1 Systematics and the Phylogenetic Revolution Chapter 25

2 The Classification of Organisms Classification organisms first classified by Aristotle over 2,000 years ago Classification scheme of the Middle Ages was replaced with a binomial system by Linnaeus about 250 years ago. binomial - two-part name for each species

3 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

4 The Taxonomic Hierarchy Species Genus Family Order Class Phylum Kingdom Domain

5 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

6 Cladistics A cladogram is constructed that depicts hypothesis of evolutionary relationships – Species that share derived characters belong to a clade. – Each node represents a hypothetical ancestral species.

7 Cladogram

8 Cladistics Outgroup comparison 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.

9 Principle of parsimony favors the hypothesis that requires the fewest assumptions

10 Cladogram Systematics and classification – A monophyletic group includes the most recent common ancestor of the group and all of its descendants.

11 paraphyletic group includes the most recent common ancestor of the group, but not all of its descendents.

12 A polyphyletic group does not include the most recent common ancestor of all the members.

13 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. Wings in birds and insects Eyes in mammals and squid

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

15 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.

16 Tree of Life

17 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

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

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

20 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.

21 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.

22 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.

23 Domain Eukarya Key characteristics – Sexuality  alternate between syngamy and meiosis

24 Impact of Molecular Cladistics Molecular systematics labels protists as paraphyletic. Insects may be a sister group of crustaceans.

25 Major Mammalian Groups Eutherian mammals may be reclassified.