Phylogeny, Systematics, and the Tree of Life: Study Notes

Phylogeny and Systematics

Introduction to Phylogeny

Phylogeny is the study of the evolutionary history and relationships among species or groups of related species. Systematics is the scientific discipline focused on classifying organisms and determining their evolutionary relationships, forming the basis for modern taxonomy and evolutionary biology.

• Phylogeny: The evolutionary history of a species or group of related species.

• Systematics: The study of biological diversity in an evolutionary context, including taxonomy and phylogenetics.

Binomial Nomenclature and Hierarchical Classification

Binomial Nomenclature

Binomial nomenclature is a standardized system for naming species, ensuring clarity and consistency in scientific communication.

• Binomial: The two-part scientific name of a species (e.g., Homo sapiens).

• Genus: The first part of the name; always capitalized.

• Specific epithet: The second part; identifies the species within the genus; not capitalized.

• The entire species name is italicized (e.g., Canis lupus).

Hierarchical Classification

Organisms are classified into a hierarchy of taxonomic groups, from broadest to most specific.

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

• Taxon: Any named group at any level of the hierarchy.

Linking Classification and Phylogeny

Phylogenetic Trees

Phylogenetic trees are diagrams that represent evolutionary relationships among organisms. They are hypotheses about the evolutionary history of species.

• Branch point (node): Represents the divergence of two evolutionary lineages from a common ancestor.

• Sister taxa: Groups that share an immediate common ancestor not shared by other groups.

• Rooted tree: Includes a branch representing the most recent common ancestor of all taxa in the tree.

• Basal taxon: A lineage that diverges early in the history of a group and lies on a branch that originates near the common ancestor.

• Hatch mark: Indicates a character shared by all groups to the right of the mark.

Important notes:

• Tree branches can be rotated around branch points without changing the evolutionary relationships they represent.

• The order of taxa at the tips is not significant; only the branching pattern matters.

• Phylogenetic trees show patterns of descent, not phenotypic similarity or the timing of evolutionary events.

• It should not be assumed that a taxon evolved from the taxon next to it.

Applications of Phylogenies

• Phylogenetic trees can be used in practical contexts, such as determining the legality of harvested whale meat by tracing its evolutionary origin.

Morphological and Molecular Homologies

Homology vs. Analogy

Understanding evolutionary relationships requires distinguishing between similarities due to shared ancestry (homology) and those due to convergent evolution (analogy).

• Homology: Similarity due to shared ancestry (e.g., forelimbs of mammals).

• Analogy: Similarity due to convergent evolution, not common ancestry (e.g., wings of birds and bats).

• Complex structures are more likely to be homologous if they are similar in detail.

Evaluating Molecular Homologies

• DNA sequences are aligned to identify similarities and differences.

• Closely related species differ at few sites; distantly related species differ at many sites and may have insertions or deletions.

• Insertions and deletions (indels) can shift DNA sequences, complicating comparisons.

Cladistics and Clades

Cladistics

Cladistics is a method of classification based on common ancestry, grouping species into clades.

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

• Monophyletic group: Consists of an ancestor and all its descendants (a true clade).

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

• Polyphyletic group: Includes distantly related species but not their most recent common ancestor.

Shared Ancestral vs. Shared Derived Characters

• Shared ancestral character: A trait that originated in an ancestor of the group.

• Shared derived character: An evolutionary novelty unique to a particular clade.

• Whether a character is ancestral or derived depends on the context of the group being studied.

Inferring Phylogenies Using Derived Characters

• Outgroup comparison helps distinguish between shared ancestral and shared derived characters.

• Outgroup: A species or group closely related to, but not part of, the group being studied (ingroup).

• Characters shared by both outgroup and ingroup are considered ancestral.

Phylogenetic Trees with Proportional Branch Lengths

Branch lengths in phylogenetic trees can represent the number of genetic changes or the passage of time.

• Shorter branches indicate fewer genetic changes; longer branches indicate more changes.

• Branch length can also be proportional to time, if calibrated with fossil data or molecular clocks.

Maximum Parsimony and Maximum Likelihood

Methods for Tree Construction

• Maximum parsimony: The simplest explanation (fewest evolutionary events) is preferred. In DNA-based phylogenies, the tree with the fewest base changes is chosen.

• Maximum likelihood: Identifies the tree most likely to have produced the observed DNA data, based on specific models of DNA change.

Phylogenetic Trees as Hypotheses

• Phylogenetic trees are hypotheses about evolutionary relationships and can be tested with new data.

• Phylogenetic bracketing: Predicts that features shared by two closely related groups were present in their common ancestor and all descendants.

• Example: Birds and crocodiles share four-chambered hearts, nest building, and brooding; phylogenetic bracketing predicts these traits in dinosaurs, supported by fossil evidence.

Gene Duplications and Gene Families

• Gene duplication increases the number of genes, providing raw material for evolutionary innovation.

• Gene family: A group of related genes within a genome resulting from repeated duplications.

• Orthologous genes: Homologous genes found in different species due to speciation events; useful for inferring phylogeny.

• Paralogous genes: Homologous genes within a species due to gene duplication; can evolve new functions.

Molecular Clocks

Molecular clocks estimate the timing of evolutionary events based on the assumption that some genes evolve at a relatively constant rate.

• The number of nucleotide substitutions is proportional to the time since divergence for orthologous genes, or since duplication for paralogous genes.

• Calibrated using fossil data or known divergence times.

• Not all genes evolve at the same rate; natural selection and other factors can cause irregularities.

From Two Kingdoms to Three Domains

The classification of life has evolved from two kingdoms (plants and animals) to five kingdoms, and now to three domains based on molecular evidence.

• Three domains: Bacteria, Archaea, and Eukarya.

• Eukarya includes three kingdoms: Plantae, Fungi, and Animalia.

• "Protista" is no longer a valid kingdom because its members are more closely related to plants, fungi, or animals than to each other.

Role of Horizontal Gene Transfer (HGT)

Horizontal gene transfer is the movement of genes between genomes, which can complicate the reconstruction of evolutionary relationships.

• Occurs via exchange of transposable elements, plasmids, viral infection, or organismal fusion.

• HGT has played a significant role in the evolution of both prokaryotes and eukaryotes.

• Disparities between gene trees may result from HGT.

Summary Table: Types of Taxonomic Groups

Group Type	Definition	Example
Monophyletic	Ancestor and all descendants	Mammals
Paraphyletic	Ancestor and some, but not all, descendants	Reptiles (excluding birds)
Polyphyletic	Distantly related species, not including most recent common ancestor	Marine mammals (whales, seals, etc.)

Key Terms and Concepts

• Convergent evolution: The independent evolution of similar features in different lineages, leading to analogy.

• Mitochondrial DNA: Often used to study evolutionary relationships due to its rapid mutation rate.

• Ribosomal DNA: Useful for studying deep evolutionary relationships due to its slow mutation rate.

• Branch length: In some trees, reflects the number of genetic changes or the passage of time; shorter branches indicate fewer changes.

• Sister group: Two taxa that are each other's closest relatives.

• Shared ancestral vs. shared derived characters: Essential for reconstructing evolutionary relationships.

Additional info: This guide expands on the original notes by providing definitions, examples, and context for key concepts in phylogeny and systematics, as well as a summary table for taxonomic groups.