OUR Ecological Footprint - 41.2. 3. 4.

Chapter 17:Coevolution and Mutualism

Yucca and yucca moth

Objectives• Types of pairwise interactions• Coevolution• Strict (one-on-one) vs. diffuse• Gene-for-gene concept• Evidence for

• Mutualism• Types• Specificity• Constraints against strict

Pairwise interspecific interactions

***Which is: +/-, +/0, +/+, -/-, -/0 ?

• 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

wasp-unpalatable(model)

Mantid + mothpalatable(mimic)

Batesian mimicry: palatable species mimic unpalatable models.

***Which is more common: model or mimic? What must predator do to make system work? Figure 4

Mullerian mimicry

Both have stingers that release toxins.

Figure 5

Mullerian mimicry: unpalatable species resemble each other. Each species is both model and mimic.Get benefit of warning from two species.

Mullerian mimics: unpalatable organismsshare 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. Includethese concepts:Genes for resistanceGenes for virulencePre-adaptationTimeIntroduce virusCoevolution

Explain the patternin terms of:changes infrequency of virulence andresistance genes

Closely relatedgroups of herbivoresfeed on closelyrelated host plants.Suggests a longevolutionary historyof interaction.

What evidence isused to infercoevolutionaryrelationships?

Coevolution between chemical defenses of plants and their herbivores; uses phylogenetic relationships to build an

inferential argument.

Insects most specialized

ExperimentalEvidence:

Question: Do ants and treehoppers have a mutualistic relationship?

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Treehoppers - herbivoresSpiders - predators of ‘hoppers

***Develop an “if…then…that addressesthis 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: dispersiveAnimal 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 plant-pollinator/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 twoalternative anti-herbivore defenses these acacias may be using.