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Lecture

introduction to animal diversity.pdf

12 Pages
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Department
Biology
Course Code
BIOL 150B
Professor
Beaulieu

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INTRODUCTION TO ANIMAL DIVERSITY Reading: Chapter 18, p. 383 (homeotic genes); Chapter 21, pp. 457-459 (homeoboxes and Hox genes); Chapter 25, pp. 535-536; Chapter 32 A. WHAT ARE ANIMALS? 1. Animals are multicellular, heterotrophic eukaryotes. 2. Animals ingest other organisms or organic debris. 3. Animal cells are not surrounded by cell walls. 4. Animals have intercellular junctions found in no other group of organisms. 5. Animals have nervous tissue and muscle tissue. 6. Animals have common features of early development. Fig. 32.2 Note the evident polarity in the gastrula stage – one end is different from the other. How might the polarity in the multicellular gastrula have its origin in the unicellular zygote? Introduction to Animal Diversity - 2 7. Almost all animals have Hox genes, which contain homeoboxes and which are homeotic in function. Hox genes function during development to specify features of the animal’s body plan. homeobox homeotic gene Hox genes encode regulators of the transcription of other genes, thereby turning on or off genes whose products build body parts during development. Variation in Hox gene activity can therefore lead to variation in animal body plan. Hox genes are of great evolutionary significance when we try to understand animal diversification at the levels of the phylum and the class. B. FUNDAMENTAL DIFFERENCES in ANIMAL FORM and DEVELOPMENT 1. Some of these differences define clades, while others define grades. A grade is a group of organisms that share an important adaptation, but not through common descent. 2. Presence of tissues – integrated groups of cells that perform specialized functions Introduction to Animal Diversity - 3 no tissues – Parazoa (sponges, placozoans) tissues – Eumetazoa Fig. 32.11 Animal phylogeny based on molecular data. You should compare this phylogeny with the one in Figure 32.10 (which is based on morphological arguments) and note the similarities and differences. Why, generally speaking, might a morphological tree differ from a molecular tree? Introduction to Animal Diversity - 4 3. Body symmetry and the number of germ layers in the embryo radial symmetry and two germ layers (ectoderm and endoderm; diploblastic) polarity of the adult animal Radially symmetric animals are often sessile (attached to a substrate) or planktonic (drifting). bilateral symmetry and three germ layers (ectoderm, mesoderm and endoderm; triploblastic) polarities of the adult animal Bilateral symmetry is associated with cephalization – the anterior concentration of neural processing power, sensory apparatus, and sometimes weapons. 4. Early development: protostomes and deuterostomes determinate cleavage – early cleavage products lose the ability to form complete embryos by themselves indeterminate cleavage – early cleavage products can form complete embryos by themselves Introduction to Animal Diversity - 5 Fig. 32.9 Consider the associations of traits pictured here. Do you think these traits are associated for functional reasons or for historical reasons? 5. Presence of a body cavity (a coelom) between the gut and the body wall. coelomate coelom is lined with mesoderm only pseudocoelomate pseudocoelom is lined with endoderm and mesoderm Introduction to Animal Diversity - 6 acoelomate no coelom; mesoderm forms a solid mass of cells Fig. 32.8 Where in your own body is your coelom? functions of the coelom Introduction to Animal Diversity - 7 C. ORIGINS of ANIMALS and EARLY ANIMAL EVOLUTION 1. The animal kingdom is monophyletic, with the divergence of animals from a colonial protist occurring long before any fossil traces of animals appear. 2. Animal near-relations alive today: choanoflagellates 3. Early animal evolution might have been stimulated by the end of the Snowball Earth phase (Cryogenian Period). 4. The Doushantuo fossils (approx. 600 – 580 million years ago) look like animal embryos. 5. The Ediacarans (585 – 542 million years ago) – the first large multicellular organisms. Introduction to Animal Diversity - 8 6. The Cambrian explosion (542 – 525 million years ago) – half of extant animal phyla were represented. Many basic body plans appeared and many diversified during the Cambrian. key fossil formation: the Burgess shale the Cambrian explosion – why? emergence of predator-prey relationships rise in atmospheric O – could support more active metabolism 2 evolution of Hox genes, which control development in early embryos Introduction to Animal Diversity - 9 DIGRESSION: GENETIC VARIABLITY in GENOME EVOLUTION The following aspects of genome evolution could apply to any type of organism, but are given here because genome evolution in animals is a topic of particular interest in evolutio
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