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
1
CQ#1: Speciation can only be observed
over millions of years:
A. True
B. False
2
CQ#2: Species are going extinct, but no
new species are forming on Earth:
A. True
B. False
3
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.
4
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.
5
But there are parasites!
Parasitoid wasps try to lay eggs in the
maggot’s body, paralyzing and ultimately
killing the maggot.
6
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.
7
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.
8
Rhagoletis Host Shift
When apples were
introduced to
North America, the
larva of Rhagoletis
pomonella started
feeding on them.
9
Rhagoletis Host Shift
♀ lays eggs on
fruit
Hawthorn (Crataegus spp)
Rhagoletis pomonella
1864:
First noted
apple
infestation
Apple
(Malus
domesticus)
10
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.
11
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.
12
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.
13
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.
14
Rhagoletis Host Shift
Hawthornraised
Appleraised
• Hybrids are viable and fertile.
No post-zygotic barriers
15
Timing of Host Fruit Ripening
Different ripening time of host fruit leads to temporal
separation of apple and hawthorn flies.
16
Rhagoletis Speciation
Hawthorn
Apple
Small fruit (13 mm)
High nutritional quality
Large fruit (70 mm)
Low nutritional quality
Shallow burrows
More parasitoid wasps
Deep burrows
Fewer wasps
Fruit available later
Fruit available early
17
CQ#3: Based on the information provided
in this case study, are hawthorn and apple
maggot flies separate species?
A. Yes
B. It depends on how the terms “species” is
defined
C. No
18
19
CQ#4: According to the biological species
concept, are hawthorn and apple maggot
flies separate species?
A. Yes
B. No
C. I cannot tell from the information
provided
20
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.
21
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.
22
CQ#6: According to the ecological species
concept, are hawthorn and apple maggot
flies separate species?
A. Yes
B. No
C. I cannot tell from the information
provided
23
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.
24
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.
25
CQ#8: According to the morphological
species concept, are hawthorn and apple
maggot flies separate species?
A. Yes
B. No
C. I cannot tell from the information
provided
26
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.
27
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.
28
CQ#10: According to the phylogenetic
species concept, are hawthorn and apple
maggot flies separate species?
A. Yes
B. No
C. I cannot tell from the information
provided
29
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.
30
Daily Quiz
• 1. List three different species concepts
• 2. What are two different selective pressures
faced by Apple Maggot Flies and Hawthorn
Flies.
31
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.
32
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:
A. Sympatric
B. Allopatric
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
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:
A. Prezygotic
B. Postzygotic
42
CQ#14: The antennae of male moths can
only detect sex pheromones released by a
female in his species.
This reproductive barrier is:
A. Prezygotic
B. Postzygotic
43
Three outcomes…
With renewed or continued contact between two
populations, there are three possible outcomes:
1. 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?
A.
B.
C.
D.
Mechanical isolation
Habitat isolation
Temporal isolation
Hybrid breakdown
45
CQ#16: Habitat isolation and temporal
isolation are:
A. Prezygotic reproductive barriers
B. 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:
A. True
B. False
48
CQ#18: Species are going extinct, but no
new species are forming on Earth:
A. True
B. False
49
Questions for Further DiscussionHomework
1. Are apple and hawthorn maggot flies separate
species? What evidence do you find most
compelling?
2. Are they in the process of speciating? What
evidence do you find most compelling?
3. At what point is it reasonable to say that speciation
has occurred?
50