Transcript The rabbit/myxoma story

administration
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Quizzes
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Quiz 3: done
Quiz 4 (Chapters 11 and 8): December 15
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If new syllabus not sent to be by the end of the day, status quo will apply
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Thus far: Homework received from: Joelle, Antoine, Angie, Mireille. You all have until midnight
tonight. Typed. With reflection pieces.
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No documentary this Thursday; lecture on 9-11 and civil society
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Make-Ups – 12.45 – 1.45 pm – Khoury 309
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Thursday December 16
Thursday January 6
Thursday January 13
Thursday January 20
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No Class: December 22
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Do read excerpt from: “Eating Animals”
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Food, Inc.
 Wikileaks and Food, Inc.
 Wikileaks and Environment
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Chapter 8: Evolutionary Ecology
Evolutionary ecology: already
discussed
 “Nothing in ecology makes sense, except in the light of
evolution”
 Many areas in ecology where evolutionary adaptation by
natural selection takes center stage
 Evolutionary ecology
 Importance of defenses that have evolved to protect plants
and prey from their predators
 Patterns in life histories that correspond to habitats in which they
have evolved
 Optimal foraging: evolution of behavioral strategies that
maximize predator fitness and thus mold their dynamic
interactions with their prey
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Evolutionary ecology: other aspects
 Coevolution
 Evolutionary differentiation within and between species
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Molecular ecology
 Knowing how much differentiation there is within species,
or between one species and another, is critical for an
understanding of their dynamics, and for managing
those dynamics
 Is the pop offspring locally born? Immigrants?
 Who is most closely related to whom?
 Relevance?
 Use of molecular, genetic markers
 Yes, read box 8.1 
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Albatross
 Largest wingspan
 21 recognized species; 19
‘threatened’ and other 2 are
‘nearly threatened’
 Why?
 - interaction with fishing
operations, particularly longlines. A
typical longlining operation
involves releasing a single line (that
may be up to 100km long) off the
stern of the boat with as many as
3,000 baited hooks along its length.
They are common ship followers
and strike at the baited hooks as
they are being set, subsequently
drowning when the line sinks below
the water.
 - pollution
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The great garbage patch, kills
over 10,000 albatross each
year.
Unfortunately, many albatross
mistake the garbage for food
and die quite painful deaths
from the consumption of
plastics and other toxins.
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Differentiation within species
 Black-browed albatross
 T. impavida (Campell Island)
 T. melanophris (sub-Antarctic)
 Gray-headed albatross
 Breeds on a number of the
same islands as T. melanophris
 ?: how connected or separate are
these populations? Should
conservation efforts be directed at
what are currently thought to be
whole species or at particular
breeding populations?
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Albatross and molecular marking
 Study confirmed that T. impavida
was a separate species
 Study demonstrated breeding
between T. impavida and T.
melanophris
 Wider ranging gray-headed
albatrosses – from all five of their sites
– represent a single breeding
population
 Conservation perspective:
Falkland Islands also
support a breeding
population of T.
melanophris
 Thus?
 Yes: Box 8.2 
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Red
wolf
 Canis rufus – decreasing
distribution (1700 to 1970)
 1970: 14 individuals were
rescued, bred in captivity to
be reintroduced
 C. rufus exists with two
related species, C. lupus
(gray wolf) and coyote C.
latrans
 Is the red wolf a hybrid from
interbreeding gray wolf and
coyote?
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Red wolf and conservation
 “Should the conservation
status of the red wolf, and
the amount of money
spent on its conservation,
be downgraded if it is
acknowledged that it is
‘only’ a hybrid and not a
full species?
 A phylogenetic tree. Most
related are placed closest
together. Lengths of
horizontal lines represent
degree of difference
 Arrow – single genotype
shared by 8 captive red
wolves sampled
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Coevolutionary arms races
 Coevolution: remember?
 Coevolution: reciprocal evolution in two or more
interacting species of adaptations selected by their
interaction
 Evolution of both consumer and prey depend crucially
on evolution of the other
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Insect-plant arms races
 Remember: attacks by herbivores select for plant-defensive
chemicals? Remember: qualitative chemicals and quantitative
chemicals?
 Qualitative – can kill in small doses; induced by herbivore attacks
 Quantitative - rely on accumulation of ill effects; digestionreducing; produced all the time
 Toxic chemicals – by virtue of their specificity – are likely to be the
foundation of an arms race
 Plants relying on toxins are more prone to becoming involved in
arms races with their herbivores (ex: beetle and legume) than those
relying on more ‘quantitative chemicals’
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Questions …
 Do specialist herbivores generally, locked in their coevolutionary
arms races, perform better when faced with their plants’ toxic
chemicals than generalists,
 Do generalists, having invested in overcoming a wide range of
chemicals, perform better than specialists when faced with
chemicals that have not provoked coevolutionary responses?
 Answered…by analyzing wide range of data set for insect
herbivores fed on artificial diets with added chemicals
 More specialized insects suffered lower mortality on chemicals that
have provoked a coevolutionary response from specialist herbivores
 More generalist insects suffered lower mortality on chemicals that not
provoked a coevolutionary response from specialist herbivores
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Coevolution of parasites and their
hosts
 Common to find high degree of genetic variation in
virulence [highly infective] of parasites and/or in resistance or
immunity of hosts
 Every few years: a new strain of influenza virus
 No strain more devastating than worldwide epidemic
[pandemic] of Spanish flu (a subtype of avian strain H1N1)
[killed 20 million, 1918/1919]
 May seem straightforward that: Parasites select for evolution
of more resistant hosts  in turn select for more infective
parasites
 More complicated…
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Coevolution of parasites and their
hosts
 Yes: examples where host and parasite drive one
another’s evolution
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The rabbit/myxoma story
Interacting
populations
evolve in
response to each
other
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Evolution of Resistance in Rabbits
Decline in lethality of the myxoma virus in
Australia resulted from evolutionary responses in
both the rabbit and the virus populations:
 genetic factors conferring resistance to the disease
existed in the rabbit population prior to introduction
of the myxoma virus:
 the myxoma epidemic exerted strong selective pressure for
resistance
 eventually most of the surviving rabbit population consisted
of resistant animals
 less virulent strains of virus became more prevalent following
initial introduction of the virus to Australia:
 virus strains that didn’t kill their hosts were more readily
dispersed to new hosts (mosquitoes bite only living rabbits)
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The Rabbit-Myxoma System Today
 Left alone, the rabbit-myxoma system in
Australia would probably evolve to an
equilibrial state of benign, endemic disease, as
in South America [pest management specialists
continue to introduce new, virulent strains to control the
rabbit population]
 Wait. Parasites favored by natural selection are
those with the greatest fitness (greatest
reproductive rate) – sometimes achieved
through decline in virulence, sometimes not
 Further declines in myxoma virus not favored
Myxoma virus
 Blood-borne
 Transmitted from host-host by blood-feeding insect vectors
 1st 20 years after introduction to Australia, main vectors were
mosquitoes, which feed only on live hosts
 Grade I and Grade II viruses kill host quickly
 As densities decline, effective transmission becomes impossible
 Selection against Grade I and Grade II in favor of less virulent grades
 Rabbit flea – favored more virulent strains
 Selection NOT for decreased virulence – but for increased
transmissibility which is maximized by intermediate grades of
virulence
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The rabbit/myxoma story: … is an example of a predator (the virus)
and prey (the rabbits).
And an example of coevolution of parasite and host
Note: Contagious diseases spread through the atmosphere or
water are less likely to evolve hypovirulence, as they are not
dependent on their hosts for dispersal.
Host-parasitic coevolution:
agricultural plants
 Increased resistance in the host – increased infectivity in
the parasite
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Mutualistic interactions
 No species lives in isolation
 Some associations are particularly close: habitat they occupy IS an
individual of another species
 Parasites
 Nitrogen-fixing bacteria
 Symbiosis -> ‘living together’ – a symbiont occupies a habitat
provided by a host; parasites usually excluded; suggestion of
mutualistic interaction
 Mutualism: organisms of different species interact to their mutual
benefit; do not have to be symbionts
 Not necessarily conflict-free relationships – but ‘net’ beneficiaries
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Mutualistic
protectors
 Cleaner fish
 45 species: feed on ectoparasites,
bacteria, and necrotic tissue
 Grouper fish
 Bullethead parrrotfish goes to the
cleaning station off the coast of
Palestine
 Green sea turtle
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Mutualistic
protectors: plants
and ants
 Ants: the protector
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Ants protecting
Acacia from
elephants? - # > size
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Mutualism:
dispersal of seeds
and pollen
 - feeding on fruits and
dispersing the seeds – and
not just disperse the seeds
 - pollination: bees,
hummingbirds, bats, small
rodents and marsupials
 - insect pollinators: the best
 Some specialist,
protected nectars (R.
bulbusos). Why?
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Mutualism: gut
inhabitants, not
just in cows
 Live entirely ‘within’ its
partner
 Crucial role of microbes in
digestion of cellulose by
vertebrate herbivores
 Gastrointestinal tracts of all
vertebrates are populated
by a mutualistic microbiota
 Major contributors:
bacteria
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mycorrhiza
 Intimate mutualisms between fungi and root
tissue. [Cruciferae are exception]
 (1) arbuscular –found in 2/3 – most nonwoody and tropical trees
 (2) ectomycorrhizal – symbioses with many
trees and shrubs – boreal and temperate
mostly
 (3) ericoid mycorrhiza- dominant plant
species of heathland
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Ectomycorrhizas (ECM)
 Infected roots: [ ] in litter layer of soil
 Fungi form a sheath of varying thickness
around the roots
 Hyphae radiate into litter layer – getting
nutrients and water
 Fungal mycelium penetrates between cells of
root cortex and establishes interface for the
exchange of products of photosynthesis
between host and fungal partner
 ECM growth: directly related to rate of flow of
hexose sugars from the plant; when nitrate
availability is high – EM degrades
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Arbuscular
mycorrhizal
 Penetrate within the roots of the host; do
not form sheath
 Initially: fungus grows between host cells
but then enters them and forms a finely
branched intercellular ‘arbuscule’
 Benefits: nitrogen and phosphorus
uptake, pathogen and herbivore
protection, and resistance to toxic
metals
 Details/extent – depend on the species
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Mutualism: fixation of atmospheric
nitrogen
 Mutualisms of rhizobia and leguminous plants – several
steps
 Be familiar with the costs and benefits
 
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End of chapter 8
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