Ch. 20 Phylogeny, Systematics, and Classification: Study Notes

Chapter 20: Phylogeny

Introduction to Phylogeny

Phylogeny is the evolutionary history of a species or group of species. Understanding phylogeny allows biologists to trace the lineage of organisms and their evolutionary relationships.

• Phylogeny: The evolutionary history and relationships among individuals or groups of organisms.

• Phylogenies are inferred from both morphological (physical) and molecular (genetic) data.

• Shared characters are used to construct phylogenetic trees.

Taxonomy and Binomial Nomenclature

Taxonomy

Taxonomy is the science of naming, describing, and classifying organisms. It provides a universal language for biologists worldwide.

• Taxonomy: The discipline of classifying organisms and assigning each organism a universally accepted name.

Binomial Nomenclature

Binomial nomenclature is a two-part scientific naming system developed by Carl Linnaeus in the 1700s. Each species is given a unique name consisting of its genus and specific epithet.

• Format: Genus species (e.g., Homo sapiens, Puma concolor, Lens culinaris).

• The genus is capitalized and italicized; the specific epithet is lowercase and italicized.

• Over 11,000 organisms were named by Linnaeus using this system.

Hierarchical Classification

Levels of Classification

Organisms are sorted into increasingly inclusive categories, known as taxa. This hierarchical system reflects evolutionary relationships.

• Species (most exclusive)

• Genus (pl. genera)

• Family

• Order

• Class

• Phylum (pl. phyla)

• Kingdom

• Domain (most inclusive)

Each level is more inclusive than the one below it, grouping organisms by shared characteristics.

Systematics and Phylogenetic Trees

Systematics

Systematics is the scientific study that combines taxonomy with phylogenetics to determine the evolutionary relationships among organisms.

• Systematists use data from fossils, morphology, genes, and biochemistry to infer relationships.

• Systematics helps answer major questions in evolutionary biology.

Phylogenetic Trees

A phylogenetic tree (also called an evolutionary tree or cladogram) is a diagram that shows the proposed evolutionary relationships among various species or groups.

• Branches represent lineages evolving through time.

• Nodes represent common ancestors.

• Phylogenetic trees are hypotheses about evolutionary relationships.

Linking Classification and Phylogeny

Relationship Between Classification and Phylogeny

The nested nature of the Linnaean classification system reflects evolutionary relationships. Organisms are classified based on similarities, and close relatives tend to be grouped together.

• Classification is based on shared characteristics, which often reflect common ancestry.

• Sometimes, classification may be based on molecular (DNA) or morphological data.

• Some organisms may appear similar due to convergent evolution, not shared ancestry.

Homology and Convergent Evolution

Homology

Homologous characteristics are traits inherited from a common ancestor. These traits are used to infer evolutionary relationships.

• Homologous structures may have different functions but share an underlying similarity due to common ancestry.

• Homology is the basis for grouping organisms in phylogenetic trees.

Convergent Evolution

Convergent evolution occurs when unrelated organisms independently evolve similar traits as a result of adapting to similar environments or ecological niches.

• Analogous traits arise from convergent evolution, not from shared ancestry.

• Example: The wings of bats and birds are analogous, not homologous.

• Homology ≠ Analogy

Constructing Phylogenetic Trees

Using Shared Characters

Shared characters, especially those inherited from a common ancestor, are used to construct phylogenetic trees. These characters can be morphological or molecular.

• Ancestral traits: Traits present in the common ancestor of a group.

• Derived traits: Traits that are new in a particular lineage and not found in the common ancestor.

Cladistics

Cladistics is a method of systematics that uses shared derived characters to determine evolutionary relationships and define groups called clades.

• Clade: A group of species that includes an ancestral species and all its descendants.

• Clades can be of any size, as long as they include a common ancestor and all its descendants (monophyletic).

Types of Groups in Cladistics

Group Type	Definition	Example
Monophyletic	Includes a common ancestor and all its descendants	All mammals
Paraphyletic	Includes a common ancestor and some, but not all, descendants	"Reptiles" excluding birds
Polyphyletic	Does not include the most recent common ancestor of all members	Warm-blooded animals (birds and mammals)

Summary Table: Key Terms and Concepts

Term	Definition
Phylogeny	The evolutionary history of a species or group of species
Taxonomy	The science of classifying organisms
Systematics	The study of biological diversity in an evolutionary context
Homology	Similarity due to shared ancestry
Analogy	Similarity due to convergent evolution
Clade	A group consisting of an ancestor and all its descendants

Key Equations and Concepts

• There are no specific mathematical equations in this section, but understanding the hierarchical structure and relationships is essential.

• Cladograms and phylogenetic trees are constructed based on shared derived characters.

Examples and Applications

• Example of Binomial Nomenclature: Homo sapiens (humans), Puma concolor (cougar), Lens culinaris (lentil).

• Example of Homology: The forelimbs of humans, cats, whales, and bats are structurally similar due to common ancestry.

• Example of Analogy: The wings of insects and birds serve the same function but evolved independently.