Chapter 32 Notes
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Date Created: 04/05/16
Biology Chapter 32 Notes An overview of animal diversity A kingdom of consumers o Most animals are mobile and use traits such as strength, speed, toxins, or camouflage to detect, capture, and eat other organisms o For example, the chameleon captures insect prey with its long, sticky, quick-moving tongue Concept 32.1: Animals are multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers o There are exceptions to nearly every criterion for distinguishing animals from other life-forms o Several characteristics, taken together, sufficiently define the group o Nutritional mode Animals are heterotrophs that ingest their food o Cell structure and specialization Animals are multicellular eukaryotes Their cells lack cell walls Extracellular matrix: Animals’ bodies are held together by structural proteins such as collagen Nervous tissue and muscle tissue are unique, defining characteristics of animals Tissues are groups of similar cells that act as a functional unit o Reproduction and development Most animals reproduce sexually, with the diploid stage usually dominating the life cycle After a sperm fertilizes an egg, the zygote undergoes rapid cell division called cleavage Cleavage leads to formation of a multicellular, hollow blastula The blastula undergoes gastrulation, forming a gastrula with different layers of embryonic tissues Most animals have at least one larval stage A larva is sexually immature and morphologically distinct from the adult; it eventually undergoes metamorphosis to become a juvenile A juvenile resembles an adult, but is not yet sexually mature Concept 32.2: The history of animals spans more than half a billion years o More than 1.3 million animal species have been named to date; far more are estimated to exist o The common ancestor of all living animals likely lived between 700 and 770 million years ago o Steps in the origin of multicellular animals Morphological and molecular evidence points to a group of protists called choanoflagellates as the closest living relatives to animals The common ancestor may have resembled modern choanoflagellates The origin of multicellularity requires the evolution of new ways for cells to adhere (attach) and signal (communicate) to each other Molecular analysis has revealed similarities between genes coding for proteins involved in adherence and attachment in choanoflagellates and animals o Neoproterozoic Era (1 Billion–542 Million Years Ago) Early members of the animal fossil record include the Ediacaran biota, which dates back to about 560 million years ago Early animal embryos and evidence of predation have also been found in Neoproterozoic rocks o Paleozoic Era (542 Million – 251 Million Years Ago) The Cambrian explosion (535 to 525 million years ago) marks the earliest fossil appearance of many major groups of living animals Most of the fossils from the Cambrian explosion are of bilaterians, organisms that have the following traits: Bilaterally symmetric form Complete digestive tract One-way digestive system There are several hypotheses regarding the cause of the Cambrian explosion and decline of Ediacaran biota New predator-prey relationships A rise in atmospheric oxygen The evolution of the Hox gene complex Animal diversity continued to increase through the Paleozoic, but was punctuated by mass extinctions Animals began to make an impact on land by 450 million years ago Vertebrates made the transition to land around 365 million years ago o Mesozoic Era (251 Million to 65.5 Million years ago) Coral reefs emerged, becoming important marine ecological niches for other organisms The ancestors of plesiosaurs were reptiles that returned to the water During the Mesozoic era, dinosaurs were the dominant terrestrial vertebrates The first mammals emerged Flowering plants and insects diversified o Cenozoic Era (65.5 Million to present day) The beginning of the Cenozoic era followed mass extinctions of both terrestrial and marine animals These extinctions included the large, nonflying dinosaurs and the marine reptiles Mammals increased in size and exploited vacated ecological niches The global climate cooled Concept 32.3: Animals can be characterized by “body plans” o Zoologists sometimes categorize animals according to a body plan, a set of morphological and developmental traits o Some body plans have been conserved, while others have changed multiple times over the course of evolution o Symmetry Animals can be categorized according to the symmetry of their bodies, or lack of it Some animals have radial symmetry, with no front and back, or left and right Two-sided symmetry is called bilateral symmetry Bilaterally symmetrical animals have A dorsal (top) side and a ventral (bottom) side A right and left side Anterior (front) and posterior (back) ends Many also have sensory equipment, such as a brain, concentrated in their anterior end Radial animals are often sessile or planktonic (drifting or weakly swimming) Bilateral animals often move actively and have a central nervous system o Tissues Animal body plans also vary according to the organization of the animal’s tissues Tissues are collections of specialized cells isolated from other tissues by membranous layers During development, three germ layers give rise to the tissues and organs of the animal embryo Ectoderm is the germ layer covering the embryo’s surface Endoderm is the innermost germ layer and lines the developing digestive tube, called the archenteron Sponges and a few other groups lack true tissues Diploblastic animals have ectoderm and endoderm These include cnidarians and a few other groups Triploblastic animals also have an intervening mesoderm layer; these include all bilaterians These include flatworms, arthropods, vertebrates, and others o Body cavities Most triploblastic animals possess a body cavity A true body cavity is called a coelom and is derived from mesoderm Coelomates are animals that possess a true coelom A pseudocoelom is a body cavity derived from the mesoderm and endoderm Triploblastic animals that posses a pseudocoelom are called pseudocoelomates Triploblastic animals that lack a body cavity are called acoelomates Terms such as coelomates and pseudocoelomates refer to organisms that have a similar body plan and belong to the same grade A grade is a group whose members share key biological features (usually a paraphyletic group, sometimes a polyphyletic group, sometimes we don’t know whether the group is monophyletic) A grade is not necessarily a clade, ( remember, a clade is a monophyletic group, i. e. an ancestor and all of its descendants) o Protostome and Deuterostome Development Based on early development, many animals can be categorized as having protostome development or deuterostome development o Cleavage In protostome development, cleavage is spiral and determinate In deuterostome development, cleavage is radial and indeterminate With indeterminate cleavage, each cell in the early stages of cleavage retains the capacity to develop into a complete embryo Indeterminate cleavage makes possible identical twins, and embryonic stem cells o Coelom Formation In protostome development, the splitting of solid masses of mesoderm forms the coelom In deuterostome development, the mesoderm buds from the wall of the archenteron to form the coelom o Fate of the blastopore The blastopore forms during gastrulation and connects the archenteron to the exterior of the gastrula In protostome development, the blastopore becomes the mouth In deuterostome development, the blastopore becomes the anus Concept 32.4: Views of animal phylogeny continue to be shaped by new molecular and morphological data o By 500 million years ago, most animal phyla with members alive today were established o The diversification of animals Zoologists recognize about three dozen animal phyla Phylogenies now combine morphological, molecular, and fossil data Five important points about the relationships among living animals are reflected in their phylogeny All animals share a common ancestor Sponges are basal animals (simple in construction and diverging early in the phylogeny of Animalia) Eumetazoa (“true animals”) is a clade of animals with true tissues Most animal phyla belong to the clade Bilateria There are three major clades of bilaterian animals, all of which are invertebrates, animals that lack a backbone, except some of the Chordata, which are classified as vertebrates because they have a backbone The bilaterians are divided into three clades: o Deuterostomia, Ecdysozoa, and Lophotrochozoa Deuterostomia includes hemichordates (acorn worms), echinoderms (sea stars and relatives), and chordates This clade includes both vertebrates and invertebrates Ecdysozoa is a clade of invertebrates that shed their exoskeletons through a process called ecdysis Lophotrochozoa is another clade of bilaterian invertebrates o Some lophotrochozoans have a feeding structure called a lophophore o Others go through a distinct developmental stage called the trochophore larva o Future directions in animals systematics Systematics, like all fields of scientific research, isan ongoing, dynamic process of inquiry Three outstanding questions are the focus of current research Are sponges monophyletic? Are ctenophores basal metazoans? Are acoelomate flatworms basal bilaterians?