Species Case Study

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“Hello dear, “ said Mom while cutting the tomatoes for dinner, “How was school

If They Can Mate, Did They Speciate?


A Case Study in Evolution and Meaning of Species


Jim Dixon

Sharon High School





How many times have teachers heard students say that an animal evolved when a member of species X mated with a member of species Y? How many times have we been asked questions such as: Is there really such as thing as a liger? Can a dog and a wolf mate?  What would happen if a Chihuahua and a Great Dane mated?  Did humans mate with chimps and that’s how we got AIDS? Haven’t we explained that a species is defined by reproductive isolation only to have students point out exceptions such as the dog and the wolf, or the recent hybrid Grizzly/Polar bear?


This unit is designed to address the vexing issue of the species concept by using these common student questions and recent biology news.  It is based on a case study approach. It is designed to offer AP Biology students an opportunity to review/relearn evolutionary concepts in a manner different than that in an introductory course. This method is being increasingly used in colleges because it fosters deeper thinking, independent learning and more sophisticated understanding.

It can be done in whole or in part and can be used as a webquest.


For further explanation of the design and use of the unit, please see the teacher notes.

If They Can Mate, Did They Speciate?


A Case Study in Evolution and Meaning of Species



Part 1:            Ligers and Chumans and Pizzly Bears! Oh, My!


“Hello dear,” said Mom while cutting the tomatoes for dinner, “How was school?”


Tammy had just arrived home from her after school volunteer work at the animal shelter.  “Kinda cool.  We had a huge debate in biology today.  Hi Dad.”


Dad looked up from his newspaper.  “Hi hon.”


“So dear, what did you debate about?” Mom said while concentrating on the tomatoes, “Not the value of genetic altered food again, I hope. I don’t care what they do to plants as long as we can get tasty tomatoes”


“It’s genetically modified organisms, and no it wasn’t that.  We were discussing evolution and species and stuff.  George, will you lower that!”


Tammy’s brother George was listening to music in the other room.


“Anyway, Mr. Murray was explaining what a species is and how they can’t breed with other species and he was all excited about a news story of how some hunters have found a bear who was a cross between a polar bear and a grizzly and another news story about how human ancestors and chimp ancestors may have interbred not long after they split into two species. Then Mel asked, ‘If there was a pizzly, and a chuman, could there really be a Liger?’”


“That’s from that movie, Napoleon Dynamite.  Someone asked the same question last year when I had bio,” said George as he entered the room. “It’s a cross between a lion and a tiger. I’ll bet Murray was really piz…”


“Don’t open that refrigerator, dear.  We’ll be eating in a little while,” said mom.  “So, is there such a thing?”




Tammy stared at George, wondering what day George actually stayed awake in bio last year. “Actually, he went on a long lecture about how we shouldn’t think of the chimp and human actually being a chimp and human yet.  The human ancestor would have seemed very ape like to us. And apparently the tiger and lion are similar enough that they can mate but since they are only together in zoos I guess it doesn’t count. And no, they aren’t magic.”


“That’s interesting dear.  In the supermarket I saw pluquats.  I think they are a cross between plums and kumquats.”

“Yeah, well, anyway, then Erica asked if a Chihuahua and a Great Dane could actually, you know, mate. And, if they couldn’t, were they different species?  She always asks the best questions.”

“Now that I’d like to see!” said George. “I bet they couldn’t fit together!”


“Hmmrph, I’m going to uhh, polish my golf clubs,” said Dad rising suddenly.


Mom giggled, “He always gets so nervous when we discuss s-e-x.  So, what did the teacher say?”


“Well, after the boys all stopped laughing, he explained that they might not be able to do it, umm naturally, but their sperm and eggs are compatible. But, if paleontologists in the future found Chihuahua bones and Great Dane bones they would probably consider them different species.”


“Sounds like the liger dear”


“That’s what I thought.  I was going to ask about it at the shelter but Dr. Edwards wasn’t in.  Anyway, that got us wondering if the Chihuahua and the Great Dane were different species like the tiger and the lion.  Then Erica asked if there were any cases of species in the wild that could mate.  Mr. Murray said he read that about 25% of plants and 10% of animals do make hybrids with closely related species.”


Mom pulled her finger away from the knife suddenly, “Hmm, if scientists could cross a tomato and a navel orange…”


“Yeah, anyway, that’s when Susan said that if they can make hybrids then they are the same species.  Sophia thought that the hybrids are sterile, like a mule but Mr. Edwards point out that there was one who just gave birth. Then Tara started chanting ‘If you can mate than you didn’t speciate’. Mr. Edwards made her stop and then said that the whole species concept is kinda murky.”


“Just like you!” muttered George as he left with a tomato slice shoved in his mouth.


“Loser,” said Tammy.  “Anyway, Elliot was saying that on his farm his family had a smallish Jack Russell terrier that mated with a medium size Brittany.  One of the puppies grew up and mated with the neighbor’s German Shepard.  His family was worried because they had always suspected that the Shepard was actually part coyote since it was so vicious and there are coyote in the area.   In fact, the terrier’s mom was eaten by a coyote.  That’s why they put up that big fence.”


“Hmm, do they grow tomatoes on that farm?”


Case analysis:


1. In your Case Study Journal, create a chart with the headings:


I Know…



I Think I Know…


I Want to Know…





Think about this case and reflect back on your homework and prior learning. Make as many entries as possible in each column of the chart.


2. Discuss your list with your group. Add notes to your lists.


3. Using your textbook make sure you understand the following concepts.  Record definitions and connections to the case in your journal.


The stages of Natural Selection (as already discussed)

Reproductive isolating mechanisms (as listed in the book)


Biological species concept

Morphological species concept

Phylogenetic species concept

Ecological species concept

Allopatric speciation

Sympatric speciation


Gene Flow

Gene Pool





4. Read these brief articles at these websites:

            Grizzly-Polar Bear Hybrid Found—But What Does It Mean?



Did Humans and Chimps Once Interbreed?



            Morocco’s Miracle Mule



            Ligers and Tigons (including pictures)




5. Discussion questions (answer these questions in your case journal in preparation for the follow-up class discussion):


A. Consider the examples of “murky species” mentioned in the case (polar bear/grizzly, dog/coyote, Great Dane/Chihuahua). Apply each of the species concepts to these examples.  Do the concepts differ in their ability to help make sense of the examples?  


B. Does the ability of two species to hybridize indicate anything about how recently they speciated?


C. Should biologists decide on set number of genetic differences or morphological differences be used to decide if a two populations are different species (or subspecies)?


D. Are populations that speciate sympatrically more likely to be able hybridize than ones that speciate allopatrically?


E. Does the presence of hybrids (especially fertile) mean that there is no species barrier?


F. Can new species be created by hybridization?


G. Be sure to review reproductive isolating mechanisms very carefully.


H. Do you think this is an important issue?


I. Relate your research to the “want to know” column.  What questions can you now answer?


6. For more information, check out the following websites:


Speciation (from the Understanding Evolution Website)



What Is A Species And What Is Not by Ernst Mayr



            A species definition for the modern synthesis by James Mallet



            Genetics and the origin of species: An introduction by Francisco Ayala and Walter Finch



Hybridization As An Invasion Of The Genome by James Mallet



  How To Define A Species.  (posts examining the dog question)






 Evolution Caught In The Act (Galapogos finches)



Ligers and Tigons




What is a species? (a good primer on speciation, includes red wolf)



Part 2: The Sisterhood of the Rings


The next morning, Tammy did her usual Saturday volunteer work at the Animal shelter.  She found the vet working on repairing the wing of a Sea Gull. 


“Someone found this bird in the mall parking lot, got hit by a car,” Dr. Edwards explained.  She had a thriving practice tending to people’s dogs and cats, but liked to help out at the shelter evenings and weekends.  The other volunteers loved her energy and knowledge.  “It has a broken humerus in this left wing. Even though its humerus is just like that of a dog or cat or us, I doubt I can fix it, bird’s bones are so fragile, but I’ll try. You know how I love birds. He probably won’t fly again. Perhaps we can send it to the zoo. How’s school?”


While handing Dr. Edwards tape and gauze, Tammy told Dr. Edwards about her bio class and thinking about species.  “So, if animal A can mate with B and B can mate with C, but A and C can’t mate, who and what is a species?  It’s so confusing!”


“You know, Tammy, this gull might be able to help.”


“Dr. Edwards, the gull seems to have too much on its mind to want to discuss the species concept.  It’d be like discussing it with my brother. Yuk, and just as messy.”


“Okay, let me explain,” said Dr. Edwards as she readjusted the gull on the table.  “Perhaps he’ll stay still long enough to learn something too.  This is an American Herring gull.  It exists in the northern part of North America, all across the continent.  In England and Northern Europe, there is the Herring gull.  It’s considered to be a different species or subspecies, but it does sometimes hybridize with the American Herring Gull in Iceland and Greenland.  The English version is a lighter grey.  In Alaska, the American Herring gull sometimes mates with a gull from far eastern Siberia.  It’s called the Vega herring gull.  It’s darker than the American version.  In Siberia, as you head west, the Vega gull is replaced by the darker Birula’s gull…”


“Let me guess,” interrupted Tammy, “They can sometimes hybridize.”


“Yup, and the Birula’s range borders on the Heuglin’s range which borders on the Siberian lesser black backed gull’s range.  Each one can hybridize and each species is a bit darker than the more eastern one.  In Northern Europe and England we find the Lesser black backed gull, which is really dark.”


“Wait, “Tammy yelled, while wiping bird droppings off her smock.  The gull was now safely in a large cage.  “You said the plain ole herring gull was there.”


“Right, but so is the lesser black backed gull, and, these two won’t mate. Seem’s the color or something keeps them from mating.”


“So, A won’t mate with G”


“Exactly! Biologists call this a ring species.  They see it as speciation in process.  Perhaps the Lesser Black Backed Gull started it, spread east and lightened up as it did. Now some say this isn’t a good example because there is some question about whether all the species are evolving from a common ancestor—the American herring gull may have evolved from a different set of gulls and has converged on the herring gull.  Some birds can hybridize you know.  But you get the idea.  Genetically, all these birds are pretty similar. Actually, when I did my Master’s degree I studied bird evolution.  Another good example is some warblers in Asia.  Oh, and there’s the famous salamanders in California.  Seems that species can interbreed for a while after their speciation.  Did you hear about how ancestors of the chimps and ancestors of humans interbred for a while after they split up?”


“Yeah, but that was when the human and chimp ancestors were barely different.  Could they have been ring species?”


“Well, I think the definition of ring species requires a bit more than that, but maybe that’s how they start. Yup, evolution is interesting! And important. I just read a paper about how evolution in heart worm is interfering with medicines.  But that’s a discussion for another day. Hope the explanation of the gull ring species cleared some things up.  Thanks for the help.”  Dr Edwards left to help a bulldog that had slipped on some ice.


‘Yeah, it helped,’ thought Tammy.  ‘But are those gulls A species or G species? And is species singular or plural, anyway?’



Case analysis:


1. In your Case Study Journal, create a chart with the headings:



I Know…



I Think I Know…


I Want to Know…







Think about this case and reflect back on your homework and prior learning. Make as many entries as possible in each column of the chart.


2. Discuss your list with your group. Add notes to your lists.


3. Using your textbook make sure you understand the following concepts.  Record definitions and connections to the case in your journal.

Ring species


Sexual selection

            Disruptive selection

            Stabilizing selection

            Directional selection

            Divergent evolution

            Convergent evolution



4. Read the brief articles at the following websites:


            The Larus Gulls Circumpolar Species Ring



Ring Species (from the PBS Evolution Website)



Evolving Before our eyes



Ring Species: Unusual Demonstrations of Speciation



Ring Species and Clinal variation




5. Discussion questions (answer these questions in your case journal in preparation for the follow-up class discussion):


            A. Why do biologists like to study ring species?


B. What would happen if a highway or shopping mall prevented the interbreeding of parts of the ring species?


C. Should members of a ring species be considered separate species, separate subspecies or just one species?


D. The expanding desert in China has broken the ring of the greenish warblers.  Should the species that has lost connection to the rest now be thought of as a different species?


E. What selections are driving the evolution of the salamanders and warblers?  Are these cases of sympatric or allopatric speciation?


F.      The two northern (incompatible) species of warblers live in similar environments and

have evolved similar traits to each other (diverging from the common southern ancestor).  Doesn’t speciation only occur when a species adapts to a different environment?


G. Relate your research to the “want to know” column.  What questions can you now answer?






6. For more information, check out the following websites:


The Greenish Warbler Ring Species



Ensatina eschscholtzi: Speciation In Progress



Ring Species



Levels Of Selection And Speciation Mechanisms

(scroll to section on ring species in gulls and ring species in salamanders)



The Auk: Overview: Circular Overlaps: Rare demonstrations of speciation.



Ring Species As Bridges Between Microevolution And Speciation.



Part 3:  The Mouse That Caused the Roar


On Sunday, Tammy and her family were having their usual Sunday breakfast at Bauer’s Diner.  The TV over the counter was tuned to a news channel.  As usual, Sandy was working the counter; pouring coffee while loudly commenting on the news stories to the men hunched over their coffees.  The news anchor was talking to a person seen in a small monitor in the upper right corner of the screen. Another person could be seen in a monitor in the lower right. Tammy heard a word that caught her attention.


“…species!  I mean if they can’t prove it is a different species I don’t see why it should be on the list,” said the person in the upper monitor.


As Tammy listened to the story she teased out some of the details.  The Secretary of the Interior wanted to remove the Preble’s meadow jumping mouse from the endangered species list.  It is found in areas of Colorado and was interfering with construction of housing and retail developments.  A biologist hired by the Interior Department had determined Preble’s mouse was not that genetically different from another, more plentiful species of jumping mouse.  Now, another study was contradicting that finding.


“The department and its scientists are just promoting the President’s pro-development agenda!  This is not how science should be done,” said the person in the lower monitor.


“I don’t think that any reasonable person wants to see the whales go extinct, but we are talking about a mouse that may not actually be a species.  The Endangered Species Act does not protect hybrids” said the upper speaker.


“This mouse is not a hybrid.  The Act protects endangered species and all subspecies of an endangered species. This mouse is either its own species or a subspecies of another…”


“…Mouse species that is not endangered and therefore should not be protected. Look at what has gone on with the Red Wolf!” the man in the upper screen interrupted.


The man in the lower corner looked frustrated, “But, it is different and part of the biodiversity! It may be an endangered species or we may be seeing speciation. It may be evolving…”


Sandy changed the channel with a stab of the clicker. “Who cares!” she said, “It’s just a mouse and don’t give me any of that evolving stuff!” She seemed to be yelling at a guy with a trucker hat who was stirring his coffee and looking at another waitress.  “Anyway, it could only be a different species if it was created that way.  One species does not become another one! And if you ask me one mouse is the same kind as another! If they can mate then they havta be the same species just the way they we created. Mice don’t change over time.”


“George, will you lower that!” yelled Mom.


George put down the pitcher of maple syrup he was holding high over extending tongue.


“I don’t think it should matter if it is its own species,” said Tammy, not sure if anyone in her family was listening.  “Species boundaries are kinda real and kinda man made, but it is important that we preserve the gene pool.”


“No, dear, you can’t wear your jeans in the pool.” said Dad as he looked up over the newspaper.


“No, I’m talking about why it shouldn’t matter what species it is.”


“Actually, Sis,” said George, with one eye on the blonde girl wearing a shirt that said ‘HERBIVORE” at the next table. “It would appear that classification and species designation is important if it can be used justify habitat destruction and promote the intelligent design agenda.”  George glanced over at the blonde with an arch to his right eyebrow.


Tammy opened her mouth.  Dad looked up over the newspaper, and Mom cried ouch as she dropped her knife. 


“Check please!” Dad called out.


That night Tammy did an internet search on Preble’s meadow jumping mouse.


Case analysis:


1. In your Case Study Journal, create a chart with the headings:



I Know…



I think I know…


I want to know…..







Think about this case and reflect back on your homework and prior learning. Make as many entries as possible in each column of the chart.


2. Discuss your list with your group. Add notes to your lists.


3. Read the brief articles at the following websites:

Status Of Threatened Mouse On Trial



Western Business Roundtable: FWS Long Past Due In Delisting Mouse



FAQ Preble’s Meadow Jumping Mouse        



Government Postpones Decision On Jumping Mouse Status



Subspecies Fight For Space On Protected List (listen to news story)



5. Discussion questions (answer these questions in your case journal in preparation for the class discussion):


A. Why should we protect endangered species?


B. Is it more important to preserve the organisms or the genes?


C. Why is protecting subspecies and hybrids more controversial than protecting species?


D. If one member of a ring species was endangered, but other members were plentiful, should the endangered member be protected?


E. Why would those who do not believe in the evolution of species (macroevolution) have difficulty with idea of preserving of subspecies or members of a ring species?


F. Relate your research to the “want to know” column.  What questions can you now answer?



6. Written Assignment:

            Write a 5 paragraph essay in which you argue for the importance of government funding of research into evolution (especially research into phylogenetic relationships and speciation).  Include three well supported arguments based in evolutionary science.



7. For more information, check out the following websites:


The Red Wolf: Is It A True Species?



Arizona Agave [a hybrid] Taken Off The Endangered Species List



Hybridization: The Double Edged Threat?



Speciation and Biodiversity (an interview with E.O. Wilson)



Systematics And Endangered Species Conservation


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