1. What is a Species? Speciation and the Maggot Fly by Joan Sharp Simon Fraser University Modified from a case by Martin G. Kelly, Buffalo State College
Image Credit
Description: Rhagoletis pomonella (male and female apple maggot flies)
Source:
Author: Jack Kelly Clark
Licensing: Photo of Rhagoletis pomonella from UC IPM Pest Management Guidelines: Apple, UC ANR Publication Used with permission from University of California Statewide IPM Program (

2. CQ#1: Speciation can only be observed over millions of years:
True
False

3. CQ#2: Species are going extinct, but no new species are forming on Earth:
True
False

4. A Case Study in Speciation
Hawthorn trees are native to North America.
The hawthorn fruit is eaten by the larvae of the hawthorn maggot fly, Rhagoletis pomonella.
Image Credits
Description: Hawthorn tree
Source: Wikimedia Commons,
Author: Sannse
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Description: Rhagoletis pomonella (male and female apple maggot flies)
Source:
Author: Jack Kelly Clark
Licensing: Photo of Rhagoletis pomonella from UC IPM Pest Management Guidelines: Apple, UC ANR Publication Used with permission from University of California Statewide IPM Program (

5. Rhagoletis pomonella life cycle
The female lays fertilized eggs in the fruit.
Maggots (larvae) emerge from the egg, feed on the fruit, and grow through several molts.
Healthy maggots drop from the tree with the fruit and burrow in the soil. Pupation takes place in the soil.
Adult maggot flies emerge from the soil and fly to fruit trees, where they mate on the surface of the fruit.
Image Credit
Description: Rhagoletis pomonella (male and female apple maggot flies)
Source:
Author: Jack Kelly Clark
Licensing: Photo of Rhagoletis pomonella from UC IPM Pest Management Guidelines: Apple, UC ANR Publication Used with permission from University of California Statewide IPM Program (

6. But there are parasites!
Parasitoid wasps try to lay eggs in the maggot’s body, paralyzing and ultimately killing the maggot.
Image Credit
Description: Parasitoid wasp
Source: Wikimedia Commons,
Author: Richard Bartz
Licensing: Licensed under the Creative Commons Attribution ShareAlike 2.5 License.

7. Hawthorns
Hawthorns are native North American shrubs in the genus Crataegus.
Hawthorn fruits range between 5 mm and 20 mm in diameter, with an average of 12.6 mm.
Image Credits
Description: Hawthorn tree
Source: Wikimedia Commons,
Author: Sannse
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Description: Hawthorn fruit
Source: UBC Botanical Garden and Centre for Plant Research,
Licensing: Licensed under the Creative Commons Attribution-Noncommercial 2.0 Canada License.

8. Apples Apples belong to the genus Malus.
Domesticated apples (Malus domesticus) were introduced to North American in the 1600s. They are the most widely grown fruit in North America.
A typical commercial apple has a diameter of 70 mm.
Image Credit
Description: Empire apples
Source: New York Apple Association,
Licensing: Courtesy of © New York Apple Association (

9. Rhagoletis Host Shift
When apples were introduced to North America, the larva of Rhagoletis pomonella started feeding on them.
Image Credits
Description: Rhagoletis on apple
Source: Penn State Department of Entomology Image Gallery,
Author: Larry Hull
Licensing: Images in this gallery have been contributed by individuals committed to the enhancement of teaching and learning. The images may be freely used by faculty, students, and staff for non-commercial educational purposes.
Description: Photo of damaged apple
Source:
Author: Jack Kelly Clark
Licensing: Used with permission from University of California Statewide IPM Program (

10. Rhagoletis Host Shift Hawthorn (Crataegus spp) Apple
♀ lays eggs on fruit
Hawthorn (Crataegus spp)
Image Credits
Description: Rhagoletis on apple
Source: Penn State Department of Entomology Image Gallery,
Author: Larry Hull
Licensing: Images in this gallery have been contributed by individuals committed to the enhancement of teaching and learning. The images may be freely used by faculty, students, and staff for non-commercial educational purposes.
Description: Hawthorn tree
Source: Wikimedia Commons,
Author: Sannse
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Description: Apple tree
Source: Wikimedia Commons,
Author: Original uploader was Mistman123 at en.wikipedia
Licensing: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation license, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
Apple
(Malus domesticus)
Rhagoletis pomonella
1864:
First noted
apple
infestation

11. Apple vs. Hawthorn: The Maggot’s Viewpoint
The large apple fruit provides 220 times more food than hawthorn fruit.
But the nutritional quality of hawthorn fruit is superior:
52% of hawthorn maggots survive vs. 27% of apple maggots.

12. Apple vs. Hawthorn: The Risk of Attack
Larger fruits of apples are much deeper than hawthorn fruits.
Apple maggots can burrow to avoid parasitoid wasps.
Apple maggots carry fewer parasitoid wasp eggs than hawthorn maggots do.
Image Credit
Description: Parasitoid wasp
Source: Wikimedia Commons,
Author: Richard Bartz
Licensing: Licensed under the Creative Commons Attribution ShareAlike 2.5 License.

13. Today: There are Hawthorn & Apple Maggot Flies
Hawthorn and apple maggot flies are physically indistinguishable.
However, they are genetically distinct, with different genetic profiles.
There is no geographic isolation or physical separation between hawthorn and apple maggot flies.

14. Hawthorn & Apple Maggot Flies
Maggot flies tend to mate with their own kind.
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 4-6% hybridization rate between hawthorn and apple maggot flies.

15. Rhagoletis Host Shift Hybrids are viable and fertile.
Apple-raised
Hawthorn-raised
Image Credits
Description: Rhagoletis on apple
Source: Penn State Department of Entomology Image Gallery,
Author: Larry Hull
Licensing: Images in this gallery have been contributed by individuals committed to the enhancement of teaching and learning. The images may be freely used by faculty, students, and staff for non-commercial educational purposes.
Description: Hawthorn fruit
Source: UBC Botanical Garden and Centre for Plant Research,
Licensing: Licensed under the Creative Commons Attribution-Noncommercial 2.0 Canada License.
Description: Red Delicious apple
Source: Wikimedia Commons,
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Hybrids are viable and fertile.
No post-zygotic barriers

16. Timing of Host Fruit Ripening
Image Credit
Description: Emergence of Rhagoletis pomonella
Source: National Center for Case Study Teaching in Science,
Author: Jim Stamos, adapted from G. L. Bush, Sympatric host race formation and speciation in frugivorous flies of the genus Rhagoletis (Diptera: Tephritidae), Evolution 23:237–251, 1969.
Licensing: Used with permission.
Different ripening time of host fruit leads to temporal separation of apple and hawthorn flies.

17. Rhagoletis Speciation
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
Image Credits
Description: Hawthorn fruit
Source: UBC Botanical Garden and Centre for Plant Research,
Licensing: Licensed under the Creative Commons Attribution-Noncommercial 2.0 Canada License.
Description: Red Delicious apple
Source: Wikimedia Commons,
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.

18. CQ#3: Based on the information provided in this case study, are hawthorn and apple maggot flies separate species?
Yes
It depends on how the terms “species” is defined No

19. Image Credit
Description: Mating Adelé penguins
Source: Wikimedia Commons,
Author: Mila Zinkova
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.

20. CQ#4: According to the biological species concept, are hawthorn and apple maggot flies separate species?
Yes No
I cannot tell from the information provided

21. CQ#5: Which information is relevant to the biological species definition?
Hawthorn and apple maggot flies are physically indistinguishable.
There is a 4-6% hybridization rate between hawthorn and apple maggot flies.
Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.
Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

22. Ecological species concept
A species is a set of organisms exploiting a single niche.
The key aspects of this definition are the resources exploited and the habitat occupied by the members of a species.
Image Credit
Description: Predatory jungle cat, Felis chaus
Source: Wikimedia Commons,
Licensing: The copyright holder has irrevocably released all rights to this image, allowing it to be freely reproduced, distributed, transmitted, used, modified, built upon, or otherwise exploited in any way by anyone for any purpose, commercial or non-commercial, with or without attribution of the author.

23. I cannot tell from the information provided
CQ#6: According to the ecological species concept, are hawthorn and apple maggot flies separate species?
Yes No
I cannot tell from the information provided

24. CQ#7: Which information is relevant to the ecological species definition?
Hawthorn and apple maggot flies are physically indistinguishable.
There is a 4-6% hybridization rate between hawthorn and apple maggot flies.
Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.
Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

25. Morphological species concept
A species is a set of individuals with morphological features in common.
The key aspect of this definition is the morphology of the members of a species. Individuals of a species are morphologically similar to one another, yet morphologically distinct from individuals from another species.
Image Credit
Description: Shiitake mushroom
Source: USDA ARS Photo Library, also at
Author: Keith Weller
Licensing: This image is in the public domain because it contains materials that originally came from the Agricultural Research Service, the research agency of the United States Department of Agriculture.

26. CQ#8: According to the morphological species concept, are hawthorn and apple maggot flies separate species?
Yes No
I cannot tell from the information provided

27. CQ#9: Which information is relevant to the morphological species definition?
Hawthorn and apple maggot flies are physically indistinguishable.
There is a 4-6% hybridization rate between hawthorn and apple maggot flies.
Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.
Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

28. Phylogenetic species concept
A species may be defined by its unique genetic history as a tip of a phylogenetic tree.
Species are defined by their unique derived features and shared ancestry.
Image Credit
Description: Cladogram
Source: Wikimedia Commons,
Author: Fred Hsu
Licensing: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation license, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

29. CQ#10: According to the phylogenetic species concept, are hawthorn and apple maggot flies separate species?
Yes No
I cannot tell from the information provided

30. CQ#11: Which information is relevant to the phylogenetic species definition?
Hawthorn and apple maggot flies are physically indistinguishable.
There is a 4-6% hybridization rate between hawthorn and apple maggot flies.
Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.
Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

31. Modes of speciation
Allopatric speciation (‘other country”) is initiated by a geographic barrier between individuals from two natural populations.
Sympatric speciation (“same country”) takes place in a single geographic area.

32. Image Credits
Description: Mountain range
Source: Wikimedia Commons,
Author: Kogo
Licensing: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation license, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
Description: River
Source: Wikimedia Commons,
Author: Fabien Léonard
Description: Glacier
Source: Wikimedia Commons,
Author: Ben W. Bell
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.
Description: Blue Jay
Source: Wikimedia Commons,
Author: Mdf

33. Sympatric speciation
In sympatric speciation, there is no geographic barrier to gene flow.
What prevents reproduction between individuals from different populations living in the same area?
Gene flow in sympatry may be prevented by polyploidy (especially in plants) or by habitat specialization.
These factors may also be important in allopatric speciation.

34. CQ#12: Speciation in Rhagoletis is:
Sympatric
Allopatric
Image Credit
Description: Rhagoletis on apple
Source: Penn State Department of Entomology Image Gallery,
Author: Larry Hull
Licensing: Images in this gallery have been contributed by individuals committed to the enhancement of teaching and learning. The images may be freely used by faculty, students, and staff for non-commercial educational purposes.

35. Genetic divergence
Genetic divergence is the accumulation of genetic differences between two populations.

36. Factors causing genetic divergence between isolated populations
Founder effect
Mutation
Genetic drift
Differential selection

37. Reproductive isolation
How do two similar species maintain genetic isolation if they come (or remain) in contact with each other?
Reproductive isolating mechanisms prevent two individuals from distinct species from interbreeding to produce viable and fertile hybrid offspring.

38. Image Credit
Description: Sperm
Source: Wikimedia Commons,
Author: Gilberto Santa Rosa from Rio de Janeiro, Brazil
Licensing: Licensed under the Creative Commons Attribution 2.0 Generic License.
Description: Egg cell
Source: Wikimedia Commons,
Author: Faithful reproduction of a lithograph plate from Gray’s Anatomy
Licensing: This image is in the public domain because its copyright has expired.
Description: Frogs in amplexus
Source: Wikimedia Commons,
Author: Rainforest_harley
Licensing: Licensed under the Creative Commons Attribution-Share Alike 2.0 Generic License.
Description: Fertilization
Source: Wikimedia Commons,
Licensing: Believed to be in the public domain.

39. Reproductive isolation: Prezygotic barriers
Habitat isolation
Behavioral isolation
Temporal isolation
Mechanical isolation
Gametic isolation

40. Reproductive isolation: Postzygotic barriers
Reduced hybrid viability
Reduced hybrid fertility
Hybrid breakdown

41. CQ#13: When a male donkey mates with a female horse, the hybrid offspring is an infertile mule.
This reproductive barrier is:
Prezygotic
Postzygotic
Image Credit
Description: Mule
Source: Wikimedia Commons,
Author: Einar Faanes
Licensing: Licensed under the Creative Commons Attribution ShareAlike 3.0 License.

42. CQ#14: The antennae of male moths can only detect sex pheromones released by a female in his species.
This reproductive barrier is:
Prezygotic
Postzygotic
Image Credit
Description: Luna moth antennae
Source: Wikimedia Commons,
Author: Pollinator, Fairfield Glade, TN July 2003
Licensing: Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.

43. Three outcomes…
With renewed or continued contact between two populations, there are three possible outcomes:
Individuals can hybridize readily.
No speciation
2. Individuals do not hybridize at all.
Full speciation
3. Individuals hybridize but offspring
have reduced fitness.
Speciation in progress. Selection for evolution of strong
reproductive barriers.

44. CQ#15: What reproductive barrier limits interbreeding between hawthorn and apple maggot flies?
Mechanical isolation
Habitat isolation
Temporal isolation
Hybrid breakdown

45. CQ#16: Habitat isolation and temporal isolation are:
Prezygotic reproductive barriers
Postzygotic reproductive barriers

46. Question
Would you expect natural selection to favor pre-zygotic or post-zygotic isolating mechanisms between sympatric species?

47. CQ#17: Speciation can only be observed over millions of years:
True
False

48. CQ#18: Species are going extinct, but no new species are forming on Earth:
True
False

49. Questions for Further Discussion
Are apple and hawthorn maggot flies separate species?
Are they in the process of speciating?
At what point is it reasonable to say that speciation has occurred?