Chapter 22: Phylogenetic Reconstruction

Introduction to Phylogenetics

Phylogenetics is the study of evolutionary relationships among biological species based on similarities and differences in their physical and/or genetic characteristics. It is a fundamental topic in General Biology, helping students understand how organisms are classified and related through evolutionary history.

Taxonomic Hierarchy and Classification

Biological classification organizes living organisms into hierarchical categories based on shared characteristics. The major ranks, from broadest to most specific, are:

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Each successive category contains fewer species. For example, Panthera (genus) contains fewer species than Carnivora (order).

• Key Point: The species category contains the fewest number of organisms, while domain contains the most.

• Example: Panthera leo (lion) is a species within the genus Panthera.

Domains of Life

All living organisms are classified into three domains:

• Bacteria

• Archaea

• Eukarya

Plantae is a kingdom within the domain Eukarya, not a domain itself.

Scientific Naming (Binomial Nomenclature)

Scientific names are written using binomial nomenclature, which includes the genus and species names. The correct format is:

• Genus species (italicized, with genus capitalized and species lowercase)

Example: Homo sapiens is the scientific name for humans.

Homologous and Analogous Structures

Structures in different species can be classified as homologous or analogous:

• Homologous structures: Features inherited from a common ancestor, often with different functions.

• Analogous structures: Features with similar functions but evolved independently (not from a common ancestor).

Key Point: Animals with homologous structures likely evolved from a common ancestor.

Phylogenetic Trees and Outgroups

A phylogenetic tree (cladogram) visually represents evolutionary relationships. An outgroup is a species or group that is closely related to but outside the group of interest, used to root the tree and infer ancestral traits.

• Example: In a tree comparing mammals, a frog or snake may serve as an outgroup.

Monophyletic, Paraphyletic, and Polyphyletic Groups

Groups in phylogenetic trees are defined as:

• Monophyletic group: Includes an ancestor and all its descendants.

• Paraphyletic group: Includes an ancestor and some, but not all, descendants.

• Polyphyletic group: Includes species with different ancestors.

Example: Describing a group with an ancestral species and some descendants is a paraphyletic group.

Shared Derived and Shared Ancestral Characters

Characters used in phylogenetic analysis are:

• Shared derived character (synapomorphy): A trait unique to a particular clade.

• Shared ancestral character (symplesiomorphy): A trait present in the ancestor of a group.

Example: Hair in mammals is a shared derived character compared to other vertebrates.

Cladistics and Character Analysis

Cladistics is a method of classifying species based on shared derived characters. It is more effective than using shared ancestral characters because it identifies evolutionary novelties unique to certain groups.

• Key Point: Shared derived characters help clarify evolutionary relationships.

Convergent Evolution

Convergent evolution occurs when unrelated species independently evolve similar traits due to similar environmental pressures.

• Example: Two trees in different classes developing thorns independently is an example of convergent evolution.

Gene Homology: Orthologs and Paralogs

Genes can be homologous in two main ways:

• Orthologous genes: Genes in different species that evolved from a common ancestral gene by speciation.

• Paralogous genes: Genes related by duplication within a genome.

Example: Hemoglobin and myoglobin genes are paralogous—they descended from a common ancestor but now have different functions.

Classification Table: Taxonomic Ranks

The following table summarizes the main taxonomic ranks and their relative inclusiveness:

Application: Phylogenetic Trees and Outgroups

When constructing a phylogenetic tree, selecting an appropriate outgroup is essential for rooting the tree and determining the direction of evolutionary change.

• Example: In a tree of mammals, a primitive mammal or a non-mammalian vertebrate may serve as an outgroup.

Summary Table: Homologous vs. Analogous Structures

Key Equations and Concepts

• Cladogram construction: Based on shared derived characters (synapomorphies).

• Gene duplication:

• Speciation:

Additional info:

• Phylogenetic analysis is central to understanding evolutionary biology and biodiversity.

• Correct scientific naming is essential for clear communication in biology.

• Cladistics is a powerful tool for reconstructing evolutionary relationships using shared derived characters.