Taxonomy

• To sort organisms into species• To classify species into higher taxonomic

levels• A taxon is a taxonomic unit at any level;

for example “Mammalia” is a taxon at the Class level (taxa = plural)

Taxonomy

• Species that appear to be closely related are grouped into the same genus (e.g., the leopard Panthera pardus; African lion Panthera leo; Tiger Panthera tigris)

• Similar genus are grouped into the same family, and so on…

Domain

Kingdom

Phylum

Subphylum

Class

Order

Family

Genus

Species

Diversification

Taxonomy

• Classification system• Domain Eukarya• Kingdom Animalia• Phylum Chordata• Class Mammalia• Order Primates• Family Hominidae• Genus Homo• species sapiens

Taxonomy

• The taxonomic system was developed by Linnaeus in the 1750’s

• Binomial (two part) system; Genus species• The scientific name includes the species

name• Example: Homo sapiens or H. sapiens• No 2 organisms can have the same scientific

name and this name is the same everywhere in the world!

Taxonomy• Scientific names are descriptive

• May describe unique characteristic, region where species is found, etc.

• Example: Humpback whale Megaptera novaeangliae – Magas = large– Pteron = wing– Novas = new– Angaliae = England

Taxonomy

• Common names can be deceiving…

http://www.koala.net/media http://www.ticam.utexas.edu/images/grizzly.jpg

“bear”

“dolphin”

Taxonomy

• Scientific names may change over time

• Many hierarchies are being re-examined based on results of molecular analysis

• Linnaean taxonomy does not take into account evolutionary relationships

• Enter phylogeny and systematics…

Systematics

• The reconstruction and study of evolutionary relationships

• Phylogeny – an evolutionary tree; species grouped by how long they’ve shared a common ancestor

• Evidence for determining these relationships must come from shared homolagous characteristics

Phylogeny

• The evolutionary history of a group is presented graphically in phylogenetic trees

• In a phylogenetic tree:– Time goes from the bottom up– Read from bottom up, not left to right– Branch length = number of changes– Depicts evolutionary relationships!

Time

Time

Remember: Read from the bottom up; not left to right!

These three versions convey the same information (phylogenies depict evolutionary

relationships)

Node 1: common ancestor of human and chimp

Node 2: c.a. of human, chimp, and gorilla

Phylogenetic trees reflect hierarchical classification of taxonomic groups

Phylogeny• Systematists prefer

monophyletic taxa

• Monophyletic – a single ancestor gave rise to all species in that taxon (and to no species in any other taxa); “single tribe”

• A monophyletic group consists of a common ancestor and all its descendants

Monophyletic group

Monophyletic group (indicated in pink) consists of most recent common ancestor and all of its

descendents

Common ancestor: “Archosaur”

Paraphyletic group

Paraphyletic group (indicated by pink) consists of the most recent common ancestor and some

(not all) of the descendents

1 descendent, the hawk, is excluded

Polyphyletic group

Polyphyletic group (indicated in pink) does NOT contain the most recent common ancestor

Birds and bats can be included because of morphological similarities (convergent evolution)

Monophyletic versus Paraphyletic and Polyphyletic groups

Old classification system for plants

New classification system for plants

Phylogeny

• What evidence is used to reconstruct phylogeny?– Comparative anatomy– Morphology– Embryology– Protein comparisons– DNA comparisons

• Problems of homology versus analogy

Phylogeny• Homology (homologous structures) –

likeness due to common ancestor• Analogy (analogous structures) – also

known as “homoplasy”; likeness due to convergent evolution; functional constraint

• Analogous structures are the result of separate evolution

Comparative Biology

Elongated canine teeth (saber teeth) evolved separately in different groups of carnivores

Cladistics• Cladistics – only shared derived

characters are used in determining evolutionary relationships

• Homoplasy (analogy) complicates cladistic analysis

• Species that share a common ancestor constitute a clade (a monophyletic group is a clade)

• Clades are hypotheses, just as phylogenetic trees are

Time

clade

Cladistics

• Principle of parsimony – favors the hypothesis that requires the fewest assumptions (the phylogeny that requires the fewest evolutionary events is the best hypothesis)

• Simplest is best (Occam’s Razor)

Parsimony and Homoplasy

Placement of frogs as closely related to salamanders requires that tail loss (in adults) evolved twice (homoplasy); if frogs are

closely related to gorillas, then tail loss evolved once, but would also have to lose amniotic membrane and hair

Ancestral versus Derived characters

• Ancestral – dates back to common ancestor (found in common ancestor and all its descendents)

• Derived – feature of distinct taxon• Hair is a shared derived feature of mammals, but

so is a backbone; the presence of a backbone cannot help us distinguish mammals from other vertebrates

• The presence of backbone is a shared ancestral feature (also present in amphibians and reptiles)

Recommended Reading

• Dictionary of Word Roots and Combining Forms; Donald J. Borror

• Best $15 you can spend

• Compiled from Greek, Latin and other languages, with special reference to biological terms and scientific names