = “top”]

In the speciation model shown at left, the __________ size of the peripheral population allows for higher amounts of ____________. However, __________ won’t occur until __________ is cut off. By stage 3, the two populations are reproductively ____________ from one another .

[l]genetic drift

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

[f*] Great!

[l]gene flow

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

[f*] Good!

[l]isolated

[fx] No. Please try again.

[f*] Great!

[l]small

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

[f*] Correct!

[l]speciation

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

[f*] Great!

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

[hangman ]

[c*] cline

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

[hangman ]

[c*] variation

[q]Number 3 below represents a _________ zone.

[hangman]

[c*]hybrid

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

[hangman]

[c*]parapatric

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

[hangman]

[c*]cline

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

[hangman]

[c*]ring

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

[hangman]

[c*]interbreed

[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]

[c*]species

[x][restart]

[/qwiz]

 

Links

  1. Sympatric Speciation (the next tutorial in this module)
  2. Species, Speciation and Extinction Menu