Case Teaching Notes for
“What Is a Species? Speciation and the Maggot Fly”

Introduction / Background

This case is a “clicker case.” It is called that because it combines the use of student personal response systems (clickers) with case teaching methods. Basically, how this work is the case is presented in class using a series of PowerPoint slides punctuated by questions (called “clicker questions”) that students respond to before moving on to the next slide. In this way, students work their way through the material to understand the problem presented. Specifically designed for large introductory science classes, the method integrates lecture material, case storylines, student discussion, (clicker) questions, lecture, and the presentation of data in an iterative fashion.

Few students realize that there are many well studied cases of recent or ongoing speciation events. Classroom discussion of well understood cases of incipient speciation, such as the apple and hawthorn maggot fly Rhagoletis pomonella, can be an excellent tool for engaging students and teaching them about the mechanisms of speciation.

This case study is designed for use in a general biology course for majors. Speciation is introduced half way through the one-semester course, after students have been introduced to natural selection and microevolution. The case is presented in class using a PowerPoint presentation. Students learn about the natural history of hawthorn and apple maggot flies, and are asked to use various species concepts to decide if apple and hawthorn maggot flies should be considered separate species and what evidence is relevant to each species concept. They are also asked whether the incipient speciation of the maggot flies is sympatric or allopatric, what reproductive barriers have arisen between the two “races,” and whether these barriers are prezygotic or postzygotic.

The case has been modified from “As the Worm Turns: Speciation and the Apple Maggot Fly” by Martin G. Kelly, Department of Biology, Buffalo State College. The original case was published by the National Centre for Case Study Teaching in Science on its website at http://www.sciencecases.org/maggot_fly/maggot_fly.asp in 2003.

Objectives

Upon completion of this case, students should be able to:

• Define several species concepts, including the biological species concept, ecological species concept, morphological species concept, and phylogenetic species concept.
• Use several species concepts to determine if two groups of organisms represent separate species.
• Distinguish between sympatric and allopatric speciation.
• Understand the significance of reproductive isolating mechanisms in reducing gene flow between two populations.
• Distinguish between prezygotic and postzygotic barriers to reproduction.

Misconceptions

• Although many species are going extinct, no new species are forming on Earth. Speciation is considered a process from the distant past that must be inferred from the fossil record and molecular data.

• Evolutionary change in a genetically isolated population results from a purposeful striving for adaptation to a new environment. Students mistakenly think that evolutionary change results from a gradual increase in favorable traits in all members of a population. Students fail to realize that individuals who inherit traits that increase their survival or reproduction in the new environment will produce more offspring; over time, their descendants will make up an increasing proportion of the population.

• The evolution of reproductive isolating mechanisms after individuals from two isolated populations come into renewed contact results from the “purposeful striving” of individuals to reduce hybridization. Students do not realize that the evolution of isolating mechanisms results from increased numbers of descendants who inherited traits or preferences that lead them to mate with “their own kind.”

Classroom Management / Blocks of Analysis

This case is presented as part of a general biology course designed for majors. The material should be introduced after students have learned about the evolution of populations, covering natural selection, genetic drift, mutation, and gene flow. The case can be delivered in one 50-minute period. Clicker questions are an excellent way to promote and facilitate small group discussion in a large lecture class. I recommend giving students one or two minutes (depending on the complexity of the question) to discuss the question before selecting their answers.

Teaching the Case

Slide 2 and Slide 3: The first two clicker questions test for two common misconceptions: (1) that speciation occurs over immensely long periods of time and (2) that speciation is not taking place at present. If a significant percentage of your students have one of these misconceptions, you might wish to return to these questions after completing the case. Ask the same clicker questions after finishing the case, pointing out that Rhagoletis apple and hawthorn maggot flies are currently speciating, and that this process was initiated by the introduction of apples to North America 400 years ago.

Slide 4, Slide 5, and Slide 6: The female hawthorn maggot fly, Rhagoletis pomonella, lays fertilized eggs in the fruit of the hawthorn tree. Maggots (larvae) emerge from the egg, eat the fruit, grow through several molts, and drop from the tree. Pupation takes place in the soil. Adult flies emerge from the soil and mate with other maggot flies on the surface of the fruit. A female parasitoid wasp may land on the hawthorn and probe the fruit with her long ovipositor. If she encounters a maggot, she lays an egg in the maggot’s body, paralyzing it. The wasp larva devours and eventually kills the maggot.

Slide 7: Hawthorn trees are native North American shrubs belonging to the genus Crataegus. Hawthorn fruits range between 4.9 and 19.5 mm in diameter, with an average diameter of 12.6 mm.

Slide 8: Domesticated apples, belonging to the species Malus domesticus, were introduced to North America in the 1600s and are now the most widely grown fruit in North America. A typical commercial apple has a diameter of 70 mm. There are at least four resident North American species of Malus, commonly known as crab apples. Crab apple fruits are small, roughly similar in size to hawthorn fruits. Rhagoletis pomonella has not been reported to parasitize North American crab apple species. This may be because maggot flies do not parasitize crab apples. However, it is more likely that this has not been studied, since native crab apples lack commercial value.

Slide 9 and Slide 10: Shortly after the introduction of the apple, the larvae of Rhagoletis pomonella started feeding on this fruit. (Berlocher and Feder, 2002; Jiggers and Bridle, 2004; Linn et al., 2003; Prokopy et al., 1988)

Slide 11 and Slide 12: A fruit of the larger apple provides 220 times more food (based on volume) than a hawthorn fruit. Apple maggot flies lay more eggs per fruit than do hawthorn maggot flies. However, the nutritional quality of hawthorn fruit is superior. 52% of hawthorn maggots survive to adulthood vs. 27% of apple maggots. Apple and hawthorn fruits provide quite different environments for the development of maggot fly larvae. Both hawthorn and apple are woody plants belonging to the rose family. The larger fruit of apple trees provide 5.5 times more depth (based on diameter) for developing maggots than do hawthorn fruits. Apple maggots are better able to escape parasitoid wasps; they do this by burrowing deeper into the fruit than the wasp can penetrate with its ovipositor. As a result, apple maggots carry 70% fewer parasitoid wasp eggs than do hawthorn maggots.

Slide 13, Slide 14, and Slide 15: Hawthorn and apple maggot flies are assigned to the same taxonomic species (Rhagoletis pomonella) and are physically indistinguishable. There is no geographic isolation or physical separation between hawthorn and apple maggot flies. However, the flies are genetically distinct and have recognizably different genetic profiles. Feder et al. (2003) describe six allozyme changes found in the apple maggot flies. These changes alter the timing of emergence of adult flies, leading to adaptive change synchronizing apple maggot fly emergence with peak fruiting of apple trees. Hawthorn maggot flies strongly prefer to mate on and lay fertilized eggs in hawthorn fruit. Apple maggot flies strongly prefer to mate on and lay fertilized eggs in apple fruit. There is only a 4 to 6% hybridization rate between hawthorn and apple maggot flies, and hybrids are viable and fertile.

Slide 16: The slide shows the timing of fly emergence (solid and dashed lines) and fruit ripening (colored filled-in curves). In this figure, the taxonomic term “race” is used to identify maggot flies that reproduce on apples or hawthorns (Bush, 1969). Adult flies emerge from pupae in the soil to reproduce before fruit mature. The flies mate on the surface of the fruit and the female lays fertilized eggs into ripe fruit. Maggots (larvae) hatch from the egg, eat the fruit, and grow through several molts. Maggots leave the tree when fruit falls from the tree and burrow in the soil, where they pupate. Apple fruits ripen approximately 1 month earlier than hawthorn fruits, but there is overlap between the end of apple fruiting season and the beginning of hawthorn fruiting season.

Slide 17: This slide provides a comparison of hawthorns and apples.

Hawthorn	Apple
Small fruit (13 mm)	Large fruit (70 mm)
High nutritional quality	Low nutritional quality
Shallow burrows	Deep burrows
More parasitoid wasps	Fewer wasps
Fruit available later	Fruit available early

Slide 18: In this clicker question, students are asked whether hawthorn and apple maggot flies are separate species. This slide introduces the various species concepts that may be used to define the term “species.”

Slide 19: This slide explains the biological species concept. A species is a group of actually or potentially interbreeding individuals from separate natural populations. These individuals can sexually produce viable, fertile offspring. A species is reproductively isolated from other species. The key aspect of this definition is reproductive isolation.

Slide 20 and Slide 21: Students are asked whether hawthorn and apple maggot flies are separate species according to the biological species concept, and what information provided in the case study is relevant to this species concept.

Slide 22: This slide explains the ecological species concept. A species is a set of individuals exploiting a single niche. The key aspects of this definition are the resources exploited and the habitat occupied by the individual members of a species.

Slide 23 and Slide 24: Students are asked whether hawthorn and apple maggot flies are separate species according to the ecological species concept, and what information provided in the case study is relevant to this species concept.

Slide 25: This slide explains the morphological species concept. A species is a set of individuals who share similar, unique morphological features. The key aspect of this definition is the shared, similar morphology of the members of a species.

Slide 26 and Slide 27: Students are asked whether hawthorn and apple maggot flies are separate species according to the morphological species concept, and what information provided in the case study is relevant to this species concept.

Slide 28: This slide explains the phylogenetic species concept. A species may be defined by its unique genetic history. Species are at branch tips in a phylogenetic tree. Species descend from shared ancestors, but are defined by their unique derived features.

Slide 29 and Slide 30: Students are asked whether hawthorn and apple maggot flies are separate species according to the phylogenetic species concept, and what information provided in the case study is relevant to this species concept.

Slide 31, Slide 32, and Slide 33: How do new species come into existence? How does a single species (with a single gene pool) ultimately produce two new species? How do two related species maintain genetic isolation if they coexist in a single geographic area? There are two contrasting modes of speciation: allopatric speciation and sympatric speciation.

• In allopatric speciation, the initiating factor for speciation is the geographic separation of two populations of individuals. Without gene flow, the two populations can diverge genetically. The barrier to individuals (gene flow) may be a lake, ocean, mountain, desert, prairie, glacier, etc. The nature of the barrier varies with the mobility of the species. Does a mountain range, a river, or a glacier pose a barrier to interbreeding between individuals from two populations of blue jays? Ask students to suggest an organism for which each barrier would prevent interbreeding between individuals from two separate populations.

• In sympatric speciation, speciation takes place with geographically overlapping populations. Reproductive isolation in sympatry may arise from polyploidy (especially in plants) or specialization to locally different habitats.

Slide 34: Students are asked if speciation in Rhagoletis is sympatric or allopatric.

Slide 35: Genetic divergence is defined as the accumulation of genetic differences between two populations.

Slide 36: Once two populations are genetically isolated, several factors can act to produce genetic divergence between isolated populations:

• Founder effect (if one population is founded by a small number of individuals)
• Mutation
• Genetic drift (if one or both populations are small)
• Differential selection (if the two populations are in different environments)

Slide 37, Slide 38, Slide 39, and Slide 40: With renewed or continuing contact between the two populations, there may be some degree of hybridization. Individuals from one population may mate with individuals from the other population, renewing gene flow between the populations and producing hybrid offspring. If the populations have genetically diverged to adapt to very different environmental conditions, the hybrid offspring may have reduced fitness. This produces strong selective pressure to prevent mating between individuals from two different populations. Such isolating mechanisms prevent individuals from two species mating and producing offspring who are not viable or sterile. Isolating mechanisms may prevent mating (prezygotic barriers) or prevent successful development or reproduction in hybrid offspring (postzygotic barriers).

Prezygotic barriers include:

• Habitat isolation
• Behavioral isolation
• Temporal isolation
• Mechanical isolation
• Gametic isolation

Postzygotic barriers include:

• Reduced hybrid viability
• Reduced hybrid fertility
• Hybrid breakdown

Slide 41 and Slide 42: Two clicker questions follow, allowing the instructor to determine if students understand the distinction between prezygotic and postzygotic isolating mechanisms.

Slide 43: When gene flow between two genetically isolated populations continues, or is renewed, there are three possible outcomes. The outcome will depend on the genetic differences that have accumulated between the two populations, as a result of founder effect, genetic drift, differential natural selection, and different mutations. Individuals from the two populations may (1) interbreed readily (they belong to a single species), (2) not interbreed at all (they belong to two separate species), or (3) interbreed, and produce hybrid offspring with reduced ability to survive or reproduce (speciation may be in progress). In the third instance, there is strong selection for effective reproductive isolating mechanisms, preventing individuals from the production of hybrid offspring with lower fitness. In this case, full speciation may follow renewed contact between populations.

Slide 44 and Slide 45: Students are asked what reproductive barriers limit reproduction between hawthorn and apple maggot flies and whether these barriers are prezygotic or postzygotic.

Slide 46: This slide asks students if they would expect natural selection to favor prezygotic or postzygotic isolating mechanisms between sympatric species. Prezygotic isolating mechanisms are much less costly to individuals. There is a cost to a female horse in bearing and nursing an infertile mule offspring.

Slide 47 and Slide 48: The final clicker questions return to the two questions that began the case.

Slide 49: This slide points out to students that apple and hawthorn maggot flies are defined as separate species according to some species concepts, but not others. Clarify that these insects are in the process of speciating, and ask your students at what point is it reasonable to conclude that speciation has occurred.

Answer Key

Answers to the questions posed in the case study are provided in a separate answer key to the case. Those answers are password-protected. To access the answers for this case, go to the key. You will be prompted for a username and password. If you have not yet registered with us, you can see whether you are eligible for an account by reviewing our password policy and then apply online or write to answerkey@sciencecases.org.

References

• Berlocher, S.H., and J.L. Feder. 2002. Sympatric speciation in phytophagous insects: Moving beyond controversy? Annual Review of Entomology 47:773–815.
• Bush, G.L. 1969. Sympatric host race formation and speciation in frugivorous flies of the genus Rhagoletis (Diptera: Tephritidae). Evolution 23: 237–251.
• Feder, J.L., J.B. Roethele, K. Filchak, J. Niedbalski, and J. Romero-Severson. 2003. Evidence for inversion polymorphism related to sympatric host race formation in the apple maggot fly, Rhagoletis pomonella. Genetics 163: 939–953.
• Jiggers, C., and J. Bridle. 2004. Speciation in the apple maggot fly: A blend of vintages? Trends in Ecology and Evolution 19: 111–114.
• Linn, C., J.L. Feder, S. Nohima, H.R. Dambroski, S.H. Berlocher, and W. Roelofs. 2003. Fruit odor discrimination and sympatric host race formation in Rhagoletis. Proceedings of the National Academy of Science (USA) 100: 11490–11493.
• Prokopy, R.J., S.R. Diehl, and S.S. Cooley SS. 1988. Behavioral evidence for host races in Rhagoletis pomonella flies. Oecologia 76: 138–147.

Slide Credits

Acknowledgements: This material is based upon work supported by the NSF under Grant No. DUE-0618570. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of NSF.

Date Posted: November 4, 2009.

Copyright © 1999–2009 by the National Center for Case Study Teaching in Science.  Please see our usage guidelines, which outline our policy concerning permissible reproduction of this work.