Trends in Evolution: Convergence and Divergence Trends in Evolution: Convergence and Divergence Purpose The purpose of this lesson is to reinforce the concepts of evolution by means of natural selection. Students will explore the evidence we use to determine evolutionary relationships. They will recognize how similar selective pressures can result in similar phenotypes and adaptive strategies and how species differentiate due to resource partitioning and competition when they pioneer a new environment. Audience This lesson was designed to be used in an introductory high school biology course. Lesson Objectives Upon completion of this lesson, students will be able to: ஃ model evolutionary relationships among species based data and revise the model based on additional information. ஃ determine the reliability of different forms of data and draw phylogenetic trees based upon these data. Key Words adaptive radiation, convergent evolution, ecomorph, niche, phenotype, phylogeny Big Question This lesson addresses the Big Question “What does it mean to observe?” Standard Alignments ஃ Science and Engineering Practices ஃ SP 2. D eveloping and using models ஃ SP 4. Analyzing and interpreting data ஃ SP 6 . Constructing explanations. ஃ SP 7. Engaging in argument from evidence ஃ MA Science and Technology/Engineering Standards (2016) HS-LS4-1. Communicate scientific information that common ancestry and biological evolution are supported by multiple lines of empirical evidence, including molecular, anatomical, and developmental similarities inherited from a common ancestor (homologies), seen through fossils and laboratory and field observations. Trends in Evolution: Convergence & Divergence 1 ஃ NGSS Standards (2013) ஃ HS-LS4-1. C ommunicate scientific information that common ancestry and biological evolution are supported by multiple lines of empirical evidence. ஃ HS-LS4-4. C onstruct an explanation based on evidence for how natural selection leads to adaptation of populations. Misconceptions Addressed ஃ In addition to probing tree-thinking and misconceptions related to modeling and interpreting evolutionary trees, this lesson addresses this common misconception about evolution and natural selection: The fittest individuals in a population are guaranteed to survive and reproduce, and the least fit are guaranteed not to survive and reproduce. (Question 4e) ஃ Further information about student misconceptions on this topic can be found h ere. You can find information about tree-thinking misconceptions o n the Understanding Evolution website . They also have a page on general evolution misconceptions . Primary Sources ஃ Bite “ Can A Computer Predict How Species Evolve? ” b ased on: ஃ Mahler, D. Luke, Travis Ingram, Liam J. Revell, and Jonathan B. Losos. 2013. “Exceptional Convergence on the Macroevolutionary Landscape in Island Lizard Radiations.” S cience 431(6143): 292-295. doi: 10.1126/science.1232392. ஃ Rabosky, Daniel L. and Richard E. Glor. 2010. “Equilibrium speciation dynamics in a model adaptive radiation of island lizards.” P roceedings of the National Academy of Sciences 107(51): 22178-22183. doi:10.1073/pnas.1007606107. ஃ Misconceptions Anderson, Diane I., Kathleen M. Fisher, and Gregory J. Norman. 2002. “ Development and Evaluation of the Conceptual Inventory of Natural Selection.” J ournal of Research In Science and Teaching 39(10): 952–978. doi: 10.1002/tea.10053 Materials ஃ Copies of the Student Handout and Science Bite for each student. ஃ Set of A nolis lizard information cards for each pair, preferably printed in color. Time This lesson should take approximately two 50-minute class periods. Additional time may be required for introductory and/or wrap-up discussions. Student Prior Knowledge Students should be familiar with evolution by means of natural selection, interpreting and drawing phylogenetic trees, and basic ecology. Trends in Evolution: Convergence & Divergence 2 Instructions and Teacher Tips ஃ General Procedure ஃ Have students find a partner. Each pair will sit with another pair to form a group of four. Discussions will happen between members of the pair and members of the overall group. ஃ Pass out sets of “species cards” representing eight species found on two islands to each pair. ஃ Student pairs will develop a phylogenetic tree model demonstrating relatedness of the eight species based on anatomical information, then anatomical and geography/niche information, and then finally based on anatomical, geographical/niche, and genomic information. ஃ After each set of evidence is introduced, student groups of four will discuss their reasoning and evidence for their model and revise if necessary. ஃ Students answer the first two analysis questions and then read the Bite and answer remaining analysis questions. This can be done individually or as a group, in class or for homework. ஃ After students have completed the activity, consider bringing them together as a class or break into small groups to compare answers and to discuss any remaining questions. ஃ Tips, Extensions, and Variations ஃ You will need to print the species cards (ideally two slides per page, so that each page has all necessary information on one species) so they can be rearranged in a phylogeny. If possible, print these cards in color. Alternatively, if you have to print the cards in black & white, you could show the species images in color over the projector so students can refer to the color images as they complete this exercise. ஃ When you first hand out the cards, have them folded so that only the lizard image and the phenotype information are visible. When they get to the niche/geography step, have them unfold to reveal that information, only completely unfolding the cards when they get to the mock genome. ஃ As the students do this activity, it may be helpful/fun to have them physically move the cards around on a whiteboard or large sheet of butcher paper and then create their phylogeny by drawing connecting lines between the cards. This creates a way for the students to interact with the phylogeny before they formalize it and write it down on the student handout. A whiteboard is a particularly appealing option because they can easily erase and redraw the connecting lines as they rearrange the cards and modify their phylogenies in each step of this lesson. ஃ When constructing trees, remind students that they should start with the most closely related lizards and then work from the ends of the tree down the branches to the root, the common ancestor for the entire group. Trends in Evolution: Convergence & Divergence 3 ஃ When they get to the genetic information, it might be helpful to review with students that an allele is a version of a segment of DNA, such as a gene. The different circles on the mock genome represent genes, so each genome model has four genes. The different letters represent different alleles. Among the lizard species we are using, Gene 1 has one allele: A. Gene 2 has three alleles: A, B, and C. Gene 3 has 5 alleles: A, B, C, D, E. Gene 4 has seven alleles: A, C, D, E, G, F, G. ஃ If students are having a difficult time visualizing different ecomorphs, consider projecting the diagram on p age 3 of Chapter 4 from researcher Jonathan Losos’ book L izards in a Phylogenetic Tree (2011, University of California Press). ஃ If you have time, you may want to conduct a class discussion on divergent and convergent evolution. This may be done after the completion of the cards activity, prior to lesson, after reading the Bite. ஃ Using classic examples of divergent evolution, discuss how species branch out into evolutionary niches. Encourage students to come up with their own examples. Discuss effects of competition and habitat/resource partitioning and how it fuels adaptive radiation and development of analogous structures. (Examples: Darwin’s finches, Galapagos Giant Tortoises, Marsupials of Australia, Lemurs of Madagascar) ஃ Using examples of convergent evolution, discuss how species can independently derive similar characteristics due to similar selective pressures. Have students brainstorm the selective pressures that cause trends toward convergence. Ask students to come up with general examples (powered flight in insects, birds and bats) and introduce some specific examples. (Examples: anteaters/ numbat, echidna/porcupine, tasmanian tiger/wolf, sugar gliders/flying squirrels/colugo, groundhog/wombat etc.) Background Information and Research Details ஃ The lesson demonstrates how models are used to study evolutionary processes. Computer modeling of evolution is a powerful tool since we can (a) “watch” evolution progress over very long time periods (millions of years) and (b) simulate the same evolutionary events many times, so we can see which outcome is most likely. ஃ Computer models of evolution use the things we know as a starting point and are designed to answer a particular research question. The Bite explores research on A nolis lizards in the Caribbean. Scientists have been studying these animals in an evolutionary context for a long time, so we understand a lot about them. We understand their lifestyles, their adaptations to their specific habitat, their phylogenetic relationships, and each species predicted genetic ‘distance’ from their most recent common ancestor. The scientists feed the phylogenetic relationships, phenotypes, and environment into a computer model. The model has ‘adaptive peaks’, which represent the ecomorphs. The scientists found that over a long period, the Anolis lizards diversified to fill the adaptive peaks, so they could predict the similar diversification we see on each Caribbean island. Then, the model allowed multiple species to fill the same adaptive peaks, and they did, which is indicative of convergent evolution. This is a powerful tool for understanding Trends in Evolution: Convergence & Divergence 4 evolutionary processes that occur over long timer periods. You can view a full phylogeny in Figure S1 in the Rabosky & Glor (2010) paper. If you focus just on the colors of the tips, you can clearly see that species are grouped by island, with a few exceptions of course! ஃ The phylogeny shown in the bite is based on genetic data.