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Animal Diversity. Biology for Majors. Eras of Earth’s History. Earliest Animal Fossils. Fossils of spongelike creatures (a) Cyclomedusa and (b) Dickinsonia date to 650 million years ago, during the Ediacaran period. The Cambrian Explosion.
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Animal Diversity Biology for Majors
Earliest Animal Fossils Fossils of spongelike creatures (a) Cyclomedusa and (b) Dickinsonia date to 650 million years ago, during the Ediacaran period.
The Cambrian Explosion The Cambrian period, 542–488 million years ago, marks the most rapid evolution animal diversity. Most of the animal phyla in existence originated then including echinoderms, mollusks, worms, arthropods (including the trilobites below), and chordates.
Causes of the Cambrian Explosion The cause of the Cambrian explosion is still debated. Environmental changes such as rising oxygen levels may have created a more suitable environment for animals.
Origins of Animalia Both Parazoa and Eumetazoa evolved from a common ancestral organism that resembles the modern-day protists called choanoflagellates. These protist cells strongly resemble the sponge choanocyte cells today
Classifying Protosomes Animals that molt their exoskeletons, such as these (a) Madagascar hissing cockroaches, are in the clade Ecdysozoa. (b) Phoronids are in the clade Lophotrochozoa. The tentacles are part of a feeding structure called a lophophore.
Modern Revisions Nucleic acid and protein analyses have greatly informed the modern phylogenetic animal tree. Scientists continue to explore molecular data and use it to revise phylogenic trees.
Common Traits of Animals All animals are eukaryotic, multicellular organisms, and almost all animals have a complex tissue structure with differentiated and specialized tissues. Most animals are motile, at least during certain life stages. All animals require a source of food and are therefore heterotrophic. As heterotrophs, animals may be carnivores, herbivores, omnivores, or parasites. Most animals reproduce sexually, and the offspring pass through a series of developmental stages that establish a determined and fixed body plan. The body plan refers to the morphology of an animal, determined by developmental cues.
Animal Food Sources All animals require a source of food and are therefore heterotrophic. As heterotrophs, animals may be carnivores, herbivores, omnivores (a), or parasites (b).
Body Plans: Asymmetry The sponge is asymmetrical. This body plan is only found in Parazoa.
Body Plans: Radial Symmetry Radial symmetry is the arrangement of body parts around a central axis. Animals in the phyla Ctenophora and Cnidaria, including jellyfish and adult sea anemones (a and b) have this body plan. It allows these sedentary or slow moving creatures to experience the environment equally from all directions.
Body Plans: Bilateral Symmetry Bilateral symmetry is the division of the animal through a sagittal plane, resulting in two mirror image, right and left halves, such as those of a butterfly. Bilateral symmetry allows for streamlined and directional motion.
Limits on body size Gravity (land) Drag (in water) Skeleton weight (endoskeletons allow for larger body size than exoskeletons, shown above) Diffusion (multicellularity solves this problem for animals) Surface area to volume ratio in supporting skeletons and heat dissipation
Bioenergetics • Endotherms maintain a relatively constant body temperature. • The absence of insulation in ectothermic animals increases their dependence on the environment for body heat. • The amount of energy expended by an animal over a specific time is called its metabolic rate.
Factors that affect energy requirements: • Smaller endothermic animals have a greater surface area for their mass than larger ones. Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. • The more active an animal is, the more energy is needed to maintain that activity. Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions.
Complex Tissues The animal kingdom is divided into Parazoa (sponges) and Eumetazoa (all other animals). Parazoa do not contain true specialized tissues; although they do possess specialized cells that perform different functions, those cells are not organized into tissues. Eumetazoa possess unique tissues, absent in fungi and plants, which allow coordination (nerve tissue) of motility (muscle tissue).
Types of Epithelial Tissues Epithelial tissues cover the outside of organs and structures in the body and line the lumens of organs in a single layer (simple epithelia) or multiple layers of cells (stratified epithelia).
Transitional Epithelial Tissue Transitional epithelia of the urinary bladder undergo changes in thickness depending on how full the bladder is.
Connective Tissues Connective tissues are made up of a matrix consisting of living cells and a non-living substance, called the ground substance. The ground substance is made of an organic substance (usually a protein) and an inorganic substance (usually a mineral or water). The principal cell of connective tissues is the fibroblast. This cell makes the fibers found in nearly all of the connective tissues. Fibroblasts are motile, able to carry out mitosis, and can synthesize whichever connective tissue is needed. Macrophages, lymphocytes, and, occasionally, leukocytes can be found in some of the tissues. The matrix in connective tissues gives the tissue its density.
Types of Cartilage • Hyaline cartilage (left) has few collagen and elastin fibers • Elastic cartilage has many elastin fibers. • Fibrocartilage contains a large amount of collagen fibers, giving the tissue tremendous strength.
Adipose Tissue Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs.
Blood Blood has a fluid matrix, the plasma.
Types of Muscle Cells Smooth muscle cells do not have striations, while skeletal muscle cells do. Cardiac muscle cells have striations, but, unlike the multinucleate skeletal cells, they have only one nucleus. Cardiac muscle tissue also has intercalated discs, specialized regions running along the plasma membrane that join adjacent cardiac muscle cells and assist in passing an electrical impulse from cell to cell.
Nervous Tissue The large structure with a central nucleus is the cell body of the neuron. Projections from the cell body are either dendrites specialized in receiving input or a single axon specialized in transmitting impulses. Some glial cells are also shown. Astrocytes regulate the chemical environment of the nerve cell, and oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently.
Asexual Reproduction in Animals The most common forms of asexual reproduction for stationary aquatic animals include budding and fragmentation, where part of a parent individual can separate and grow into a new individual. Parthenogenesis, found in certain insects and vertebrates, is where unfertilized eggs can develop into new male offspring. This type of parthenogenesis is called haplodiploidy. These types of asexual reproduction produce genetically identical offspring, which is disadvantageous from the perspective of evolutionary adaptability.
Animal Development The zygote (fertilized egg) develops into a blastopore. Different cell layers (called germ layers) are formed during gastrulation. These germ layers are programmed to develop into certain tissue types, organs, and organ systems during a process called organogenesis.
Embryological Development Diploblasts display radial symmetry and have a non-living layer between the endoderm and ectoderm. More complex animals (those with bilateral symmetry) develop three tissue layers.
Mouth Development Eucoelomates can be divided into two groups. In protostomes, part of the mesoderm separates to form the coelom in a process called schizocoely. In deuterostomes, the mesoderm pinches off to form the coelom in a process called enterocoely.
Hox Genes Hox genes are highly conserved genes encoding transcription factors that determine the course of embryonic development in animals. They determine the animal’s body plan. What makes Hoxgenes so powerful is that they serve as master control genes that can turn on or off large numbers of other genes. Hoxgenes do this by coding transcription factors that control the expression of numerous other genes. Hox genes are homologous in the animal kingdom, that is, the genetic sequences of Hox genes and their positions on chromosomes are remarkably similar across most animals because of their presence in a common ancestor.
Practice Question Why are Hox genes “highly conserved?” How does that conservation help scientist understand evolutionary relationships?
Homeostasis Physiologically, it is the body’s attempt to maintain a constant and balanced internal environment, which requires persistent monitoring and adjustments as conditions change. Adjustment of physiological systems within the body is called homeostatic regulation, which involves three parts or mechanisms: • The receptor receives information that something in the environment is changing. • The control center receives and processes information from the receptor. • The effector responds to the commands of the control center by either opposing or enhancing the stimulus. Adjustments must be made continuously to stay at or near a specific value: the set point.
Positive Feedback Loops Positive feedback loops are rare but important. They include blood clotting and childbirth.
Quick Review • How did the animal kingdom evolve? • How do scientists understand animal phylogeny? Can you use this system to classify animals? • What are common features of the animal kingdom? • What are the various types of body plans that occur in animals? • What tissue structures are found in animals? • How do animals reproduce and develop?
