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Week_7_Answers - STUDY QUESTIONS(Lectures 23-25 1...

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STUDY QUESTIONS (Lectures 23-25) 1) Biologists have claimed that the jaws and limbs of terrestrial vertebrates evolved from structures that had a different function in their aquatic ancestors. What structures do biologists believe the jaw and limb evolved from, what is the evidence, and what was the significance of these events for vertebrate evolution? Biologists think the jaw evolved from gill arches and the support for this idea is morphological (both are hinged bony bars moved by muscle cells coming from the same embryonic cell population) and developmental (both are derived from neural crest cells). The jaw plays a huge role in animal feeding (in the capture and processing of food items). Biologists think the limb originated as a fish fin and the support for this idea is morphological (number and arrangement of bones is similar in both), developmental (Hox gene protein products are found at the same time and places in mouse limbs and zebrafish fins) and paleontological (a transitional “fishapod” has been unearthed in Canada). The limb plays an important role in tetrapod movement (e.g., on land). 2) You discover an organism you believe to be some sort of land snail laying eggs. You bring these eggs back to the lab with you and carefully observe the cellular development of the zygote and embryo under a powerful microscope. You observe radial cleavage at the 4-cell stage and that the pore formed by gastrulation in the developing embryo eventually ends up being the anus of the embryo. Why is this unexpected and what do you now know? Radial cleavage at the 4-cell stage of development and anus formation through gastrulation are diagnostic features of the deuterostomes (the “second mouths”). Because snails are gastropod mollusks,
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