CLADISTICS Student Packet SUMMARY PHYLOGENETIC TREES AND CLADOGRAMS ARE MODELS OF EVOLUTIONARY HISTORY THAT CAN BE TESTED Phylogeny is the history of descent of organisms from their common ancestor. Phylogenetic trees/cladograms are constructed by employing computer programs that measure and model relatedness among organisms. When constructing phylogenies, scientists consider fossils record, molecular similarities such as DNA or amino acid sequences, comparisons of embryology, metabolic pathways, reproductive strategies, behavioral traits, biogeography and homologous structures. Homologous structures are any features shared by two or more species that are inherited from a common ancestor. Vestigial structures are examples of homologous structures; they are remnants of functional structures and when compared to fossils, they provide evidence for evolution. On the other hand, analogous structures should not be used in establishing phylogenies. Analogous structures are those that are similar in structure but are not inherited from a common ancestor. Analogous structures evolve through convergent evolution, the independent evolution of similar features in different lineages, as a result of similar selection pressures. The placental mole and the southern Australian mole are both burrowing mammals, yet are not closely related. Bird and bat wings are analogous because they have separate evolutionary origins, but they evolved to serve the same function. Birds and bats did not inherit wings from a common ancestor with wings. However, forelimbs of birds and bats are homologous because they inherited forelimbs from a common ancestor with forelimbs. INTERPRETING PHYLOGENETIC TREES/CLADOGRAMS Groups (taxa) of evolutionary related organisms are represented as branches in cladograms. The tips of the tree represent groups of descendent taxa and nodes (branching points) represent the common ancestors of those descendants. When a lineage splits into two branches, a speciation event occurred. Phylogenetic trees/cladograms can represent traits that are either derived or lost due to evolution. The original shared trait is termed the ancestral trait and the trait found in the newly evolved organism is called the derived trait. An example of a lost trait is lack of legs in sea mammals. Sister taxa are groups or organisms that share an immediate common ancestor and are very closely related. Outgroup is a group/taxon that is less closely related to the other organisms. 1
A clade (a.k.a. monophyletic group) is any taxon that consists of all the evolutionary descendants of a common ancestor. All clades are subsets of larger clades, with all life as the most inclusive taxon. Every node in a phylogenetic tree should give rise to two lineages. If more than two lineages are shown, it indicates an unresolved pattern of divergence (polytomy). Branches can rotate and still represent the same phylogeny. The two phylogenetic trees below illustrate the same evolutionary relationships. (The vertical branches have been rotated). If there are several hypotheses about relatedness of organisms, the principle of parsimony is used: the simplest explanation with fewer evolutionary events is more likely to have occurred. Phylogenetic trees may include estimates of times of divergence of lineages determined by the molecular clock analysis. The molecular clock hypothesis states that among closely related species, a given gene usually evolves at a reasonably constant rate. The number of mutation events in a gene can be used to predict times of evolutionary divergence. Phylogenetic trees are constantly being revised based on the biological data used, new mathematical ideas and emerging knowledge. 2
MULTIPLE CHOICE QUESTIONS Questions 1-2 The cladograms below show possible evolutionary relationships among four main groups of plants: nonvascular plants such as mosses, seedless vascular plants such as ferns, naked seed vascular plants such as pines and flowering plants such as oaks. 1. What is the outgroup in the cladograms? a. Mosses b. Ferns c. Pines d. Oaks 2. Based on the information shown in tree (b), which plants have a vascular system and seeds? a. Mosses and ferns b. Ferns and pines c. Pines and oaks d. Oaks 3. The cladogram below shows the evolutionary relationships in vertebrates. Which if the four letters corresponds to evolution of lungs? a. A b. B c. C d. D 3
Question 4-5 The heart is a muscular organ which pumps blood throughout the circulatory system. In vertebrates, there are three types of hearts: a two-chambered heart (consisting of one atrium and one ventricle), a three-chambered heart (consisting of two atria and one ventricle) and a four-chambered heart (consisting of two atria and two ventricles). The cladogram below depicts the evolution of the heart. 4. Which of the following statements is supported by the cladogram above? a. Zebrafish and lampreys have identical circulatory mechanisms. b. The heart of clawed frog is more similar to the human heart than to the heart of pufferfish. c. Birds have a three-chambered heart. d. The circulatory system of zebrafish is more closely related to the circulatory system of lampreys than to the circulatory system of green anole lizards. 5. Which of the following is an incorrect interpretation of the information presented in the cladogram? a. Homeostatic control systems, such as the circulatory system, reflect common ancestry. b. The hearts of animals evolved new mechanisms due to adaptations to different environments. c. Organisms share conserved core processes that evolved and are widely distributed today. d. Several mammals, such as mouse and opossum, evolved circulatory mechanisms independently from other mammals. 4
6. The table below depicts characters of several animal species. Character A - Turtle B - Horse C - Wolf D - Cat E - Leopard Hair + + + + Carnivorous + + + Retractable Claws + + Mammary glands + + + + Ability to purr + Backbone + + + + + Which of the following cladograms is the most accurate representation of the information in the table? a. Cladogram 1 b. Cladogram 2 c. Cladogram 3 d. Cladogram 4 7. The North American flying squirrel can be found at night gliding among the tree-tops and gathering insects for food. The Australian sugar glider essentially leads the same lifestyle. If these two species of squirrels were placed side by side, they would look almost the same. However, the North American flying squirrel is a placental mammal while the Australian sugar glider is a marsupial mammal. How is it possible that these organisms look identical? a. The two species of squirrels are displaying adaptive radiation. When Pangaea broke apart, the squirrels moved away from each other and adapted to different climates. b. Flying squirrels and sugar gliders have undergone sympatric speciation during Pangea. c. This is an example of convergent evolution. The two species appear to be one because they have evolved in similar environments and similar traits were favored for survival. d. These two species share a distant common ancestor. The process of divergent evolution forced them to evolve into two different species over time. 5
MATH GRID IN 1. The molecular clock hypothesis states that among closely related species, a given gene usually evolves at a reasonably constant rate. The number of mutation events in a gene can be used to predict times of evolutionary divergence. Cytochrome c is an electron carrier of the electron transport chain located in the inner mitochondrial membrane. On average, this protein mutates every 17.7 million years. In mammals, cytochrome c contains 15 different amino acids than the reptilian cytochrome c. Using the molecular clock hypothesis, calculate how many million years ago mammals and reptiles diverged. Give your answer to the nearest whole number. SHORT FREE RESPONSE QUESTIONS 1. Mammalian milk contains antibodies that are produced by the mother s immune system and passed to offspring during feeding. Mammalian milk also contains a sugar (lactose) and may contain proteins (protein A, protein B, and casein), as indicated in the table. MILK COMPONENTS IN DIFFERENT MAMMALS Character Cat Cow Horse Human Pig Lactose + + + + + Protein A + + + + + Protein B + + + Casein + + + + indicates the presence of the character and indicates the absence of a character Using the data in the table, construct a cladogram on the template provided to indicate the most likely evolutionary relationships among the different mammals. Indicate on the cladogram where each of the characters most likely arose in the evolutionary process, and justify the placement of the characters on the cladogram. 6
2. Based on the data in the table below, draw a phylogenetic tree that reflects the evolutionary relationships of the organisms based on the differences in their cytochrome c amino-acid sequences and explain the relationships of the organisms. Identify which organism is most closely related to the chicken and explain your choice. THE NUMBER OF AMINO ACID DIFFERENCES IN CYTOCHROME C AMONG VARIOUS ORGANISMS Horse Donkey Chicken Penguin Snake Horse 0 1 11 13 21 Donkey 0 10 12 20 Chicken 0 3 18 Penguin 0 17 Snake 0 7
LONG FREE RESPONSE QUESTION Phylogeny reflects the evolutionary history of organisms. a) Explain THREE methods that have been used to investigate the phylogeny of organisms. Describe a strength or weakness of each method. b) The two phylogenetic trees represent the relationship of whales to six other mammals. All of the organisms shown have a pulley-shaped astragalus bone in the ankle except for the whale. i. For each tree, describe a monophyletic group, the closest relative to the whale, and the point at which the pulley astragalus was lost or gained. ii. Based on the principle of parsimony (the simplest explanation is the best) and the genomic information in the table shown, identify which tree is the best representation of the evolutionary relationship of these animals, and justify your answer. 8
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