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43284_Ward's World+MGH Animal Evolution

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1 Animal Evolution Article by: J.W. Valentine, Department of Geology, University of California, Santa Barbara, California and Keith S. Thomson, Oxford University Museum of Natural History, Oxford, United Kingdom. Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource that provides immediate, authoritative answers to students' thirst for scientific knowledge on topics such as climate change, virology, pollution, and more. Ward's World and McGraw Hill have partnered to offer educators a no-obligation, free trial subscription to this product. Request your free trial today and discover how valuable AccessScience can be for you and your students. The processes of biological and organic change in animals by which descendants come to differ from their ancestors. Animal evolution encompasses the theory that modern animals are the modified descendants of animals that formerly existed and that these earlier forms descended from still earlier and different organisms. The concepts of evolutionary biology and their relationship to phylogeny (the genealogical history of organisms, both living and extinct), adaptation, and speciation were formulated by Charles Darwin. Much of Darwin's conclu- sions emanated from his investigative study of animals. Animals are multicellular organisms that feed by ingestion of other organisms or their products, being unable to derive energy through photosynthesis or chemosynthesis. Animals are cur- rently classed into approximately 36 phyla, and each of these phyla has evolved a distinctive body plan or architecture. Repre- sentatives of the major animal groups are depicted in Fig. 1. Interrelationships of animal groups All phyla began as invertebrates, but lineages of the phylum Chordata developed the internal skeletal armature, with spinal column, which was exploited in numerous fish groups and which eventually gave rise to terrestrial vertebrates. The num- ber of phyla is uncertain partly because most of the branching patterns and the ancestral body plans from which putative phyla have arisen are not yet known. For example, arthropods (including crustaceans and insects) may have all diversified from a common ancestor that was a primitive arthropod, in which case they may be grouped into a single phylum; or sev- eral arthropod groups may have evolved independently from nonarthropod ancestors, in which case each such group must be considered a separate phylum. So far as known, all animal phyla began in the sea. Several lines of evidence bear significantly upon the ancestry of these animals and of their major subdivisions. Classically, animals were grouped according to the similarities of their adult body plans; however, although the body plans have provided a basis for the recognition of most of the phyla and other distinc- tive animal groups, they have not provided definitive evidence of the interrelations of the phyla. A second approach is to com- pare patterns of early development, which should be more con- servative than adult features; such comparisons have permitted the grouping of some phyla into likely alliances. Hypotheses as to the body plans of phylum ancestors have been erected from these morphologic data. It has not proven possible to corrobo- rate any hypotheses, however, because the earliest members of + ward ' s science Key Concepts • Most animal phyla that exist today trace back to the Cambrian Period, more than 500 million years ago. • Although all animal phyla originated in the oceans, members of a few phyla, including bacteria, insects, and vertebrates, have colonized the land. • Members of a clade, such as a phylum or class, descend from a common ancestor and share certain inherited characteristics. • Vertebrates have adapted to life on land as a result of the evolution of tetrapod limbs and an enclosed amniotic egg that allows embryonic development outside of a body of water. • Vertebrates share structures, such as a notochord, pharyngeal slits, and a dorsal, hollow nerve cord, with other members of the phylum Chordata.

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