Building Cladograms in Biology

Cladogram and Phylogenetic Relationships

• A cladogram shows different organisms at the tips and different characteristics that separate the organisms.
• Moving forward in the cladogram indicates getting more exclusive different traits.
• A cladogram shows common ancestry and lineage of the descendants.
• A cladogram shows relatedness of the organisms.
• Shared derived characteristics help us to identify which organisms are distantly or closely related.

Building Cladogram

• Clades are built based on the appearance of new traits.
• Outgroup – a group that has the least characters, and there might be a trait that pre-dates the outgroup.
• Shared derived character (synapomorphies) – every organism in the cladogram beyond that point shares the trait.
• The most primitive is the outgroup.

Key Concepts in Cladogram

• The branches at every node can be rotated, and branches do not infer any sort of order, they indicate recency of descent.
• Two lineages branching from a single ancestral node are known as sister taxa.
• There is no such thing as a "most highly evolved species"; all extant species descended from successful ancestors and evolved to survive and reproduce in the context of their specific environment.

Taxonomy and Systematics

• Taxonomy is the theory and practice of classifying organisms.
• Systematics is the study of the diversification of life forms over time, both past and present, and their relationships between other species.
• The method of grouping organisms is based on evolutionary relationships.

Carolus Linnaeus

• Devised a taxonomic system, a hierarchical system, since organisms are grouped into ever more inclusive categories from species up to kingdom.

Binomial Nomenclature

• Refers to the practice of assigning scientific names, which consists of two parts – the genus or generic name and the specific epithet.
• Major rules:
  • Names should be written in Latin.
  • The scientific name is always written with the genus capitalized and the specific epithet in lower case letters.
  • Should be italicized or if handwritten, underlined.
  • All taxa must have an author when described.

Dichotomous Key

• A tool that helps identify unknown organisms to some taxonomic level (e.g., species, genus, family, etc.).
• A series of choices is made that leads the user to the correct identity of a sample organism.
• "Dichotomous" means "divided into two parts".

Tips in Dichotomous Key

• List down all the characters – differences of the organisms.
• Organize the characters in order, starting with the most general.
• Divide the specimens into two groups based on the most general character.
• Use statements (i.e., has feathers and no feathers) or questions (does it have feathers).

Systematics and Phylogeny

• Phylogeny is the evolutionary relationships among organisms.
• Cladogram is a phylogenetic tree that shows the relationship of taxa based on shared derived characters.
• Character is any trait of an organism that can be described or measured.
• Homologous characters are characters having similar structures because they were derived from a common ancestor.
• Analogous characters are characters that have separate evolutionary origins but are superficially similar because they perform the same function.

Definition of Terms

• Clade is a group of taxa consisting of an ancestor and all of its descendant taxa.
• Phylogeny is the history of evolutionary descent of extant and extinct taxa from ancestral forms.

Phylogenetic Tree

• Branch point is a point where a split occurs and represents where a single lineage evolved into a distinct new one.
• Basal taxon is a lineage that evolved early from the root that remains unbranched.
• Sister taxa are two lineages stemming from the same branch point.
• Polytomy is a branch with more than two lineages.

Primitive vs Derived Characters

• Traits of the descendant species may involve a descent with modification of traits.
• Primitive character (plesiomorphy) is any character that shows little or no change from the ancestral trait.
• Derived character (apomorphy) is any character that undergoes modification in terms of appearance and/or function in relation to its ancestral trait.

Symplesiomorphy vs Synapomorphy

• Symplesiomorphy is a shared primitive character between two or more taxa.
• Synapomorphy is a shared derived character between two or more taxa.
• Symplesiomorphies cannot be used to separate members of different taxa since everyone has them.
• Synapomorphies can inform about relative recency of common descent.

Recency of Common Descent

• The more synapomorphies two taxa have in common, the more recently they evolved from a common ancestor.
• Recency of common descent is a key concept in building a cladogram.