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Evolution Simulation Littorina obtusata, the flat periwinkle http://www.arkive.org/flat-periwinkle/littorina-obtusata/video-00.html Darwinian Snails http://www.arkive.org/common-shore-crab/carcinus-maenas/video-08.html Carcinus maenus, the green crab http://www.pbs.org/wgbh/evolution/educators/teachstuds/svideos.html Range of Carcinus maenas. Blue areas are the native range; red areas are the introduced or invasive range. Black dots represent single sightings that did not lead to invasion, and green areas are the potential range of the species. Appledore Island •! One of seven islands forming Isles of Shoals •! About 7 mi off coast of Maine Robin Seeley’s Experiment 14 Shorter & thicker 7 •!compared shells in two populations of snails on Appledore Island •!1871- before arrival of green crab •!1980s - after arrival •!1980s shells: thicker and narrower range of variation Taller & thinner Snail Population Evolved Based on his analysis, Seeley predicted that the snail population had evolved Evolution: change in composition of population •! Average shell thickness •! Range of variation •! Spire height Part I: Simulation Based on Seeley’s Experiment A Model of Evolution by Natural Selection Starting population 4th Generation Part II: Requirements for Evolution by Natural Selection •! Examine three conditions which may affect evolution by selection •! Variation •! Inheritance •! Selection •! See what effects each of these conditions has on evolution of the snail population Part II: Requirements for Evolution by Natural Selection Variation Would you expect the snail population to evolve if all the snails had the same shell thickness at the start? No Variation Starting Population No Variation 2nd Generation No Variation Part II: Requirements for Evolution by Natural Selection Inheritance Would you expect the snail population to evolve if shell thickness were not inheritable; i.e. if there were no genetic basis for shell thickness? Would the population evolve if each snail grew its shell to a random thickness that had nothing to do with its parent’s shell thickness? No Inheritance No Inheritance Starting Population No Inheritance 4th Generation Part II: Requirements for Evolution by Natural Selection Selection Would you expect the snail population to evolve if predator crabs were especially large and could crack snail shells no matter how thick they are? These “super crabs” would eat snails randomly, without any preference for thinner shells. http://www.sml.cornell.edu/sml_mm_video_smlnew1.html No Selection Trial 1: No Selection Starting Population Trial 1: No Selection 4th Generation Trial 3: No Selection Starting Population Trial 3: No Selection 4th Generation Part III: Darwin's Theory of Evolution by Natural Selection Variation + Heredity + Differential = Natural Selection Reproduction Under which conditions will the snail population evolve toward thicker shells? Part IV: What is the Source of Variation? MUTATIONS •!Mutation: an error during reproduction •!Result: Mutant offspring do not resemble parents. There is an equal probability that shells may be thicker or thinner than parents •!How can the population be driven towards thicker shells with mutations than without mutations? Part IV: Source of Variation Among Individuals: MUTATIONS Starting Population 3rd generation 9th generation 12th generation 6th generation Part V: What makes populations evolve? a)! After they were born, did the individual snails ever change their shell thickness? If the individuals didn’t change, how was it possible for the population to change? b) Did snails grow thicker shells because the snails needed them in order to survive? If not, where did new thicknesses come from? c) Did the predators create a need for the snails to change - a need to which the snails responded? Or did the predators simply determine which snails survived to reproduce and which didn’t? Part VI: CHALLENGE - Evolution by natural selection in flat periwinkles Seeley’s two experiments 1.!Lab experiment Average time to eat thin-shelled snail - 42 sec. a)! Only one of eight crabs was able to eat thick-shelled snail within 8 min. b)! The other crabs (7/8) gave up within 8 min. Field Experiment Results a)! Tethered 15 pairs of snails (one thick-shelled and one thinshelled) to seaweed with fishing line b)! Placed 15 pairs at 3 different locations on Appledore Island •!Gleason Point - crabs abundant •!Sipp Bay - crabs rare •!Timber Cove - crabs absent c)! Observations •! Checked each locations at 6, 9 and 16 days •! Could determine if snails were eaten by crabs, died (other cause) or escaped •! If eaten, part of shell would still be attached to fishing line Field Experiment Results •! Red circles=thick shells •! Blue circles= thin shells Part VI-A: CHALLENGE 1. Review the requirements for evolution by natural selection. a) What evidence, if any, does Seeley have that the flat periwinkles of Appledore island vary in the thickness of their shells? b) What evidence, if any, does Seeley have that snails with thick shells are more likely to survive than snails with thin shells? (Think carefully about this.) c) What evidence, if any, does Seeley have that shell thickness is heritable? (Again, think carefully about this.) Part VI-A: CHALLENGE 2. Seeley’s data provide direct evidence that two out of three of Darwin’s conditions for evolution by natural selection are true of the flat periwinkle population on Appledore Island. •! For the third condition, the evidence is indirect at best. •! For which of the three conditions is the evidence you have seen the weakest? Explain. Part VI-B: CHALLENGE - Designing Your Own Experiment •! Do the snails from the crab-infested environment have thicker shells, on average, than the snails from the crab-free environment? •! Do the snail populations meet all three requirements for evolution by natural selection? •! Variation •! Inheritability of shell thickness •! Some snails survive to reproduce more successfully than others •! Do the snails from the two environments differ because one or both has evolved by natural selection, or do they differ simply because snails can smell crabs and grow thicker shells when they need them? Part VI-B: CHALLENGE - Designing Your Own Experiment Question 1: Do the snails from the crab-infested environment have thicker shells, on average, than the snails from the crab-free environment? Distribution of shell thicknesses for snails living in crabfree waters (West coastline). Distribution of shell thicknesses for snails living in crab-infested waters (East coastline). Part VI-B: CHALLENGE - Designing Your Own Experiment Question 2: Do the snail populations meet all three requirements for evolution by natural selection? •! For evolution by natural selection to occur, the snail population must contain variation in shell thickness. •! For evolution by natural selection to occur, the variation must be at least partly inheritable. •! For evolution by natural selection to occur, some snails must survive to reproduce at higher rates than others. Part VI-B: CHALLENGE - Designing Your Own Experiment Question 3: Do snails from the two environments differ because one or both have evolved by natural selection, or do they differ simply because snails can smell crabs and grow thicker shells when they need them? Part VI-B: CHALLENGE - Designing Your Own Experiment •! For each question design experiments to test the hypothesis. •! Use controls: e.g., same numbers and sizes of snails in each experimental tank, same numbers of crabs + bands on claws, tanks with and without crabs •! Limit the snails of different thicknesses (e.g., use one or a few thicknesses •! Describe your experiments •! Use images of experimental set-up •! Make sure your histograms are numbered and labeled. •! Keep them in sequence. Is Shell Thickness Inheritable? Plan for today 1.! Complete the Darwinian Snails packet and hand it in along with the labeled histograms. 2. Take the quiz on Darwinian Snails packet. 3. Download Mitosis Demystified and complete the exercise. 4. Take the self-test; omit question 11. 5. Remove the adult flies (F1) from your vial. Links to download Mitosis Demystified: http://simbio.com/downloads/AppFilesFall2010/MM/ MitosisandMeiosisMac.dmg http://simbio.com/downloads/AppFilesFall2010/MM/ MitosisandMeiosisWin.zip Pre-Lab Assignment 1.! Review the life cycle of Drosophila 2.! Read the Laboratory Exercise: Analyzing Eye Pigment Mutations in Wild and Mutant Strains of Drosophila 3.! Read the material from Campbell, Concepts and Connections. 4.! Complete Mitosis Demystified, take the diagnostic test and post or send your score. 5.! Prepare for the quiz on Darwinian snails, cell cycle and mitosis