Phylogenetics & Systematics: Taxonomy, Phylogenetic Trees, and Evolutionary Relationships

Systematics

Overview of Systematics

Systematics is the scientific study of the diversity of life and the relationships among living things through time. It encompasses two major components: taxonomy and phylogenetics.

• Taxonomy: The science of naming, describing, and classifying organisms into groups (taxa) such as species, genus, family, etc.

• Phylogenetics: The study and estimation of evolutionary relationships, often represented as phylogenetic trees.

Taxonomy

Hierarchical Classification

Taxonomy organizes living organisms into a nested hierarchy of groups, each called a taxon (plural: taxa). This system reflects evolutionary relationships and is consistent with the concept of a tree of life.

• Major Taxonomic Ranks: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

• Example: Panthera pardus (the leopard) belongs to all the following taxa: Domain Eukarya, Kingdom Animalia, Phylum Chordata, Class Mammalia, Order Carnivora, Family Felidae, Genus Panthera, Species pardus.

Binomial Nomenclature

Each species is assigned a two-part scientific name (binomial), consisting of the genus and the specific epithet.

• Format: Genus species (e.g., Panthera pardus)

• Hierarchy to Know: Family > Genus > Species

Phylogenetics

Phylogenetic Trees

Phylogenetic trees (or phylogenies) are diagrams that represent evolutionary relationships among taxa. They can be drawn in various ways but convey the same information.

• Tips (Terminal Nodes): Represent examined units, often living species.

• Branch Points (Internal Nodes): Represent common ancestors.

• Sister Taxa: Groups that share an immediate common ancestor.

Types of Phylogenetic Trees

• Cladogram: Branch lengths have no particular meaning; shows relationships only.

• Phylogram: Branch lengths represent the amount of inferred evolutionary change, often used in molecular phylogenies.

Monophyletic, Paraphyletic, and Polyphyletic Groups

Definitions and Examples

• Monophyletic Group (Clade): Includes an ancestor and all its descendants. Example: Land plants (Plantae/Embryophyta).

• Paraphyletic Group: Includes an ancestor and some, but not all, of its descendants. Example: Traditional 'Reptiles' excluding birds.

• Polyphyletic Group: Does not include the most recent common ancestor of all members; consists of taxa from different branches.

Inferring Phylogenies

Character States and Homology

Phylogenies are inferred by analyzing character states (traits or features) among organisms. These can be morphological (physical features) or molecular (DNA/protein sequences).

• Homology: Similarity due to shared ancestry.

• Distribution of Character States: Patterns of traits among taxa reflect evolutionary relationships.

Cladistic Reasoning

• Shared Derived States (Synapomorphies): Indicate evolutionary relationships and define clades.

• Shared Ancestral States: Do not define clades.

• Outgroup Comparison: Used to distinguish ancestral and derived states.

Example: Claws/Nails Character

• Character: Claws/Nails

• States: Non-retractable (ancestral, 0), Retractable (derived, 1)

• Outgroup: Turtle (non-retractable claws)

• Evolutionary Change: From non-retractable to retractable claws ($0 \rightarrow 1$)

Character Matrix/Table

Multiple characters are analyzed together in a character matrix to infer phylogenetic trees.

TAXA	Hair	Meat-cutting teeth	Retractable claws	Purring
Turtle (outgroup)	0	0	0	0
Horse	1	0	0	0
Wolf	1	1	0	0
Leopard	1	1	1	0
Domestic Cat	1	1	1	1

Parsimony in Phylogenetic Inference

Principle of Parsimony

When character state distributions are not all consistent with the same tree, the principle of parsimony is used. The most parsimonious tree is the one that requires the fewest evolutionary changes.

• Convergent Evolution: Similar traits evolve independently in different lineages (analogy).

• Reversals: Loss of a derived state, returning to the ancestral state.

• Method: Compare all possible trees and select the one with the minimum number of changes.

Example: Dinosaur-Bird Relationship

TAXA	Gait	Beak	Pubis	Hollow bones	Wishbone
Crocodile*	0	0	0	0	0
Parasaurolophus	1	0	1	1	0
Tyrannosaurus	1	0	0	1	1
Hawk	1	1	1	1	1

Tree with the fewest changes (most parsimonious) best reflects evolutionary relationships.

Molecular Phylogenetics

Using Molecular Sequence Data

Phylogenies of living taxa are often estimated by comparing molecular sequences, such as DNA or proteins. Each site in an aligned sequence is treated as a character, and different bases at a site are considered different states.

TAXA	1	2	3	4	5	6	7	8	9	10
Species W	G	T	A	C	G	C	A	C	T	T
Species X	G	T	A	C	G	C	A	C	C	C
Species Y	G	T	A	T	G	C	A	C	T	T
Species Z	G	T	A	T	G	-	A	C	T	T

• Example: Site #4 shows variation (C or T), indicating at least one evolutionary change.

Phylogenies of Genes

Phylogenetic analyses can also be applied to genes themselves, tracing the history of gene duplication, endosymbiotic events, or the origins of organelles such as mitochondria and plastids.

Summary of Key Concepts

• Taxonomy: Naming and identification of taxa (species and higher categories).

• Phylogenetics: Estimation of evolutionary trees.

• Monophyletic, Paraphyletic, Polyphyletic: Formal taxa should be monophyletic.

• Shared-derived character states: Denote clades; shared ancestral states do not.

• Parsimony: Method for inferring evolutionary trees from many characters.

• Molecular sequences: Used to infer relationships among organisms and evolutionary histories of genes.

Additional info: Most modern phylogenetic analyses of molecular data use methods beyond parsimony, such as maximum likelihood or Bayesian inference, but the logic of using character state data remains foundational.