Great Transformations Answer Key

Evolution: Great Transformations ANSWER KEY You are responsible for answers to the questions highlighted below. 1. What is a ‘Great [evolutionary] transformation’? Key evolutionary change in a lineage that opens the door for new ways of life and new forms of life. Here we focus on 3 great transformations: A. Evolution of whales B. Evolution of Tetrapods C. Evolution of Animal Body plans A. Evolution of Whales 2. Why is the evolution of whales puzzling? Whales are mammals, mammals evolved on land [approx 200mya]. Whales are aquatic but evolved later, after the evolution of terrestrial mammals. (note: mya = million years ago) Puzzle = How/when did mammals return to an aquatic existence? How did marine mammals evolve? 3. What was Darwin’s hypothesis about whale evolution? Whales evolved from terrestrial mammals. 4. Our understanding of whale evolution was aided by the discovery of the fossil of a wolf-like skull of a terrestrial organism (in the 1980’s). Why was this fossil important to the study of whale evolution? The distinctive shape of the inner ear on this fossilized skull is found only in modern and extinct whales. This suggested this terrestrial animal was related to, and likely an ancestor of, modern whales. 5. The video describes two types of evidence that were used to reconstruct whale evolution. Which of the following types of evidence were important? The fossil record Limb homology The fossil record (paleontology) Seeking transitional forms Mechanics of swimming Biogeography 6. Gingerich sought transitional forms to test predictions of Darwin’s hypothesis. What transition was Gingerich seeking to understand? He was seeking to understand the transition from land mammals to aquatic mammals. 7. What types of organisms did Gingerich expect to find if Darwin’s hypothesis was correct? He expected to find a series of related organisms appearing in sequence in the fossil record. These should show the changes in traits necessary to go from the wolf-like ancestor found in Pakistan to the modern whale. NOTE: As we discussed in class, it is not necessary that every organism was a ‘link’ in a chain that led in a direct line of descent to modern whales. There might also be some organisms that showed transitional features (some adaptations for land in addition to some adaptations for aquatic life) which left no living descendants but are nevertheless evidence that the predicted evolutionary sequence is plausible. 8. Why did Gingerich seek transitional forms that would be informative about whale evolution in the Sahara Desert? (there may be more than one correct answer) a. Fossils are more likely to be well-preserved in the hot-dry dessert compared to an aquatic habitat. b. The Sahara Desert was a sea 40 million years ago. c. One lineage of the wolf-like ancestor of all whales migrated to the Sahara Desert prior to the evolution of whales. d. There is evidence that terrestrial ancestors of whales lived in the Sahara Desert millions of years ago. Although the Sahara is currently a desert, it was a sea 40 mya. There is an area there called ‘whale valley’ that contains the remains of many whale-like animals. NOTE: This is a good example of the radical change in environmental conditions over geological time that was discussed in the first few lectures. 9. What did Gingerich discover about Basilosaurus that was not previously known, and what did this suggest about the origin of this animal? Basilosaurus had a pelvis, complete set of leg bones, and toes, unlike modern whales. This strongly suggested terrestrial origins. 10. Which of the following were major changes in body plan in the transition series leading to modern whales? (select all correct answers) Loss of legs Origin of a swim bladder Movement of nostrils to top of the head Streamlining of body shape 2. Mechanics of swimming In addition to the fossil record, commonalities between living aquatic mammals and terrestrial mammals suggest common ancestry, particularly when contrasted with aquatic animals that are more distantly related. 11. How does swimming differ between fish/sharks compared to whales/dolphins, and how do these compare to movement patterns of terrestrial mammals? a) Fish/Sharks: flex spine from side to side b) Whales/Dolphins: undulate spine up and down (dorso-ventral) c) Terrestrial mammals moving on land or in the water: undulate spine up and down (dorso-ventral) when running or swimming d) Movements of Whales/Dolphins is most similar to: Terrestrial mammals (e.g., compare otter swimming and dog running to whales/dolphins) B. Evolution of Tetrapods Life evolved in the sea. This section concerns how, where and when the transition to land began. The focus is on tetrapod evolution. Tetrapods are animals with 4 limbs (e.g., amphibians, reptiles, birds, mammals). The presence of 4 limbs in all of these organisms suggests an early common ancestor with 4 limbs. 12. The discovery of a fossil of an aquatic animal with a tetrapod shoulder contradicted old hypotheses about tetrapod evolution. Why? The old hypothesis was that fish moved onto land, then, fins evolved into legs to allow terrestrial movement. Under this hypothesis, aquatic animals should not have limbs (or tetrapod shoulders). 13. What new hypothesis about tetrapod evolution did Clack propose? Hypothesis: limbs evolved in an aquatic ancestor and only later were used on land. NOTE: It is key here to understand that evolution is not ‘forward looking’. That is, the limbs did not evolve ‘so that they could later be used on land’. Rather, limbs were useful in the water, and when these animals later ended up on land, the limbs could be co-opted (changed by natural selection) for use as terrestrial locomotory organs (i.e., they were exaptations, a type of adaptation that is currently used for a different function than it was when it originally evolved). The process here is the same slow process we have been discussing, based on some existing variants conferring higher lifetime reproductive success than other variants; and the average trait in the population shifts. Then additional mutation can additional variation, selection acts on the new variants, and the process continued over generations can lead to limbs well-adapted to terrestrial life. 14. What key features of the early tetrapod skeleton discovered by Clack supported her new hypothesis about tetrapod limb evolution? It was a ‘fish with fingers’. It had a complete, paddle-like hand—tetrapod hand bones on appendages, but it also had a fishlike tail and gills, so was clearly aquatic. C. Evolution of Animal Body plans 15. What was the Cambrian explosion (570 million years ago)? = sudden explosion of diversity in animal body plans Genetic basis of body plan evolution Fossils show major changes in animal body plans over time. This section focuses on how these changes may have occurred. 16. Organisms show a pattern of repeating segments, and there are sometimes errors or repetition of segments. Bateson studied developmental ‘errors’ where body parts developed in the wrong place. What did these developmental errors suggest? Errors suggested some underlying ‘blueprint’ for animal development might exist. This might sometimes be disrupted to lead to misplaced body parts. This was inferred because the body part was built correctly, suggesting it was not the gene for building those parts that caused the problem, rather the error was in something else that determined where those parts would be built. These novel arrangements of body parts introduced radically different variants to populations. These variants might sometimes be viable and trigger the evolution of diverse of body plans. 17. Lewis proposed an initially controversial hypothesis about body plan development. Under his hypothesis, why could relatively few mutations lead to big differences in body plans? Lewis proposed that development of each body segment or area (e.g., insect head or abdomen) is directed by a single gene (‘architect’ gene), so a small set of genes are responsible for the layout of an entire body plan. These genes turn on other genes that are appropriate to the body part being constructed (so they are like ‘architects’ in the sense that they direct the construction of the body). Even small errors in ‘architect’ genes or in turning on ‘architect’ genes can lead to substantial differences in body plans as a result of relatively few mutations. (NOTE: these ‘Architect’ or Homeobox (or Hox) genes are developmental regulatory genes – these regulate the transcription of other genes in embryos. Homeobox products provide positional information in a multicellular embryo and induce the formation of particular parts or structures in the right position. They do not contain the information necessary to build the structure itself, but they do ‘turn on’ the genes that then create the structure. 18. Levine & McGinnis tested Lewis’ hypothesis using the Antennapedia gene in Drosophila. What did their new technique demonstrate? Antennapedia (which controlled leg development) was normally expressed / turned on in the thorax of developing embryos. They showed that Antennapedia was like a master switch that was turned on in areas that would become insect thorax. Genes like antennapedia could be the architects of body plans—turning on genes in the wrong place would lead to radical changes in body plans, and provide variation on which natural selection could act. 19. What is the evidence for ‘architect’ genes in other organisms? (Note that the ‘architect’ genes referred to in this film are Hox (or homeobox) genes—developmental regulatory genes –these regulate the transcription of other genes in embryos. The use of the term 'architect' gene is to make this documentary accessible to a broader audience). The ‘eyeless’ gene controls eye development in Drosophila A very similar gene is present in the mouse, and is involved in eye development. Transplanting the ‘eye control’ gene from the mouse to Drosophila leads to development of a normal (compound) insect eye. Conclusion: same gene directs eye growth in both flies and mammals. This ‘architect’ gene turns on other genes that direct eye development (i.e., the gene itself does not determine eye structure, since the Drosophila did not develop a mouse eye). Rather, the gene turns on other genes that are necessary for eye growth. 20. What do ‘architect’ (Hox) genes suggest about evolution of body plans across taxa? Apparently animals use a very similar set of ‘architect’ genes to build bodies. This suggests evolutionary conservation of mechanisms for building body plans among animals. That is, the ‘architects’ turn on other genes—these other genes may have been modified over evolutionary time (e.g., mouse eye versus a Drosophila eye)--but the architects are very similar across taxa. ‘Architect’ genes were likely inherited from common ancestors to most organisms with complex body plans. The evolution of new body plans could be fueled by relatively simple changes in these genes or pattern of activation of these genes.