Download 16 Coevolution Mutualism 2009

OUR Ecological Footprint - 4
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Chapter 17:
Coevolution and Mutualism
Yucca and yucca moth
Objectives
• Types of pairwise interactions
• Coevolution
•
Strict (one-on-one) vs. diffuse
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Gene-for-gene concept
•
Evidence for
• Mutualism
•
Types
•
Specificity
•
Constraints against strict
Pairwise interspecific interactions
***Which is: +/-, +/0, +/+, -/-, -/0 ?
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Mutualism
Facilitation (Commensalism) +/0
Amensalism -/0
Herbivory
Predation
Parasitism
Disease
Competition
Fluidity of interspecific relationships:
• Can evolve from one type to another.
• Switch + and - signs of interaction,
e.g. +/+ to +/-.
Symbiosis:
• an intimate and often obligatory association
of two species, usually involving
coevolution. May be parasitic or mutualistic.
Lichen =
algae + fungus
***Define ‘coevolution’:
• Interacting species evolve in response
to each other (how many species?)
• Traits of each species affect fitness of
individuals of other species.
• Traits have variation and a genetic
basis.
• May be mutualistic or antagonistic
relationship.
• Strict coevolution: narrow sense
• One species stimulates evolution in
one other species and vice versa.
• Limited to a pair of species
• May be rare and limited to very
strong interactions
• Diffuse coevolution: broad sense
• Species simultaneously respond to
an array of complex interactions with
many other species.
Batesian mimicry: “a sheep in wolf’s clothing”
shows 1/2 of coevolution equation; selection
by predator on prey
Batesian mimicry: palatable species mimic
unpalatable models.
waspunpalatable
(model)
***Which is more
common: model or
mimic? What must
predator do to
make system
work?
Mantid +
moth
palatable
(mimic)
Figure 4
Figure 5
Mullerian mimicry
Both have stingers that release toxins.
Mullerian mimicry: unpalatable species
resemble each other. Each species is both
model and mimic.
Get benefit of warning from two species.
Mullerian mimics: unpalatable organisms
share pattern of warning coloration.
Gene - for - gene concept:
‘evolutionary arms race’
• E.g. plant - pathogen interaction
•
Based on:
single gene conferring resistance
•
vs. single gene for virulence
•
Back and forth:
•
change in gene, then selection:
favor plant, then pathogen,
then plant, then pathogen…
•
Race escalates with addition of each new
trait.
Describe the coevolutionary ‘arms race’
between rabbits and myxoma virus. Include
these concepts:
Genes for resistance
Genes for virulence
Pre-adaptation
Time
Introduce virus
Coevolution
Explain the pattern
in terms of:
changes in
frequency of
virulence and
resistance genes
What evidence is
used to infer
coevolutionary
relationships?
Closely related
groups of herbivores
feed on closely
related host plants.
Suggests a long
evolutionary history
of interaction.
Coevolution between chemical defenses
of plants and their herbivores; uses
phylogenetic relationships to build an
inferential argument.
Insects
most
specialized
Experimental
Evidence:
Question:
Do ants and
treehoppers
have a
mutualistic
relationship?
Treehoppers - herbivores
Spiders - predators of ‘hoppers
***Develop an “if…then…that addresses
this question.
Hypothesis:
Prediction:
Experimental Design??
What is independent?
dependent variable?
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What is major conclusion?? Mutualism or not?
Figure 1
Mutualism: two species specialized to
perform a complementary (positive)
function for each other.
• 1) Mutualism: trophic
• Partners specialized in complementary
ways to obtain limiting energy and
nutrients
• ***Examples?
Legumes and N-fixing Rhizobium
• plant
gives bacteria shelter
• bacteria gives plant usable N
Figure 2
Endomycorrhizae in root cells of orchid
• fungus absorbs nutrients, e.g. P for plant
• plant gives carbos, vitamins, amino acids
to fungus
Figure 3
2) Mutualism: defensive
Species receive food or shelter in return
for defending against natural enemies.
• Cleaners of
parasites and
diseased tissue
Figure 4
Ant-plant defense mutualism: Acacia and
Pseudomyrmex ants
Figure 5
***What is evidence that Acacia benefits
from presence of ants?
***Predict a possible evolutionary
response when a plant, but not the ant,
invades an island. On the mainland they
are mutualists.
What is the assumption?
3) Mutualism: dispersive
Animal vectors move plant pollen and
seeds (gene flow) in return for food
rewards.
Pollination syndromes provide
circumstantial evidence of at least diffuse
coevolution.
Some plant-pollinator interactions are
highly specific (obligate mutualism).
Orchid and
male
euglossine bee
Seed dispersal (no or diffuse mutualism)
Plant-seed disperser mutualism
Ant-plant mutualism: elaisomes
Constraints on evolution of strict mutualisms
• Species diversity diffuses selection
from any one species.
• Succession, disturbance, and flux in
species’ ranges change selection over
time and space.
• Complex genetics promotes uneven
rates of evolution among mutualists;
one of pair has more potential to
respond than other.
Another obligate strict mutualism of a plantpollinator/seed predator.
Yucca and its
moth
Figure 8
Some adaptations were present before the
establishment of the mutualism
(preadaptations) and occur in close relatives
that are not mutualists.
Exam question:
Acacia trees have a mutualistic relationship with
ants.
1. Develop an “if…then…” relating to the benefit of
the mutualism to the plant.
2. Does the existence of the mutually beneficial
traits in this mutualism confirm this relationship as
an example of coevolution? Explain, including what
is the best evidence of coevolution.
3. Not all species of acacia form a mutualism with
ants. In a setting where herbivore pressure is low,
why might the mutualism not have arisen?
4. In some areas where herbivore pressure is high,
the mutualism with ants has not arisen. Predict two
alternative anti-herbivore defenses these acacias
may be using.