What is a Species?Speciation and the Maggot Fly

by Joan SharpSimon Fraser University

Modified from a case by Martin G. Kelly, Buffalo State College1

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

A. TrueB. False

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

A. TrueB. False

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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.

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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.

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But there are parasites!

Parasitoid wasps try to lay eggs in the maggot’s body, paralyzing and ultimately killing the maggot.

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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.

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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.

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Rhagoletis Host Shift

When apples were introduced to North America, the larva of Rhagoletis pomonella started feeding on them.

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Rhagoletis Host Shift

Rhagoletis pomonella

Hawthorn (Crataegus spp)

♀ lays eggs on fruit

1864: First notedappleinfestation

Apple (Malus

domesticus)

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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.

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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.

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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.

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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.

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Rhagoletis Host Shift

• Hybrids are viable and fertile. No post-zygotic barriers

Hawthorn-raised

Apple-raised

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Timing of Host Fruit Ripening

Different ripening time of host fruit leads to temporal separation of apple and hawthorn flies.

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Rhagoletis Speciation

Small fruit (13 mm) Large fruit (70 mm)High nutritional quality Low nutritional quality

Shallow burrows Deep burrowsMore parasitoid wasps Fewer wasps

Fruit available later Fruit available early

HawthornApple

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CQ#3: Based on the information provided in this case study, are hawthorn and apple maggot flies separate species?

A. YesB. It depends on how the terms “species” is

definedC. No

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CQ#4: According to the biological species concept, are hawthorn and apple maggot flies separate species?

A. YesB. NoC. I cannot tell from the information

provided

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CQ#5: Which information is relevant to the biological species definition?

A. Hawthorn and apple maggot flies are physically indistinguishable.

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

C. Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.

D. Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

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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.

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CQ#6: According to the ecological species concept, are hawthorn and apple maggot flies separate species?

A. YesB. NoC. I cannot tell from the information

provided

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

A. Hawthorn and apple maggot flies are physically indistinguishable.

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

C. Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.

D. Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

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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.

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CQ#8: According to the morphological species concept, are hawthorn and apple maggot flies separate species?

A. YesB. NoC. I cannot tell from the information

provided

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

A. Hawthorn and apple maggot flies are physically indistinguishable.

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

C. Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.

D. Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

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Phylogenetic species conceptA 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.

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CQ#10: According to the phylogenetic species concept, are hawthorn and apple maggot flies separate species?

A. YesB. NoC. I cannot tell from the information

provided

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

A. Hawthorn and apple maggot flies are physically indistinguishable.

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

C. Hawthorn and apple maggot flies are genetically distinguishable and have a distinct genetic profiles.

D. Hawthorn and apple maggot flies strongly prefer to mate and lay fertilized eggs in hawthorns and apples, respectively.

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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.

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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.

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CQ#12: Speciation in Rhagoletis is:

A. SympatricB. Allopatric

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Genetic divergence

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

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Factors causing genetic divergence between isolated populations

• Founder effect• Mutation • Genetic drift• Differential selection

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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.

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Reproductive isolation: Prezygotic barriers

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

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Reproductive isolation:Postzygotic barriers

• Reduced hybrid viability• Reduced hybrid fertility • Hybrid breakdown

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CQ#13: When a male donkey mates with a female horse, the hybrid offspring is an infertile mule.

This reproductive barrier is:A. PrezygoticB. Postzygotic

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CQ#14: The antennae of male moths can only detect sex pheromones released by a female in his species.

This reproductive barrier is:A. PrezygoticB. Postzygotic

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Three outcomes…

With renewed or continued contact between two populations, there are three possible outcomes:

1. Individuals can hybridize readily.

2. Individuals do not hybridize at all.

3. Individuals hybridize but offspring have reduced fitness.

No speciation

Full speciation

Speciation in progress. Selection for evolution of strongreproductive barriers.

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CQ#15: What reproductive barrier limits interbreeding between hawthorn and apple maggot flies?

A. Mechanical isolationB. Habitat isolationC. Temporal isolationD. Hybrid breakdown

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CQ#16: Habitat isolation and temporal isolation are:

A. Prezygotic reproductive barriersB. Postzygotic reproductive barriers

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Question

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

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

A. TrueB. False

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

A. TrueB. False

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Questions for Further Discussion

1. Are apple and hawthorn maggot flies separate species?

2. Are they in the process of speciating?

3. At what point is it reasonable to say that speciation has occurred?

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