7.9 - Phylogeny

Phylogenetic trees and cladograms are tools used to show the evolutionary relationships among species or lineages. While both provide insights into evolutionary pathways, they differ in how they represent time and change.

Key Concepts

1. Phylogenetic Trees:

   • Definition: Diagrams that represent evolutionary relationships among species, calibrated using fossil records and molecular clocks to indicate the amount of change over time.

   • Time Representation: Include a temporal aspect, showing the approximate timing of divergence events.

2. Cladograms:

   • Definition: Diagrams that show the relationships among species based on shared derived traits, without indicating the amount of time that has passed. Cladograms just show related characteristics but are not as supported as phylogenetic trees. 

   • Time Representation: Do not provide information about when divergence events occurred.

Structure of Phylogenetic Trees and Cladograms

• Base of the Tree: Represents the past, the common ancestor of all the species depicted.

• Branches: Represent the lineages leading to different species.

• Tips (End of Branches): Indicate the species being studied.

• Branch Points (Nodes): Represent speciation events, where a common ancestor diverged into two or more species.

• Lowest Point of Divergence: Indicates the most recent common ancestor of all species on the tree.

Interpretation

• Common Ancestry: Species that share a branch point closer to the base of the tree have a more recent common ancestor.

• Shared Evolutionary History: Species above a particular node share a common evolutionary history from that node onward.

• Rotating Species: Rotating the branches at any node does not change the evolutionary relationships. The distance and relatedness between species remain the same.

Constructing Phylogenetic Trees and Cladograms

• Data Sources:

  • Morphological Data: Physical traits and structures of organisms.

  • Biochemical Data: Chemical processes and substances within organisms.

  • Molecular Data: DNA or amino acid sequences, considered the most accurate and reliable.

  • Behavioral Data: Observable behaviors and patterns of species.

• Analyzing Data: By comparing these types of data, scientists can group organisms based on shared derived traits, indicating common ancestry.

Key Terms

• Shared Derived Traits: Characteristics that indicate common ancestry and are used to group organisms in phylogenetic trees and cladograms.

• Outgroup: The lineage least closely related to the remainder of the organisms in the tree or cladogram. It helps to root the tree and provides a point of comparison.