2021 B1502 Lecture 1-3 - Classification (1) PDF

METHODS OF INVERTEBRATE CLASSIFICATION AND OTHER ORDERING SYSTEMS Definition of classification – Process or result (product) of arranging or grouping organisms into groups on the basis of their relationships (similarity) What is the need for a classification sys...

METHODS OF INVERTEBRATE CLASSIFICATION AND OTHER ORDERING SYSTEMS Definition of classification – Process or result (product) of arranging or grouping organisms into groups on the basis of their relationships (similarity) What is the need for a classification system? – Makes sense of the diversity of organisms (**>1.3 million species of animals) = to reflect nature Categories should reflect structural, functional, developmental, genetic/molecular, and ecological cohesiveness = provide the basis of evolutionary interpretation – Provides the framework by which biologists communicate information about organisms – Predicts properties of newly discovered or poorly known organisms  List of Classification methods 1. Classification by Cell Number 2. Classification of Metazoa based on Presence of True Tissues 3. Classification by Number of Germ Layers formed during embryogenesis 4. Classification by Body Symmetry 5. Classification of Triploblastic Eumetazoa by presence and nature of Body Cavity 6. Classification of Eucoelomates based on COELOM FORMATION and other developmental features 7. Classification by Evolutionary Relationships A. Classification by Cell Number i. Unicellular -- Protozoa ii. Multicellular -- Metazoa B. Classification of Metazoa based on Presence of True Tissues i. Without true tissues 1. Mesozoa -- Phyla: Placozoa, Rhombozoa, Orthonectida 2. Parazoa -- Phylum Porifera ii. With true tissues 1. Eumetazoa phyla 2. All pass through distinct embryonic stages during which tissue layers form C. Classification by Number of Germ Layers formed during embryogenesis (Germ layer def: group of cells behaving as a unit during early stages of embryonic development & giving rise to distinctly different tissue and/or organ system in the adult) I. Diploblastic Eumetazoa -- Phylum Cnidaria & Phylum Ctenophora - Ectoderm and Endoderm II. Triploblastic Eumetazoa -- Refers to the rest of Eumetazoa - Ectoderm - Mesoderm - Endoderm D. Classification by Body Symmetry 1. Assymetrical Body Plans – Having no ordered pattern to the gross morphology – Rarely encountered – Phylum Porifera 2. Radially Symmetrical Body Plans – Bodies can be divided into 2 equal halves by any centred cut thru body – Radial animals have no head – tail or anterior – posterior axis or dorso-ventral axis – Phylum Cnidaria & Phylum Ctenophora 3. Bilaterally Symmetrical Body Plans – Possess right & left sides that are approx mirror images – Highly correlated with cephalization (concentration of nerves & sensory tissues & organs; and feeding structures at one end of animal – Distinct anterior – posterior axis – ****Read Ruppert et al about the Ecological and evolutionary significance of Bilateral Asymmetry E. Classification of Triploblastic Eumetazoa by presence and nature of Body Cavity 1. Acoelomates – Lack an internal body cavity – The area between outer body wall & gut is filled with mesoderm – Considered the most primitive triploblastic eumetazoa – Acoelomate Phyla: Platyhelminthes, Gnathostomulida, Rhynchocoela 2. Pseudocoelomates – Area between body wall muscles & gut filled is a fluid-filled cavity – Cavity derived from blastocoel – ***Read Brusca & Brusca about the Ecological and evolutionary significance of body cavity with respect to acoelomate ancestral condition – Pseudocoelomate Phyla: Nematoda, Rotifera, Kinorhyncha, Acanthocephala, Nematomorpha 3. Eucoelomates – Have a true coelom = internal fluid-filled body cavity between gut & outer body wall muscles – Cavity lined entirely by mesodermally derived PERITONEUM F. Classification of Eucoelomates based on COELOM FORMATION and other developmental features 1. Protostomes Cleavage is spiral Cleavage is determinate Blastopore becomes mouth Mesoderm from 4d mesentoblast of the 64-cell stage embryo Coelom formation by schizocoely = gradual enlargement of mesoderm split Phyla: Annelida, Sipuncula, Echiura, Mollusca, Arthropoda, Onychophora, Tardigrada 2. Deuterostomes Cleavage is radial Cleavage is indeterminate Blastopore does not become mouth; often become anus Mesoderm from archenteron Coelom formation by enterocoely = evagination of archenteron into blastocoel Phyla: Echinodermata, Hermichordata, Chordata Radial cleavage Spiral cleavage Schizocoely Enterocoely G. Classification by Evolutionary Relationships – Formal or taxonomic methods – Taxonomy and Systematics Taxonomy = the theory & practice of identifying, describing & naming (nomenclature) and classifying organisms – Identification » Process of determining that a particular organism belongs to a recognized taxon – Classification = rearrangement of organisms into groups or taxa – Nomenclature » Concerned with assignment of names to taxonomic groups » Naming is done according to published rules (ICZN) set by the International Commission on Zoological Nomenclature » NB: Taxonomic names give clues to phylogenetic relationships Systematics = study of the kinds and diversity of organisms & the relationships among them Taxonomy & systematics sometimes used as synonyms Taxonomy of living organisms is largely based on external structures – There is increasing use of physiological, developmental, behavioural, cytogenetic and molecular biological data – Carolus Linnaeus (1707 - 1778) Developed a classification system of every (then) known organism Linnaean system was a hierarchical dual system – Organisms were classified into two kingdoms, Plantae and Animalia – For animals the hierarchical system consists of the following taxonomic groups (taxa): » Kingdom Animalia Animalia More Specificity » Phylum Arthropoda Arthropoda Less Specificity » Class Insecta Insecta » Order Hymenoptera Hymenoptera » Family Apidae Formicidae » Genus Apis Anoplolepis » Species Apis mellifera Anoplolepis custodiens Also developed binomial nomenclature – A latinized two-part name for species – Rarely a three-part name  Modern Classification Systems Based on Evolutionary Relationships A classification system based on evolutionary relationships is supposed to reflect natural relationships between organisms Natural classifications are based on genealogy (relationship by descent) But taxonomists disagree about which principle to emphasize, criteria, terminology, as well as type of evidence to use Taxonomists essentially fall into three major groups – Phyletic or Evolutionary Classification system – Phenetic or Numerical Classification system – Cladistic Classification system  Phenetic or Numerical Classification system Groups all organisms on the basis of overall similarity (outward appearance = phenotypic similarity) Each character is recorded in numerical form Clustering of taxa & their taxonomic distance from each other is calculated by computer algorithms Major principles are: – The more characters studied the better – All characters are of equal weight – The greater the proportion of similar characters, the closer are two groups related Classification results presented as phenograms Similarity does not always reflect evolutionary relationship (phylogeny)  Sources of similarity in organisms: 1. Homology (similarity in traits that’s due to descent from a common ancestor i.e., shared characters descended from a common ancestor) 2. Convergence (similar but nonhomologous characters that evolve independently in response to a particular environmental challenge in unrelated taxa 3. Parallelism = phenotypic similarities independently acquired owing to an inherited ancestral propensity of their lineage to develop these characters 4. Reversal = Occurs when further evolution of a character results in a derived condition that is similar to the ancestral (earlier) condition – Similarities 2 – 4 are called homoplasy  Cladistic Classification system Developed by Hennig, 1966 (German Entomologist) Taxa are grouped only on the basis of relative recency of common ancestry Relies on the critical distinction between primitive and advanced homologous characters (character polarity) and the recognition of sister groups (taxa) – Advanced or derived characters = apomorphies – Primitive characters = plesiomorphies – Apomorphies shared by taxa = synapomorphies – Plesiomorphies shared by taxa = symplesiomorphies Character polarity is determined by an outgroup comparison method – Comparison of character states in the group under study (ingroup) w/ those in a sister group that clearly branched off earlier (outgroup) – The Character state common to the largest sister groups is generally taken to be the primitive condition = symplesiomorphy Groups organisms based on presence of synapomorphies, not the overall similarity of potential group members – Synapomorphies are important because they identify monophyletic groups – Synapomorphies are nested or hierarchical – Classification systems based on cladistics are presented as cladograms  Distinguishing Homology from Homoplasy in Cladistics Analysis and use of many traits in reconstructing evolutionary relationships – Example, ray-finned fish & other vertebrates have a bony skeleton & many other traits that separate them from octupuses & other mollusks – Using only eye structure to group animals would suggest a close octopus- vertebrate relationship but such a hypothesis would not prevail when many traits are considered. Use of the Principle of Parsimony = Ockham’s razor = KISS – The shortest number of steps or character changes is most likely correct  Phyletic or Evolutionary Classification system Employs both cladistic and phenetic information on which to base the classification system Recognize both monophyletic and paraphyletic groups Not entirely objective Still employs the Linnaean ranking or hierarchical system