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.

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 (

Charles Darwin (1809-1882) Voyaged around the world 1831-1836
Wrote On the Origin of Species which reveals his ideas on Evolution by means of Natural Selection

Natural Selection Summarized:
Darwin’s theory suggests that in a species: There is a tendency towards overproduction Variation exists Variations are inherited Individuals survive in their environments with varying degrees of success Best adapted, survive and pass favorable variation on to next generation In time, great differences arise, until a new species evolved from an old species

Survival of the Fittest….
Fitness - the genetic contribution of an individual to succeeding generations relative to the contributions of other individuals in the population

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

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

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 (

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 (

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.

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.

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 (

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 (

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

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.

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.

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.

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.

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

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.

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.

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

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

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

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.

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.

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

CQ#7: Which information is relevant to the ecological species definition?

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.

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

CQ#9: Which information is relevant to the morphological species definition?

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.

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

CQ#11: Which information is relevant to the phylogenetic species definition?

Modes of speciation Allopatric speciation is initiated by a geographic barrier between individuals from two natural populations. Sympatric speciation takes place in a single geographic area.

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

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.

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.

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

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

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.

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.

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

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

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.

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.

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.

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

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

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

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

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

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?

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.

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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.

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Presentation on theme: "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."— Presentation transcript:

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