22Understanding Diversity: Systematics

22Understanding Diversity: Systematics

Lecture Outline

1. Organisms are named using the binomial system of nomenclature, which began with Linnaeus
2. Taxonomy is the science of naming and classifying organisms
3. The binomial system is the unique assignment of a genus and species name to all extant and extinct organisms
4. Both names must be used together to correctly identify an organism
5. The genus name is always capitalized, the species epithet is not, and both are either underlined or italicized
6. The genus name used alone designates all of the species classified in that genus
7. Abbreviations may be made using the initial of the genus name with the species name, e.g., H. sapiens, but never without the generic name
8. Names are typically derived from Greek or Latin terms or latinized names of persons, places, or characteristics
9. Scientific names cross language barriers

1. Each taxonomic level is more general than the one below it
2. A taxon is any taxonomic classification
3. Similar species are grouped into genera, genera into families, families into orders, orders into classes, classes into phyla, and phyla into kingdoms
4. Recently, the term phylum has been adopted instead of division for fungi and plants
5. A recently suggested classification system is known as PhyloCode

a) This system groups organisms into clades

b) This system is developed due to difficulties in classifying organisms in this inflexible mode of hierarchical slots

c) This mode attempts to take into account modern findings such as molecular data

1. New data influence traditional classification into kingdoms
2. Early taxonomists classified organisms into 2 kingdoms, Plantae and Animalia
3. Haeckel established the kingdom Protista
4. Whittaker proposed the 5-kingdom classification in 1969
5. Woese and colleagues proposed the splitting of prokaryotes into domains, Eubacteria and Archaebacteria, based on RNA sequencing, hence a 3 domain/5 kingdom scheme
6. The domains Archaea, Eubacteria, and Eukarya may be identified
7. Eukaryotic kingdoms include Fungi, Plantae, Animalia, and Protista
8. Systematics is concerned with reconstructing phylogeny (evolutionary relationships)
9. Homologous structures are important criteria for classification
10. Homologous structures suggest a common ancestor
11. Homoplastic characters are independently derived, but appear similar
12. Shared derived characters provide clues about phylogeny
13. Ancestral, or pleisomorphic, characters are essentially unchanged from ancestral species
14. Derived, or synapomorphic, characters were not present in ancestral species
15. Biologists carefully choose taxonomic criteria
16. Homologous characters are used to classify organisms in the same taxa
17. A combination of traits is used to classify organisms
18. Molecular biology provides taxonomic tools
19. Variations in macromolecular structure results from mutations
20. Nucleotide and amino acid sequences may be used as molecular clocks
21. Studies of ribosomal RNA structure may indicate phylogenies
22. Mitochondrial DNA may also indicate relationships

1. Taxa should reflect evolutionary relationships
2. If all of the members of a taxa share a common ancestor, it is monophyletic and reflects true evolutionary relationships
3. A taxon containing the ancestor and all organisms derived from it is called a clade
4. Many taxa are polyphyletic, and may misrepresent evolutionary relationships

1. There are 2 major approaches to systematics
2. How data are analyzed and interpreted depends on the systematist's approach
3. Phenetics was an approach tried earlier, and is based on phenotypic similarities
4. The phenetic approach is also called numerical taxonomy
5. Pheneticists do not attempt to distinguish between homoplastic and homologous characteristics
6. The inclusion of homoplastic characteristics may lead to inaccurate taxonomic classifications
7. Phenetic techniques may be useful in molecular systematics
8. The 2 major approaches used today are evolutionary systematics and cladistics (phylogenetic systematics)
9. Classical evolutionary taxonomy allows paraphyletic groups

a) Phylogenetic trees examine the branching and the extent of divergence

b) Most taxa recognized are monophyletic

1. Cladistics emphasizes phylogeny

a) Cladists use shared derived characters to reconstruct phylogenies by outgroup analysis

b) Cladograms illustrate branching between taxa

c) Branching points are called nodes

d) Ancestral and derived characters are used to construct cladograms

1. Cladistics is the newer, but currently most employed tool in modern taxonomy
2. Critical to cladistic analysis is outgroup analysis, guided by parsimony
3. Groups that share similar evolved traits, called derived characters, are considered to be closely related
4. An outgroup is a taxon, which is considered to have diverged earlier than the ingroup taxa of inconsideration

a) A classic outgroup is the closest relative of the taxon being studied

b) An ingroup shares many characters and is closer to the ancestral condition

1. Outgroup analysis is used in building and interpreting cladograms

a) After selection of taxa, homologous characters are selected

(1) Characters must be defined

(2) Characters must be organized in evolutionary order

b) Groups are characterized as relative outgroups if they lack the ancestral condition, or ingroups if they possess the ancestral character

1. A cladogram is constructed by considering shared derived characteristics

a) Taxa are grouped by shared derived characters

b) The base of the cladogram represents the common ancestor for the taxa being analyzed

c) Nodes represent branching point

d) The branching process is continued until all clades are represented

1. In a cladogram, each branch point represents a major evolutionary step

a) Organisms connected at a node share a common ancestor at that point

b) By comparing the distance from the node to the ancestral organism, evolutionary distances cannot be measured exactly; it is a relative measure only

c) Cladograms show that taxa share a common ancestor, but that ancestor remains unspecified (note that there is not a specific organism named at the node, nor at the base of the cladogram)

Research and Discussion Topics

• Investigate the changes in taxonomy since Linnaeus. Note major changes, such as the recognition that sponges are not plants (Linnaeus thought they were), the changing classification of barnacles (Linnaeus thought they were bivalves), and the breakdown of Linnaeus’ phylum, Vermes (worms).
• Compare and assess the various methods of determining the total number of species on the earth. Refer to May, R.M. "How Many Species Inhabit the Earth?" Scientific American. October 1992. 267 (4): 42-48.
• Approximately how many animal phyla are currently recognized? Which are the most speciose, based on your knowledge so far? Which are the least speciose?
• Draw a cladogram of the primates, referring to chapter 21. Include Homo sapiens and the other extinct species of the genus Homo to the best of our current knowledge. Compare to the cladograms in several of the most recent references listed in chapter 21.