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Review Sheet Ch. 32 Intro to Animal Evolution 1) Characteristics of Kingdom Animalia: multicellular, chemoheterotrophic, method of taking in nutrients is ingestion (in contrast to fungi, for example, where nutrients are absorbed); 2) no cell walls 3) most have nerves and muscles, other taxonomic groups lack these tissues 4) most reproduce sexually (large non-motile egg; small flagellated sperm); Zygote divides by mitosis (cleavage) into a blastula (a hollow ball of cells). One end of the blastula folds inward in a process called gastrulation, forming a blind pouch called the archenteron. After gastrulation the blastula is called a gastrula The opening to the archenteron is called the blastopore. The archenteron becomes the digestive cavity in most animals. 5) Animals are found in most habitats on earth. Most phyla are found in the ocean. On land, vertebrates and insects are dominant. 6) The traditional taxonomic groupings of animals are as follows: Animalia Parazoa Eumetazoa Radiata Bilaterata Acoelomates Animals with body cavities Pseudocoelomates Coelomates Protostomia Deuterostomia 7) A newer model based on molecular data (which we will emphasize over the traditional model): Animalia Parazoa Eumetazoa Radiata Bilateria Deuterostomia Protostomia In this newer model, the acoelomate and pseudocoelomate conditions evolved secondarily in Protostomes that had true coeloms. 8) Parazoa have no true tissues, organs or nerves. Sponges (Porifora) are the only phylum in this group. All other animals are the Eumetazoa group. 9) Radiata all have radial symmetry. This means they have no head or tail end. They do have a top and bottom (the oral and aboral ends). The main groups of Radiata are jellyfish and hydras (Phylum Cnidaria) and comb jellies (Phylum Ctenophora). Radiata are also diploblastic. This means they lack a mesoderm during embyonic development. The mesoderm gives rise to muscles and most other organs besides the skin, nerves, and gut. Radiata do have an ectoderm (develops into skin and nerves) and an endoderm (develops into gut). That means Radiata has skin, nerves and a gut, but not most of the other organs found in Bilateria. 10) Bilateria have bilateral symmetry. This means they have a head, a tail, and distinct right and left sides. Some Bilateria have radial symmetry as adults (e.g. sea stars, sea cucumbers), but all are bilateral at some point during early development. Bilateria are triploblastic: they have endoderm, ectoderm, and they also have a mesoderm and the tissues that develop from the mesoderm. 11) Bilateral symmetry is an adaptation for an active, mobile lifestyle, while radial symmetry is more suited for a sessile lifestyle. Bilateral symmetry is associated with cephalization, which is a concentration of sensory organs at one end of the animal (forming a head). 12) Bilateria can be divided into two major taxonomic groups: protostomes and Deuterostomia. 13) Protostome means “mouth first”. This means that the blastopore develops into the mouth. In protostomes with complete, one-way guts (i.e. a mouth and an anus), a second infolding of the gastrula pushes in until it meets the archenteron, forming a complete tube (the digestive tract). At this point the embryo is shaped like a donut. The second pore becomes the anus. See fig32.7. 14) Deuterostome means “mouth second.” This means that the blastopore does not develop into the mouth. The blastopore of deuterstomes develops into the anus. While the second pore that develops following gastrulation becomes the mouth. 15) Other key difference between protostomes and deuterostomes: Protostome Deuterostome Spiral cleavage (planes of cell division are diagonal Radial cleavage (planes of cell division parallel or to vertical axis of embryo) perpindicular to vertical axis of embryo) Determinate cleavage (developmental fate of Indeterminate cleavage (developmental fate of individual embryonic cells is set; i.e. no twins) individual embyonic cells is not set; i.e. a split embryo may develop into twins) 16) The coelom (“sea-lum”) is the body cavity, a fluid filled space separating the gut from the outer body wall. This is where our viscera (heart, lungs, kidneys), muscles, etc. are located. Some bilateria, like the flatworms, are acoelomates. They have a solid body with no cavity between their gut and outer body wall. Organisms with body cavities are divided into coelomates and pseudocoelomates. In coelomates, the body cavity is completely lined with mesoderm. In pseudocoelomates, the cavity is not completely lined with mesoderm. The main example of pseudocoelomates are the roundworms. Coelomates include chordates, echinoderms, arthropods, mollusks, annelids and other phyla. 17) A large question in animal phylogenetics is whether the acoelomate phyla (e.g. flatworms) and the pseudocoelomate phyla (e.g. roundworms) evolved from non-coelomate or coelomate ancestors. For example, it was originally believed that the flatworm’s lack of body cavity was a primitive condition. However, recent evidence suggests that flatworm’s descend from coelomate ancestors, and evolved to a simpler body plan. This would mean that the flatworm’s lack of body cavity is a derived condition. 18) Origins of animal phyla: The animal phyla that exist today did not appear gradually in the fossil record over geological time, rather they all originated during a ~40 million year period. This was about 565 to 525 million years ago. This period is called the Cambrian explosion, because so many new phyla suddenly appeared. 19) The history of life on Earth is divided into four eras. The first is also the longest and is called the Precambrian era. It starts with the origin of the Earth, and extends to the time when animals first began leaving easily recognizable fossils, from shells and other hard body parts (the beginning of the Cambrian era). The time when Kingdom Animalia originated is uncertain, but it was definitely in the Precambrian era (possibly at the very end, but also possibly a billion years ago or earlier). 20) Why did the animal phyla evolve so rapidly? Possible explanations: a) ecological: predator prey relationships caused coevolution, biological arms races that promote rapid change; b) environmental: it is possible the concentration of O2 in the atmosphere increased to a level sufficient to sustain the more active metabolisms of mobile animals; c) Genetic: the appearance of Hox genes (genes that regulate embryonic development) may have opened the door for many different combinations of major genotypes resulting in different major body plans (e.g. phyla). 21) Why have no new phyla appeared since the Cambrian explosion? It appears that by the end of the Cambrian explosion, the animal phyla had become fixed into development patterns such that offspring with major changes in developmental characteristics or body plan would be strongly selected against (i.e. not be able to survive and or reproduce).