OUR Ecological Footprint

Chapter 20: 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 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 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.

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.

Closely related groups of herbivores feed on closely related host plants. Suggests a long evolutionary history of interaction. What evidence is used to infer coevolutionary relationships?

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?

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? Figure 6

***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 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 two alternative anti-herbivore defenses these acacias may be using.