Systematics and the Phylogenetic Revolution Chapter 25

Outline The Classification of Organisms Systematics Analogy versus Homology Kingdoms Domains Viruses Impact of Molecular Cladistics Protists

The Classification of Organisms Classification - multilevel grouping of individuals organisms first classified by Aristotle over 2,000 years ago Eventually groups started to be formed and referred to as genera (singular, genus). Starting in the middle ages, names began to be systematically written down using Latin.

The Classification of Organisms Classification scheme of the Middle Ages (polynomial system) was replaced with a binomial system by Linnaeus about 250 years ago. polynomial - strings of Latin words and phrases containing up to 12 words binomial - two-part name for each species

Species Names Taxa - group of organisms at a particular level in a classification system (taxonomy) By convention: first word of binomial name is genus and is always capitalized second word refers to particular species and is not capitalized together form scientific name, written in italics

The Taxonomic Hierarchy Species Genus Family Order Class Phylum Kingdom Domain

Evolutionary Relationships Systematics - reconstruction and study of evolutionary relationships construct phylogeny by looking at similarities and differences between species Cladistics - distinguishes ancestral from shared characters only shared derived characters are considered in determining evolutionary relationships

Cladistics Outgroup comparison is used to assign polarity. A species or group of species closely related to, but not a member of, the group under study is designated an outgroup. Character states exhibited by the outgroup are assumed ancestral, and other states are considered derived.

Cladistics A cladogram is constructed that depicts hypothesis of evolutionary relationships Species that share derived characters belong to a clade. A derived character shared by clade members is a synapomorphy of the clade. Each node represents a hypothetical ancestral species.

Cladogram

Cladogram Plesiomorphies - ancestral states Symplesiomorphies - shared ancestral states Homoplasies - derived characters that are lost as species re-evolve the ancestral state Principle of parsimony favors the hypothesis that requires the fewest assumptions

Cladogram Systematics and classification A monophyletic group includes the most recent common ancestor of the group and all of its descendants. A paraphyletic group includes the most recent common ancestor of the group, but not all of its descendents. A polyphyletic group does not include the most recent common ancestor of all the members.

Analogy Versus Homology Examination of the characters on a cladogram can provide insight on how they evolved, how many times they evolved, and how complex characters evolved.

The Kingdoms of Life Most biologists use a six-kingdom system. Animalia Plantae Fungi Protista Archaebacteria Bacteria Domains - taxonomic level above kingdoms

Classifying Living Organisms

Tree of Life

Domain Archaea Share key characteristics: cells wall lack peptidoglycan possess unusual lipids and ribosomal RNA Three general categories methanogens - obtain energy using hydrogen gas to reduce carbon dioxide to methane gas

Domain Archaea extremophiles - grow under extreme conditions thermophiles (heat) halophiles (salt) pH tolerant pressure tolerant nonextreme archaebacteria - grow in same environment as eubacteria

Domain Bacteria most abundant organisms on earth most taxonomists recognize 12-15 major groups as different from archaebacteria as from eukaryotes

Domain Eukarya Appear in fossil record only about 1.5 billion years ago. complex cellular organization Fungi, Plants, and Animals are well-defined evolutionary groups. largely multicellular Diversity among protists is much greater than within or between Fungi, Plants, and Animals.

Domain Eukarya With few exceptions, all modern eukaryotic cells possess energy-producing organelles (mitochondria). Some protist phyla have also acquired chloroplasts and are photosynthetic. Mitochondria and chloroplasts are both believed to have entered early eukaryotic cells by endosymbiosis.

Domain Eukarya Key characteristics Compartmentalization Discrete compartments provide evolutionary opportunities for increased specialization within the cell. Multicellularity Activities of individual cells are coordinated and the cells themselves are in contact.

Domain Eukarya Key characteristics Sexuality alternate between syngamy and meiosis

Viruses: A Special Case Viruses do not satisfy the basic criteria of life, thus are not living organisms. cannot logically be placed in any of the kingdoms appear to be fragments of nucleic acids originally derived from genome of a living cell infect organisms at all taxonomic levels

Viral Diversity

Impact of Molecular Cladistics Traditional classification systems are based on similar traits, but do not take into account evolutionary relationships. no clear basis for determining relative importance of traits Systematic phylogenetics is based on evolutionary relationships using cladistics. Each clade has a single common ancestor.

Impact of Molecular Cladistics Molecular systematics labels protists as paraphyletic. Land plants are members of Streptophyta. Segmentation may have arisen twice. Insects may be a sister group of crustaceans. Eutherian mammals may be reclassified.

Fall of Kingdom Protista

Major Mammalian Groups

Summary The Classification of Organisms Systematics Analogy versus Homology Kingdoms Domains Viruses Impact of Molecular Cladistics Protists