The Apple Maggot Fly: A Case of Speciation

 

Adapted from As the Worm Turns: Speciation and the Apple Maggot Fly (Kelly 2004)

 

Age: 11th and 12th grade students

 

Purpose: 

The apple maggot fly is one of the many pests that feed on developing apples in orchards across the country.  They are very similar to hawthorn maggot flies, which feed on the small fruit produced by hawthorn trees.  The two flies are physically indistinguishable and are not geographically isolated, thus they have been considered to belong to the same species.  Recent studies, however, have led scientists to re-examine this classification.  The following case study provides real data for students to analyze and decide for themselves whether the apple and hawthorn maggot flies should remain as one species, or should be considered separate species.

 

Objectives: 

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

-Determine if the hawthorn and apple maggot flies belong to the same species, or separate ones.

-Provide reasonable data/reasons for this decision

-Explain, if the two are thought to be separate species, how speciation might have occurred

 

Standards:

            National:

-         A2-5, B1-2, D3, D4, D6, F2-3, G3

-         See National Science Education Standards for descriptions

            New Hampshire:

-         1a, 3a, 3d, 6d

-         See New Hampshire Science Curriculum Framework for descriptions

 

Background knowledge needed:

            By teacher: Hawthorn and apple maggot flies are assigned to the same taxonomic species, Rhagoletis pomonella.  The apple maggot fly is native to eastern North America, and it originally bred in the larger fruits of the hawthorn tree.  Hawthorn and apple maggot flies are physically indistinguishable from each other, and are each about 5 mm in length.  The tip of the female’s abdomen is more pointed than the males.

            The female fly (both hawthorn and apple) will lay her fertilized eggs into the fruit (either hawthorn or apple, respectively), and the larvae will emerge from the egg, eating the fruit, and will eventually pupariate and emerge from the fruit to reproduce.

            The hawthorn and apple trees are both woody plants belonging to the Rose Family.  Hawthorns are composed of a large and complex group of trees and shrubs native to North America, and are usually identified because each species has a different sized fruit.  They belong to the plant genus Crataegus.

            Apple trees produce an edible fruit, and belong to the plant genus Malus.  These trees are not native to North America, as early European colonists brought seeds to the continent as early as the mid-1600s.  Apple fruits tend to ripen about one month earlier than hawthorn fruits, but the two trees produce fruit simultaneously at the end of the apple fruiting season.  Apples are also larger than hawthorn fruits, as the typical commercial apple has a diameter of 2.75 inches, while the hawthorn fruit’s diameter is usually about 0.5 inches.  Thus, apples provided approximately 5.5 times more depth (determined by diameter measurements) to growing maggots than do hawthorn fruits.  Apple maggots can therefore escape predators more easily by burrowing into the apple, and typically bear about 70% less parasitoid wasp eggs (deposited by an adult wasp and will eventually kill the maggot) than do hawthorn maggots.  A larger size also means a larger volume of fruit, and apples provide about 220 times more food to developing maggots than the smaller hawthorn fruits do.

            The hawthorn fruit, even though it is smaller in diameter and volume compared to the apple, provides more nutrients to the maggot.  Maggots (both apple and hawthorn) have been shown to survive better in the hawthorn fruit, with 52% survival of maggot eggs in hawthorn fruits, compared to only 27% survival in apple fruits.

            Dependability to fruit choice acts as a very strong barrier to gene flow between the hawthorn and apple maggot flies.  Hawthorn maggot flies strongly prefer to lay fertilized eggs in the fruit of hawthorn trees, while apple maggot flies strongly prefer to lay eggs in apples.  There is only a 4-6% hybridization rate between hawthorn and apple maggot flies.  Hawthorn and apple maggot flies are also genetically distinguishable, with recognizable genetic profiles.

By Student: The concept of species is hard to grasp, as a formal definition of the word has been debated on for centuries.  Three essential concepts of species has evolved, however, and includes:

-         Species contain groups of interbreeding populations

-         Species are a natural and fundamental unit of evolution

-         Each species is a product of an independent evolutionary pathway

Two main characterizations of species in use today are the biological and morphological species concepts.  The biological species concept defines a species as a population or group of populations whose members have the potential to interbreed with one another in nature to produce fertile offspring, but cannot successfully interbreed with members of other species.  The morphological species concept states that a species is defined by measurable anatomical standards.  (Campbell and Reece 2002).

            Speciation is the mode by which new species are formed, and there are essentially two forms of speciation that students should understand.  Allopatric speciation occurs when a single species is separated by a geographic barrier (Campbell and Reece 2002).  The isolated population can then be effected by natural selection, including, or as well as, the founder effect, bottlenecking, genetic drift, inbreeding, and mutations.  Each plays a role in the development of a new and distinct species.  Sympatric speciation is the formation of a new species as a result of a genetic change within the species that causes a reproductive barrier between the “parent,” or original species, and the new, mutant organisms (Campbell and Reece 2002).

 

 

Materials Needed:

By teacher:

-         Pictures of both hawthorn and maggot flies

-         Pictures of a hawthorn and apple tree

-         Pictures of hawthorn and apple fruit

-         Attached data sheets

 

Grouping:

Students will be placed in groups of 3-4 to review and analyze the case study.

 

Preparation for Experience:

The case study will take two class periods to complete.  On the first day, students will come to class having read material from an appropriate textbook or handout provided by the instructor on the different definitions of a species and speciation (allopatric and sympatric).  The instructor will then review this material (see background knowledge necessary for students) and give a short quiz.  The class will then be broken down into the groups of 3-4 students and each group will be handed the data sheets provided.  Before the end of the period, each group should determine if the two flies belong to the same species or to separate ones, and these theories should be discussed with the whole class.

            On the second day, the students should return to their groups and, if necessary, provide a means by which speciation occurred.  Multiple sources of reference should be provided, and will need to be obtained prior to the start of class.  Students may also be allowed to go to the library to conduct further research.   If the flies were considered to be the same species after all, these groups should be able to make a feasible argument as to why they believe the two flies are classified as one species.  Once each group has either developed a method of speciation or determined that the two should remain classified as is, the instructor should open the remaining time up to discussion.  During the discussion, each group will present their findings and theories to the rest of the class and be prepared to possibly answer questions about their hypothesis.

 

Outline of Experience:

Introduction: To get the students motivated and interested in the case study, the instructor should introduce the topic by explaining how apple maggots can ruin an entire crop of apple trees, and why farmers should use IPM methods to control these pests.  This can be tied to hawthorn flies, as they do the same to hawthorn trees, the different being that hawthorn fruit is not eaten by humans, and does not have as much of an effect on us.  The students should be aware that they will be deciding on the classification of these two insects, determining whether they belong to the same species or not, and will be using actual data collected from field work performed by authentic scientists.

Body of Lesson:

Day 1

1.)   Entire class comes prepared by reading about evolution, definitions of species and modes of speciation

2.)   Teacher gives short lesson on necessary background knowledge, provided above

3.)   Short quiz given on material covered in reading and previous lecture

4.)   Class breaks into groups of 3-4 students and each group receives the data sheets (see attached data sheets)

5.)   Groups decided whether the flies belong to the same species, or to separate ones, and come up with reasons for this decision

6.)   Each group presents their theory and explains why they made their choice

Day 2

1.)   Teacher gives brief review from last class

2.)   Class divides into same groups and use data sheets and other references to decide how speciation could have occurred, or why the two flies should be considered one species

3.)   Discuss different theories as a whole class discussion, lead by teacher, but with the hypotheses developed by the students being discussed.

Conclusion: The instructor will summarize the discussion, making sure to tell the students that there really is no right or wrong answer here.  The concept of species is fairly broad and open-ended, and with new data being discovered everyday, science is constantly changing and evolving.  Students should be encouraged to find other studies like this one involving organisms that may affect their everyday lives.  

 

Assessment Plan:

Students will write a 1-2 page report on their findings, including the following aspects:

-         Explain their hypothesis

-         Define a species in their own words, using information they already know as well as information found while doing the case study

-         Provide a scenario for the mode of speciation, or give accurate reasons for why the two flies are the same species

 

 

Extensions, Adaptations, Alternatives, Next Steps:

The students who finish early will be asked to find more information about each of the hawthorn and apple trees, as well as the hawthorn and apple maggot flies.  All students will be involved in this lesson, including those with documented learning disabilities.  These students will be placed in groups with the other students, and will not form a separate group.  The data sheets can be given to certain students prior to the lesson so they have more time to process the information before participating in the actual activity.

 

References:

Campbell, N., Reece, J. (2002). Biology. 6th ed.  Benjamin Cummings: San Francisco, California.

Kelly, M.G.  As the Worm Turns: Speciation in the Apple Maggot Fly. National Center for Case Study Teaching in Science, University at Buffalo, State University of New York.  Retrieved January 21, 2004 from the World Wide Web: http://www.sciencecases.org/maggot_fly/maggot_fly.asp

 

 

 

Data Sheets for Apple Maggot Case Study

 

            Similarities between the hawthorn and apple maggot flies:

 

-         Hawthorn and apple maggot flies are physically indistinguishable.  They are both about five millimeters long.  The tip of a female’s abdomen is more narrowly pointed than a male’s.

-         There is no geographic isolation or any other natural physical separation between the adult hawthorn and apple maggot flies.

-         Both hawthorn and apple maggot flies have the same patterns of reproduction.  The female lays her fertilized eggs into the fruit.  The maggot larvae emerge from their eggs, eat the fruit as they grow, and pupariate inside the fruit.  They eventually develop into adult flies and emerge from the fruit to reproduce.

 

Differences between the hawthorn and apple maggot flies:

 

-         Hawthorn maggot flies strongly prefer to mate on and lay their fertilized eggs into the fruit of hawthorn trees

-         Apple maggot flies also strongly prefer to mate on and lay their eggs into the fruit of apple trees.

-         Hawthorn and apple maggot flies are genetically distinguishable (they have separate, recognizable profiles)

-         There is only a 4-6% hybridization rate between hawthorn and apple maggot flies

-         Preference of fruit types acts as a stout barrier to gene flow between the two types of flies.

-         Apple maggot flies lay more eggs per fruit compared with hawthorn maggot flies.

-         The hawthorn fruit has more nutritional quality than an apple.

-         Apple fruit is larger than hawthorn fruit, and apple maggot flies are able to burrow deeper into an apple due to its larger diameter.  Therefore, apple maggot flies are more able to escape predators, like parasitoid wasps, than the hawthorn flies can.