University of Louisiana at Lafayette
Popular in Evolutionary Biology
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Joseph Merritt Ramsey
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This 13 page Class Notes was uploaded by Lauren Notetaker on Saturday February 13, 2016. The Class Notes belongs to EBIO 1010 - 02 at Tulane University taught by Bruce Fleury in Spring 2016. Since its upload, it has received 27 views. For similar materials see Evolutionary Biology in Science at Tulane University.
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Date Created: 02/13/16
Problem: How to disperse your young? Solution: Have a motile larval stage Problems: Limited to food that floats by Can’t escape danger How to reach your mates? How to disperse your young? Solutions: Become a filter feeder Defend yourself (stinging cells etc.) Rely on external fertilization, be a hermaphrodite Have a motile larval stage Motile organisms tend to be bilaterally symmetric More efficient shape for moving through the environment (especially in water) Actively seek out food and mates Run away from predators Animals in motion have a specific direction Anterior Posterior Dorsal Ventral Animal awareness becomes concentrated in the direction of motion – sense organs are right up front Animals tend to become cephalized – they develop a head, a brain, and a central nervous system Aquatic or Terrestrial Problems posed by transition from water to land Transition posed new problems that both plants and animals had to solve Required a radically different set of evolutionary adaptations to make that first step onto dry land Desiccation drying up Gravity Excretion Desiccation Problem: Tissues dry out Solution: Develop a protective layer of epidermal cells – evolve a skin Problem: Need moisture to exchange gases Solution: Keep respiratory surfaces on the inside Problem: Gametes dry out, can no longer rely on external fertilization in water Solution: Internal fertilization Problem: Embryos dry out Solution: Amniotic eggs enclosed in shell for plants: Seeds Problems: Tissues dry out Need moisture to exchange gases Gametes dry out, can no longer rely on external fertilization in water Embryos dry out Solutions: Develop a protective layer of epidermal cells – evolve a skin Keep respiratory surfaces on the inside Internal fertilization Amniotic egg, seed Gravity Problem: You can no longer rely on the natural buoyancy of water Solutions: Skeletal system – endoskeleton or exoskeleton Rootshoot system for plants Excretion Problem: Aquatic organisms rely on ammonia, which requires a large amount of water to dissolve Solution: Use urea or uric acid, which require less water to dissolve Problem: When they excrete, animals lose essential salts that are dissolved in waste water Solution: Pass waste water through simple tubes (nephridia) to recover the salts Pass waste water through simple tubes (nephridia) to recover the salts Small or Large Problem Small organisms can rely on diffusion to move materials in and out Diffusion is too slow for large organisms, interior cells would starve or poison themselves in their own wastes Allometric relationship: Surface area of a sphere = 4 ∏ r2 Volume of a sphere = 4/3 ∏ r Problems: Larger animals have more volume relative to their surface area Some cells or tissues will be far away from the outside surface Diffusion will not be fast enough to move food, gases wastes to and from inner cells Solutions: Fold the digestive, respiratory, and excretory surfaces to increase surface area Be very thin or very flat Develop a vascular system – tubes to carry materials back and forth Develop a coelom – hollow fluidfilled core All of the amazing diversity we see in nature, all of the millions of different ways to be a living thing, represent the many ways in which organisms have solved these basic environmental challenges Kingdom Animalia 36 Phyla Over one million species Invertebrates (over 99%) Vertebrates (less than 1%) Heterotrophic Diploid Eukaryotic Multicellular Motile (at some stage in life cycle) Able to respond to external stimuli Able to reproduce sexually Ingest/digest food in internal cavity • Animals belong to a larger monophyletic group called Opisthokonta • Opisthokonts include: > Phylum Choanoflagellata > Kingdom Fungi (we share a common ancestor with fungi) > Kingdom Animalia • All three share a common ancestor • Phylum Choanoflagellata – choanoflagellates • These curious organisms are usually placed in the Kingdom Protista • They are identical to the feeding cells of the common sponge (more later) • Coloniality is an essential evolutionary step • Bridges the gap between unicellular protists and multicellular animals • Colonial organisms show > specialization of cells > division of labor > communication between cells Subkingdom Parazoa Sponges, Placozoa Subkingdom Eumetazoa All other animals Have cells organized into tissues, organs Share a common pattern of development Have some type of symmetry Radial symmetry Bilateral symmetry Have cells organized into tissues, organs Sexual Reproduction Haploid gametes of unequal size – egg and sperm Gametes > Zygote Zygote > Blastula Blastula > Gastrula (blastopore, blastocoel) Bilateral Eumetazoans Have three types of embryonic tissues: Endoderm = “skin within”, forms gut, internal organs Mesoderm = “skin in the middle”, forms skeleton and muscles Ectoderm = “the skin outside”, forms epidermis, nervous system Have three different body plans Body plans of higher animals is a ‘tubeinatube’ connecting mouth to anus Outer tube of skin, muscle etc… Central tube of digestive tissue (gut) Fluidfilled space in between (coelom) Coelom is an internal body cavity, a fluid filled space that runs throughout the body of higher animals This innovation opened up many new possibilities for the evolution of animals Have three different body plans: Acoelomate flatworms Pseudocoelomate roundworms, rotifers Coelomate all other higher animals who has which body plans? which falls which into what category? three tissue layers and what they become? Have three different body plans: Acoelomate flatworms Body lacks a coelom, solid except for crude internal “pouch” or GVC (gastrovascular cavity) Have three different body plans: Pseudocoelomate nematodes, rotifers Coelom is actually a fluidfilled remnant of the blastocoel Have three different body plans: Coelomate (eucoelomate) all others Coelom is formed from the mesoderm, and lined by mesodermal membranes (the peritoneum) Ancient split in Kingdom Animalia All coelomate animals share one of two basic patterns of embryonic development Protostomes annelids, mollusks, arthropods Deuterostomes echinoderms, chordates This split in animal evolution occurred at least 540 million years ago Protostomes = first mouth, blastopore becomes the mouth, anus opens opposite the mouth later on Deuterostomes = second mouth, blastopore becomes the anus, mouth opens opposite the anus later on Protostomes Annelids, Mollusks, Arthropods Spiral cleavage in embryo Determinate cells – fate set early on Schizocoels – “split”; coelom forms as a split in the mesoderm what are the differences between protostomes and deuterostomes? Deuterostomes (us) Echinoderms, Chordates Radial cleavage in embryo Indeterminate cells – fate not set early on Enterocoels “gut” Advantages of a Coelom“Tube in a Tube” Allows fluid circulation within coelom Fluids are relatively incompressible, so the coelom acts as a hydrostatic skeleton Open digestive tract (mouth anus) Digestion independent of movement More space for internal organs More space to store gametes Ingest/gigest food in internal cavity The combination of bilateral symmetry and a linear “tube in a tube” body plan opens up a new evolutionary pathway segmentation Parts of the body can become specialized to perform different functions, leading ultimately to the most successful organisms, the arthropods and chordates Kingdom Animalia Subkingdom Parazoa , Placozoa Subkingdom Eumetazoa All other animals Have cells organized into tissues, organs Have some type of symmetry Radial symmetry (Radiata) Bilateral symmetry (Bilateria) Share a common pattern of development Eumetazoans are divided into two monophyletic groups Radiata – Cnidaria, Ctenophora Bilateria – everything else Bilateria is in turn divided into two groups Protostomia – most invertebrates (may be a paraphyletic group) Deuterostomia – echinoderms, chordates Protostomes in turn are divided in two Spiralia – flatworms, rotifers, molluscs, annelids) Ecdysozoa – animals that molt (nematodes, arthropods) Within the protostomes Spiralia is divided into two groups Platyzoa – flatworms, rotifers etc. Trochozoa – annelids, molluscs etc. Animalia Parazoa Eumetazoa Protostomia Spiralia Platyzoa Trochozoa Ecdysozoa Deuterostomia Subkingdom Parazoa We’ll start with Subkingdom Parazoa = “animals set aside” Multicellular Primitive level of organization, or secondary loss of more advanced features? Little Placozoa is a big mystery… Phylum Porifera – Sponges Most parazoans are sponges – Porifera Latin porus = pores, ferre =to bear Over 10,000 species Primitive, diverged 550 mya or earlier (?) Lack symmetry Aquatic, mostly marine Sessile – rely on motile larvae to disperse Colonial organisms Lack tissues and organs Simple body, 2 layers of epithelial cells enclosing a gelatinous matrix Long lived – temperate species only a few years, though some live 200 years or more Larger sponges may live 5,000 years or more! Colonial organisms show a division of labor Several specialized cell types Amoebocytes – wandering amoeboid cells, unspecialized, totipotent Choanocytes – feeding cells (collar cells) Amoebocytes Digest, transport, and store food Transport sperm to the eggs Secrete spicules (= “skeleton”) Spicules Help sponges keep their shape Used by taxonomists to classify sponges Several different types of spicules: Calcium Silica Spongin – protein fibers Choanocytes – Feeding cells Joint action of choanocyte flagella moves water through the sponge Delivers food, oxygen, gametes Carries off wastes (mostly ammonia) matching with body parts Water enters through the ostia (outer pores) Water passes over the choanocytes Water enters the spongocoel (central cavity) Water exits through the osculum Flagella of choanocyte pulls food up against the outside mesh of the collar Choanocytes feed by phagocytosis Amoebocytes Feeding cells transfer food to amoebocytes Amoebocytes do most of the digestion Amoebocytes transport food to other cells Amoebocytes also store food Asexual reproduction in sponges occurs by fragmentation Pieces of sponge break off, fragments grow into new adult sponges Sexual reproduction poses a problem for sponges why? Problem: Sponges are sessile animals – how to be sure your nearest neighbor is a potential mate? Solution: Rely on external fertilization Be a hermaphrodite Sponges release vast clouds of sperm Choanocytes capture sperm cells, lose their flagella and become amoebocytes Amoebocytes transport sperm to the eggs Fertilization Zygote à Motile Larvae Larvae are balls of collar cells, with their collars facing outward Larvae settle down, turn inside out, and mature into a sponge Sponges similar to colonial choanoflagellate protozoans (except they are inside out!) Sponges illustrate the problem of getting larger Three “grades” of sponges tiny, small, large Asconoid – very small, flagella can move water through the spongocoel Syconoid – small, with radial canals, extra surface area for more choanocytes Leuconoid – all large sponges No central cavity, narrow canals Many small interconnected chambers Sold as bath sponges most modern sponges, however, are made of plastic Sponge diving has been practiced in the Mediterranean for centuries Small local community of Greek sponge divers in Tarpon Springs, Florida Sponges are evolutionary pioneers First “stem cells” (amoebocytes) Cell adhesion Self vs. nonself (immunity) Programmed cell death We share 70% of our genes with sponges! Phylum Cnidaria Subkingdom Parazoa Parazoa = “animals set aside” Sponges, Placozoa Subkingdom Eumetazoa Radially symmetric animals – Radiata Phylum Cnidaria hydrozoans, coral, anemones Phylum Ctenophora – comb jellies Bilaterally symmetric animals Bilateria Phylum Cnidaria Hydrozoans, coral, anemones 9,100 species, from Greek knide = nettle Ancient group – Precambrian (580 mya) Mostly marine, some freshwater Radially symmetric Carnivorous – capture live prey Sessile and motile (dimorphic humans) Cnidarians were the first animals to develop a gut cavity (GVC) Cnidarians are the first animals to develop differentiated tissues Cnidarians were the first truly mobile creatures Jellyfish pulse gracefully through the water Polyps bend to and fro, some can even detach and hop or somersault short distances Cnidarians move by means of primitive muscle fibers arranged in two directions Circular muscles can contract the exposed body Longitudinal muscles can stretch the body out, bend the body in any direction The combination of these muscles leads to the smooth movements of all higher animals These muscles are controlled by the first true nerve cells True nerve cells, but no CNS, just a simple nerve net (why??) because it’s not really going anywhere (sea anemone) Only two of the three eumetazoan tissue layers ectoderm, endoderm, but no mesoderm (hence no “body plan” per se) Space between ectoderm and endoderm is filled with mesoglea (“jelly in the middle”) Primitive senses – eyespots, statocysts, mechanical and chemical receptors Mostly small Hence can rely on diffusion for gas exchange and excretion Most primitive animals with a body cavity gastrovascular cavity (GVC) Digestion is extracellular Gland cells break down food in the GVC Can eat things larger than their cells Stinging cells called cnidocytes contain coiled nematocysts Cell membrane of cnidocyte responds to chemical or mechanical stimuli Cell membrane changes permeability osmotic pressure ? Water rushes in, explodes nematocyst out at 40 atmospheres of osmotic pressure! Discharge without additional energy Many different types of nematocysts Sticky threads, used for attachment, or moving short distances Long coils that whip around the prey and entangle it Long threads with spines or poison barbs to ensnare and stun prey Dimorphic sessile polyps or motile medusa jelly fish sting major artery and drown Both forms alternate in the life cycle Both are diploid, so not true alternation of generations Medusa is the sexual stage Phylum Cnidaria Reproduction Asexual reproduction in polyps by budding Sexual reproduction Polyps bud off tiny medusae (ephyra) Medusae produce gametes (mostly hermaphroditic) Sexual reproduction External fertilization Zygote develops into planula larva Larvae mature into new polyp Phylum Cnidaria – Colonial Forms Many colonial forms – Obelia, Physalia Colonial forms have specialized feeding polyps and reproductive polyps Physalia – Portuguese Man of War, bell is dead medusa, inflated by numerous individual polyps hanging beneath Class Hydrozoa – Hydra, Obelia, Physalia Class Scyphozoa – true jellyfish, Aurelia Class Anthozoa – coral, sea anemones Class Cubozoa – sea wasps Phylum Cnidaria – Class Hydrozoa 3,100 species, from Latin Hydra, the immortal serpent Hydra is immortal! Actually only one species thought to be immortal, Turritopsis dohrnii Medusa stage reverts to polyp when damages, restarts life cycle Can still die from injuries, predation etc. Sessile and motile forms Polyp is the dominant stage in the life cycle Polyp tentacles lined with cnidocytes Phylum Cnidaria – Class Scyphozoa what is man o war? True jellyfish Aurelia 200 species, from Latin scyphus = drinking cup Medusa is the dominant stage Polyp occurs only as small postlarval stage Medusae release gametes Zygote forms a planula larva Planula develops into small polyp, which buds off tiny medusae won’t ask details of life cycle matching terms for animals yes for life cycle don’t have to be diverse to be important Tentacles hang from edge of bell or from around the mouth Tentacles are studded with nematocysts Prey is stung, tentacles pass it to mouth Digested in complex GVC Prey consists of crustaceans, fish Bell can be contracted to swim through water Phylum Cnidaria – Class Anthozoa Corals, sea fans, sea anemones 6,200 species, from Latin anthos = flower Occur only as polyps Most advanced cnidarians, complex body Symbiotic photosynthetic dinoflagellates, so limited to shallow water Anemones are large solitary polyps Anemones feed on invertebrates and small fish Phylum Cnidaria – Importance Colonial forms form vast coral reefs of CaCO3 (calcium carbonate) Coral reefs are among the most productive ecosystems on Earth Reefs support commercial fisheries around the world Reef fish and invertebrates are harvested for the aquarium pet trade Phylum Cnidaria – Class Cubozoa Sea wasps – 20 species Among the deadliest animals on Earth!! Over 50 fatalities recorded from Australia, death occurs in 320 minutes!! Phylum Ctenophora Comb jellies – once classed with cnidarians in the Phylum Coelentrata Similar to jellyfish, but: Simpler life cycle Lack nematocysts, capture prey with sticky cells on two long tentacles Combs of cilia beat in sequence to move the jelly around – beautiful to see!!
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