Chapter Outline
THE CLASSIFICATION OF ORGANISMS Early Naming of Organisms Necessary as a point of reference for scientific discussions Genus (genera, pl.): basic unit of grouping Names written in, or given Latin form Classification specialists called systematists or taxonomists The Polynomial System Additional descriptive terms added to genus names to designate a species Polynomial name: string of Latin words and phrases Extremely long and cumbersome Lack of uniformity caused confusion The Binomial System Developed by Carl Linnaeus Derived two-part naming system from polynomials Example: Quercus phellos and Quercus rubra fig 29.1 Convenience of shorter names secured their use by scientists Two-part name called a binomial Format Genus species or G. species Genus name is capitalized and may be abbreviated by initial Species name is not capitalized and cannot be used alone Names established by rigid set of rules Provide uniform means of communication Reduce confusion as local names may describe different organisms THE TAXONOMIC HIERARCHY Binomial Classification System Is Hierarchal Family: unit one step more inclusive than a genus A single genus includes many related species A single family includes many related genera Family Fagaceae: oaks, beeches, chestnuts and others Family Sciuridae: tree squirrels, marmots and others fig 29.2 Certain features can be surmised from unit associations Taxonomic System Kingdom, phylum, class, order, family, genus, species fig 29.3 In plants, fungi and algae phylum also called division Comparative hierarchal descriptions of various organisms tbl 29.1 Categories may include several or only one taxon Taxon implies set of characteristics and group of organisms Printing conventions Genus capitalized, species not capitalized Both genus and species italicized or underlined All other taxonomic unit names capitalized, but no distinctive print style What Is a Species? Criteria not absolute Individuals of one species may appear quite dissimilar Capable of hybridizing with one another Offspring may appear different from one another Individuals from different species do not generally hybridize Criteria apply for organisms that regularly outcross Different characterizations for asexually reproducing organisms Compare morphological features Compare ecology and distribution How Many Species Are There? 1.4 million species currently named and described Some groups well known: flowering plants, vertebrate animals, butterflies More than 90% of species in these groups already named Other groups less well known only 5% of nematode, fungi, mite species recognized Actual number of species estimated at 10 million 6 to 7 million in tropics alone Only 400,000 tropical species now described Estimates apply for eukaryotes only, functionally impossible to estimate number of prokaryote species THE HISTORY OF LIFE ON EARTH The Six- Kingdoms of Life Originally only two kingdoms: animals and plants Most biologists now identify six kingdoms Four kingdoms are eukaryotic Animalia and Plantae are mostly multicellular Fungi contain multicellular forms and single-celled yeasts Fundamental differences among multicellular kingdoms Different morphology, motility and nutrition Each kingdom evolved from different single-celled ancestor Protists are unicellular Arbitrary grouping Include algae Archaebacteria and Eubacteria contain prokaryotic organisms The Evolution of Prokaryotes Most fundamental differences not between plants and animals, but between prokaryotes and eukaryotes Prokaryotes though to be uniform group lacking membrane-bound nucleus Molecular DNA analysis Shows distinction between Archaebacteria and Eubacteria Indicates first eukaryotes evolved from Archaebacteria fig 29.4 Later forms acquired mitochondria in form of symbiotic Eubacteria Similar acquisition of chloroplasts Evolution of Eukaryotes Only bacteria existed on earth for 2 billion years First appeared 1.5 billion years ago Fungi, plants and animals are well-defined evolutionary groups Each stems from different single-celled ancestor Largely multicellular, derived from ancestor classified as Protista Greater metabolic diversity between two prokaryotic groups than among all eukaryotic groups Unicellular eukaryotes lumped together in Protista, lacking rationale to put them with fungi, plants or animals Characteristics of the six kingdoms tbl 29.2 Origins Almost all modern eukaryotes possess mitochondria derived from purple sulfur bacteria Some protists acquired chloroplasts and are photosynthetic Chloroplasts derived from symbiotic cyanobacteria Defining characteristic of groups that possess them Endosymbionts evolved and adjusted to new environment Lost redundant genes, kept only those needed for survival in cell Both contain own ribosomes, more similar to bacterial ribosomes Manufacture own membranes Divide independently of cell Contain chromosomes similar to those found in bacteria Other symbionts: basal bodies, centrioles, flagella, cilia Multicellularity Bacteria occur in nearly every habitat Protists diverse in form and biochemistry fig 29.5 Multicellularity allows novel adaptations to environment Distinct cell differentiation possible Greater complexity of activities True multicellularity Occurs only in eukaryotes Coordinates activities of individual cells Bacteria and some protists may form colonial aggregates Some protists exhibit simple multicellularity fig 29.6 Green algal protists were ancestors of plants Fungi and animals arose from unicellular ancestors Groups giving rise to these kingdoms still exist Sexuality Major characteristic of eukaryotes Process is regular, results are predictable Alternation between syngamy and meiosis Syngamy: produces cell with two sets of chromosomes Meiosis: produces cells with one set of chromosomes Differs greatly from genetic exchange in bacteria Cells of animals and plants are diploid during some part of life cycle Few eukaryotes complete life cycle in haploid condition Offspring of sexual eukaryotic organisms vary widely Due to segregation during meiosis Resulting from crossing over in meiosis Provides raw material for evolution Sexual organisms evolve rapidly in relation to demands of environment Protist sexual reproduction May only occur in times of stress Many are haploid throughout entire life, an ancestral condition Life cycles fig 29.7 Zygotic meiosis Zygote is the only diploid cell Zygote immediately undergoes meiosis All other stages are haploid Gametic meiosis Gametes are only haploid cell Gametes fuse giving rise to a zygote Sporic meiosis: alternation of generation Exhibited by plants Multicellular diploid form undergoes meiosis to produce haploid spores Spores give rise to haploid phase Haploid form produces haploid gametes Gametes fuse to produce diploid zygote Viruses: A Special Case Viruses not classified as living organisms Viruses not included in any kingdom Capable of replication within a cell Machinery of host cells directed to manufacture viral material Nucleic acid fragments derived from prokaryotes or eukaryotes Non-living when outside of host fig 29.8 EVOLUTIONARY TAXONOMY Should Taxonomy Reflect History? Not an old, inactive science, but active and controversial Defining its role in biology Linnaean approach of classifying and naming Darwinian approach of tracing evolutionary history Classifying by Morphological Similarity Observations of characteristics to distinguish and name new species Must make subjective judgement on which characteristics are more important Numerical taxonomy, phenetics, applies numbers to evaluation of characteristics Use as many characteristics as possible No additional emphasis initially prescribed to any one character Avoids confusion associated with parallel evolution Analogous characteristics are only small set of the whole Homologous characteristics associated with common evolutionary descent Subsequent applications assign weight (emphasis) to certain characteristics Classifying by Evolutionary Relationships Cladistic school of taxonomy at opposite end of spectrum from phenetic school Cladistics consider only evolutionary relatedness, not morphological comparisons Classifies organisms by historical order in which evolutionary branches arise through history of group Employs specialized analytical methods to determine significant characters Results in testable hypotheses Complex comparisons requires use of computers Both phenetics and cladistics use biochemical characteristics along with morphology Basic object of cladistics Ascertain characteristics that indicate common ancestry Construct hypotheses about group's ancestral condition and derived characters Derived characters are shared by all members of branch, but not existent before branch Example: vascular plant cladogram (evolutionary tree) All vascular plants have vascular tissue, others don't fig 29.9 All seed plants on same branch of vascular plant cladogram Flowers are unique characteristic of angiosperms Ancestral angiosperm had two cotyledons, specialized monocots have one, a derived character Construction of accurate cladograms requires correct interpretation of features Cladistic approach seems most appropriate to analyze evolutionary history Cladistics shows order of descent, not extent of divergence Taxonomy Today Utilizes information from phenetics and cladistics Accounts for degree of differences and evolutionary history Example of conflicts Birds in own class, crocodiles grouped with reptiles Crocodiles more closely related to birds, share derived features fig 29.10 Birds retain own class due to degree of divergence from common ancestor with crocodiles A LOOK AHEAD