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Chapter 32

BIOL 1030 Chapter 32: Chapter 32 An Introduction to Animal Diversity

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University of Manitoba
Biological Sciences
BIOL 1030
Scott Kevin

Chapter 32 An Introduction to Animal Diversity Lecture Outline Overview: Welcome to Your Kingdom • Biologists have identified 1.3 million living species of animals. • Estimates of the total number of animal species run far higher, from 10 to 20 million to as many as 100 to 200 million. Concept 32.1 Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers • There are exceptions to nearly every criterion for distinguishing an animal from other life forms. • However, five criteria, taken together, comprise a reasonable definition. 1. Animals are multicellular, ingestive heterotrophs. • Animals take in preformed organic molecules through ingestion, eating other organisms or organic material that is decomposing. 2. Animal cells lack cell walls that provide structural support for plants and fungi. • The multicellular bodies of animals are held together by extracellular structural proteins, especially collagen. • Animals have other unique types of intercellular junctions, including tight junctions, desmosomes, and gap junctions, which hold tissues together. • These junctions are also composed of structural proteins. 3. Animals have two unique types of cells: nerve cells for impulse conduction and muscle cells for movement. 4. Most animals reproduce sexually, with the diploid stage usually dominating the life cycle. • In most species, a small flagellated sperm fertilizes a larger, nonmotile egg. • The zygote undergoes cleavage, a succession of mitotic cell divisions, leading to the formation of a multicellular, hollow ball of cells called the blastula. • During gastrulation, part of the embryo folds inward, forming layers of embryonic tissues that will develop into adult body parts. • The resulting development stage is called a gastrula. • Some animals develop directly through transient stages into adults, but others have a distinct larval stage or stages. • A larva is a sexually immature stage that is morphologically distinct from the adult, usually eats different foods, and may live in a different habitat from the adult. • Animal larvae eventually undergo metamorphosis, transforming the animal into an adult. • Animals share a unique homeobox-containing family of genes known as Hox genes. • All eukaryotes have genes that regulate the expression of other genes. • Many of these regulatory genes contain common modules of DNA sequences called homeoboxes. • All animals share the unique family of Hox genes, suggesting that this gene family arose in the eukaryotic lineage that gave rise to animals. • Hox genes play important roles in the development of animal embryos, regulating the expression of dozens or hundreds of other genes. • Hox genes control cell division and differentiation, producing different morphological features of animals. • Hox genes in sponges regulate the formation of channels, the primary feature of sponge morphology. • In more complex animals, the Hox gene family underwent further duplication. • In bilaterians, Hox genes regulate patterning of the anterior-posterior axis. • The same conserved genetic network governs the development of a large range of animals. Concept 32.2 The history of animals may span more than a billion years • Various studies suggest that animals began to diversify more than a billion years ago. • Some calculations based on molecular clocks estimate that the ancestors of animals diverged from the ancestors of fungi as much as 1.5 billion years ago. • Similar studies suggest that the common ancestor of living animals lived 1.2 billion to 800 million years ago. • The common ancestor was probably a colonial flagellated protist and may have resembled modern choanoflagellates. Neoproterozoic Era (1 billion–542 million years ago) • Although molecular data indicates a much earlier origin of animals, the oldest generally accepted animal fossils are only 575 million years old. • These fossils are known as the Ediacara fauna, named for the Ediacara Hills of Australia. • Ediacara fauna consist primarily of cnidarians, but soft-bodied mollusks were also present, and numerous fossilized burrows and tracks indicate the presence of worms. Paleozoic Era (542–251 million years ago) • Animals underwent considerable diversification between 542–525 million years ago, during the Cambrian period of the Paleozoic Era. • During this period, known as the Cambrian explosion, about half of extant animal phyla arose. • Fossils of Cambrian animals include the first animals with hard, mineralized skeletons. • There are several hypotheses regarding the cause of the Cambrian explosion. 1. The new predator-prey relationships that emerged in the Cambrian may have generated diversity through natural selection. • Predators acquired adaptations that helped them catch prey. • Prey acquired adaptations that helped them resist predation. 2. A rise of atmospheric oxygen preceded the Cambrian explosion. • More oxygen may have provided opportunities for animals with higher metabolic rates and larger body sizes. 3. The evolution of the Hox complex provided the developmental flexibility that resulted in variations in morphology. • These hypotheses are not mutually exclusive; all may have played a role. • In the Silurian and Devonian periods, animal diversity continued to increase, punctuated by episodes of mass extinction. • Vertebrates (fishes) became the top predators of marine food webs. • By 460 million years ago, arthropods began to adapt to terrestrial habitats. • Vertebrates moved to land about 360 million years ago and diversified into many lineages. • Two of these survive today: amphibians and amniotes. Mesozoic Era (251–65.5 million years ago) • Few new animal body plans emerged among animals during the Mesozoic era. • Animal phyla began to spread into new ecological niches. • In the oceans, the first coral reefs formed. • On land, birds, pterosaurs, dinosaurs, and tiny nocturnal insect-eating mammals arose. Cenozoic Era (65.5 million years ago to the present) • Insects and flowering plants both underwent a dramatic diversification during the Cenozoic era. • This era began with mass extinctions of terrestrial and marine animals. • Among the groups of species that disappeared were large, nonflying dinosaurs and the marine reptiles. • Large mammalian herbivores and carnivores diversified as mammals exploited vacated ecological niches. • Some primate species in Africa adapted to open woodlands and savannas as global climates cooled. • Our ancestors were among these grassland apes. Concept 32.3 Animals can be characterized by “body plans” • Zoologists may categorize the diversity of animals by general features of morphology and development. • A group of animal species that share the same level of organizational complexity is called a grade. • Certain body-plan features shared by a group of animals define a grade. 2. Animals can be categorized according to the symmetry of their bodies. • Sponges lack symmetry. • Some animals, such as sea anemones, have radial symmetry. • Many animals have bilateral symmetry. • A bilateral animal has a dorsal (top) side and a ventral (bottom side), a left and right side, and an anterior (head) end and a posterior (tail) end. • Linked with bilateral symmetry is cephalization, an evolutionary trend toward the concentration of sensory equipment on the anterior end. • Cephalization also includes the development of a central nervous system concentrated in the head and extending toward the tail as a longitudinal nerve cord. • The symmetry of an animal generally fits its lifestyle. • Many radial animals are sessile or planktonic and need to meet the environment equally well from all sides. • Animals that move actively are generally bilateral. • Their central nervous system allows them to coordinate complex movements involved in crawling, burrowing, flying, and swimming. 3. The animal body plan
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