Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 26.3 Shared Characters Are Used to Construct Phylogenetic Trees
Cladistics is a method of systematics that organizes species based on shared ancestry. By comparing traits among species—especially shared derived characters—scientists can infer evolutionary relationships and build accurate phylogenetic trees.
Cladistics and Clades
Cladistics groups organisms into clades—each clade includes an ancestor and all its descendants.
A clade is monophyletic if it contains an ancestral species and all descendants.
Paraphyletic groups include some, but not all, descendants of a common ancestor.
Polyphyletic groups consist of unrelated species and exclude their common ancestor—these are avoided in classification.
Shared Ancestral vs. Shared Derived Characters
Shared ancestral character: Trait present in ancestor and all descendants, but not unique to the group (e.g., backbone in mammals).
Shared derived character: Evolutionary novelty unique to a particular clade (e.g., hair in mammals).
The same character can be ancestral or derived, depending on the group being studied.
Using an Outgroup for Comparison
An outgroup is a species or group closely related to the group being studied (the ingroup) but not part of it.
Traits shared by outgroup and ingroup are considered ancestral.
Traits present only in some ingroup members are considered derived, used to infer evolutionary relationships and build trees.
Tree Construction with Shared Derived Characters
Phylogenetic trees are built by mapping when each shared derived trait appeared.
Example: Vertebrate traits like hinged jaws, four limbs, and amniotic eggs are used to construct a tree showing evolutionary relationships from lampreys to leopards.
Branch Lengths: Change vs. Time
Branch lengths can indicate:
Genetic change (e.g., more mutations in one lineage).
Chronological time, if calibrated with fossil data.
Even if organisms look “unchanged” (e.g., bacteria), they’ve undergone evolution for the same amount of time as other lineages.
Building Trees: Maximum Parsimony and Likelihood
Maximum parsimony: The simplest explanation (fewest evolutionary changes) is most likely correct.
Maximum likelihood: Calculates the most probable tree based on known models of DNA evolution.
These principles help scientists choose the best-supported phylogenetic tree.
Phylogenetic Bracketing: Making Predictions
If two living species share a trait, we can infer (by parsimony) that their common ancestor had that trait too.
Example: Birds and crocodiles both build nests and care for eggs → prediction: dinosaurs did too.
Supported by fossils showing Oviraptor dinosaurs sitting on nests in bird-like positions.
Molecular Clocks and Gene Types
Different genes evolve at different rates:
rRNA genes evolve slowly—used to study distant evolutionary relationships.
mtDNA evolves rapidly—useful for tracing recent evolutionary events (e.g., Native American ancestry).
Gene duplication creates gene families and enables evolution of new functions.
Two types of homologous genes:
Orthologous genes: Same gene in different species due to speciation.
Paralogous genes: Gene copies within the same genome due to duplication.
In a Nutshell
Cladistics builds evolutionary trees using shared derived characters. Phylogenies can reflect either the amount of genetic change or chronological time. Parsimony and likelihood help determine the best tree, and phylogenetic bracketing allows scientists to make evolutionary predictions. Gene duplication further contributes to genetic innovation and complexity.