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Evolution Lecture 27 Chapter 18 (last chapter in course) Topics for today: Coevolution • Example of tight coevolution • Phylogenetic test of coevolution • Expectations and examples: • Enemy-victim • Predator-prey – stop here • Host-parasite • Mutualism • Competition An example of tight coevolutionary dynamics: figs and fig wasps o 750 species of fig o 300 known fig wasps o Both highly specialized o 60 my of coadaptation Life cycle of the fig wasp Other kinds of interactions among organisms 1. Resources o Nutrition or habitat 2. Competitorso Food, space, or habitat 3. Enemies o One is the consumable resource 4. Commensals o One benefits with neutral effects on the other 5. Mutualistic o Both benefit Interactions may vary o Over time o Different environments o Mutually beneficial in low N environments o Disadvantageous to the plant in high N soil Do interacting species coevolve? o Yes, if reciprocal genetic change occurs between interacting species due to natural selection they have imposed upon each other • Specific coevolution • Diffuse coevolution • Escape and radiate coevolution Fig. 18.2 How can we test for coevolution? Example 1 Aphids and endosymbiotic bacteria: a tightly codependent relationship o Aphids suck plant sap Ï carbohydrates Ð amino acids o Aphid can’t make some essential amino acids o Supplied by bacteria symbionts o Plant supplies sugars, C, N & glutamine o Interaction is 150 to 250 myo o In nature, can’t exist alone • Examine phylogenies and look for parallel change Matching  Association dates from the origin of the group  Diverged in concert  Little cross infection between Mismatching  Cross infection and independent evolution  Extinction and recolonization  More likely if organism disperses Fig. 18.3 Example 2 Gophers and Lice o Solitary life in individual tunnels o Rarely meet except to mate o Lice specialized to cling to hairs of hosto Not highly mobile otherwise o Areas of congruence and lack of congruence Fig. 18.3 Coevolution: Enemies and victims Examples 1. Predator-prey 2. Disease-host 3. Plant-herbivore What traits should evolve? o Offensive traits o Capturing prey o Subduing prey o Infecting hosts Fig. 18.4 o Defensive traits o Cryptic patterning o Chemical defense o Immune system o Strict coevolution difficult to show because trait may be valuable in more than one interaction Fig. 18.5 Outcomes of coevolution in enemy-victim systems o Unending evolutionary arms race o Offensive and defensive weaponry expensive o Costs eventually outweigh benefits o Stable equilibrium o Defensive compounds cost the plant 10% of energy budget Ð fitness in absence of insect o Compounds attract other enemies o Continuous cycling o Gene-for-gene interactions o Quantitative trait interactions o Extinction Example: Gene-for-gene model of coevolution o Negative frequency-dependent selection o 1 or few loci controls resistance and infectivity o If resistance has some cost, then it will decline with a change in infectivity Fig. 18.6 Fig.


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U of M BIOL 4802 - Coevolution

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