A mind is a fire to be kindled, not a vessel to be filled.

Transcription

1 A mind is a fire to be kindled, not a vessel to be filled. - Mestrius Plutarchus, or Plutarch, a leading thinker in the Golden Age of the Roman Empire (lived ~ A.D.) Lecture 2 Distinction between Taxonomy and Systematics Terms used in hierarchical classification Information needed for creating a phylogeny Species concepts (BSC vs. ESC vs. PSC) Major divisions within Kingdom Animalia Unicellular vs. multicellular Cellular aggregations vs. specialization Body symmetry: radial vs. bilateral Larval development: proto- vs. deuterostome Classification and Phylogeny of Animals Taxonomy: a formal system for naming and classifying species Systematics: study of evolutionary relationships, reconstruction of phylogenies for all animal taxa Theory of Common Descent: underlying principle in the development of phylogenies 1

2 Taxonomy is a Hierarchical Classification Scheme Deer Mouse Bush Katydid Kingdom Animalia Animalia Phylum Chordata Arthropoda Subphylum Vertebrata Uniramia Class Mammalia Insecta Order Rodentia Orthoptera Family Muridae Tettigoniidae Genus Peromyscus Scudderia Species maniculatus furcata Some Conventions Family Muridae Tettigoniidae Genus Peromyscus Scudderia Species maniculatus furcata Suffixes often give hints about level of taxonomy Genus and species are based on system of binomial nomenclature of Carolus Linnaeus Genus and species are italicized Specific epithet lower-cased, all other levels of taxonomy are treated as proper nouns Sometimes authority and date are reported: Didelphis marsupialis, Linnaeus, 1758 The Science of Systematics Sources of phylogenetic information: Morphological: shapes and sizes of structures from living and fossilized organisms Behavioral: postures used in courtship display, foraging modes may be conserved Cytology: variation in numbers, shapes and sizes of chromosomes; exclusive to living rather than fossilized organisms Biochemistry: DNA sequences (ndna vs. mdna) and amino acid sequences of proteins; some fossils contain sufficient DNA for comparative study 2

3 Challenges for Systematics: The Branches Common ancestor? All descendants? Yes Yes No Yes No No In cladistics, taxonomic groups must be monophyletic Challenges for Systematics: The Twigs Do these represent one species or two? Sexual dimorphism within one species of Eclectus Parrot Male Female Criteria for Species Recognition What themes do all modern species concepts have in common? 1. Common descent Members of a species must be able to trace ancestry to a common population 2.Smallest distinct grouping that shares patterns of ancestry and descent Helps to differentiate species from higher taxa, whose members also share common descent 3. Reproductive community This community excludes members of other species 3

4 Three Major Species Concepts 1. Biological Species Concept: community of interbreeding natural populations that is reproductively isolated from other such groups Reproductive isolation leads to closure of a lineage Easily tested for sympatric populations Problems: No discrete temporal boundaries between species how to convince fossils to attempt mating? Species as both a unit of evolution AND rank in taxonomic hierarchy Interbreeding NOT going to happen in asexual animals Species Concepts 2. Evolutionary Species Concept: single lineage of ancestor-descendant populations that maintains its identity from other such lineages and has its own evolutionary tendencies and historical fate accommodates sexual AND asexual forms, fossils lineage-based concept adds evolutionary time component, helps define species boundaries in time Problems: All traits equally relevant, but traits can provide contradictory information Idea of common evolutionary fate somewhat ambiguous Species Concepts 3. Phylogenetic (Cladistic) Species Concept: smallest diagnosable cluster of individuals with a common pattern of ancestry and descent Species must be monophyletic Interbreeding not a concern Character-based concept; no inferences about historical relationships Problems: Could designate a cluster of individuals by characters of trivial importance Dealing with discrepancies between morphology and newer molecular data Analysis required quite imposing; will lead to highest species count 4

5 Species Concepts Why all the fighting over the definition of species? 1. Species the basic unit of evolution 2. Conservation strategies often aimed at saving species Concept of species an artificial construct used to help us organize nature Existence of multiple viable concepts and disagreement a sign that systematics is a dynamic field of research No one concept is comprehensive, works in all cases, or is final Major Divisions of Life: The Traditional View Five major kingdoms (Whittaker 1969) Problem Two paraphyletic groups: Protists and Monerans do not contain all descendants! Major Divisions of Life: Three Domains * * * Prokaryotes: Bacteria Archaea Eukaryotes: Eucarya Protista is paraphyletic (and polyphyletic?): Could be 3 kingdoms ( * ) 5

6 Protozoa Eight Phyla Metazoa Three Major Branches of the Metazoa Mesozoa Parazoa Eumetazoa small, marine parasites sponges germ layers, tissues Phylum Mesozoa Phyla Porifera All other phyla and Placozoa Metazoa 6

7 Two Major Grades of the Eumetazoa Planes of symmetry Radiata: animals with radial symmetry Phylum Cnidaria (jellyfish, sea anemones, coral) Phylum Ctenophora (comb jellies) Bilateria: animals with bilateral symmetry; cephalization All remaining phyla Protostomes Deuterostomes Two Major Divisions of Bilateria 1. Protostomia (Protostomes) phyla: Platyhelminthes (flatworms) Nemertea (nematodes) Annelida (segmented worms) Mollusca (clams) Arthropoda (lobsters, spiders, insects) 2. Deuterostomia (Deuterostomes) phyla: Echinodermata (sea stars) Hemichordata (acorn worms) Chordata (vertebrates) 7

8 Protostome Deuterostome Mouth first Spiral cleavage Coelom splitting Mosaic embryo Anus first Radial cleavage Coelom outpocket Regulative embryo Major Points Distinction between Taxonomy and Systematics Terms used in hierarchical classification Information and goals for creating a phylogeny Species concepts (BSC vs. ESC vs. PSC) Major divisions within Kingdom Animalia Unicellular vs. multicellular Cellular aggregations vs. specialization Body symmetry: radial vs. bilateral Larval development: proto- vs. deuterostome Next Time Origins of Eukaryotes and Multicellular Organisms Protozoans (Kingdoms or Phyla?) Sponges (Phylum Porifera) Readings: Chapters 5 and 6 8