Phylogeny (College Board AP® Biology): Study Guide

Phylogenetic trees and cladograms

• Phylogenetic trees and cladograms are types of diagrams that show evolutionary relationships among groups of organisms or lineages

• Both diagrams represent hypotheses about evolutionary relationships based on shared characteristics

• These relationships are not finalised but are continuously revised as more evidence arises over time

Constructing phylogenetic trees and cladograms

• Information that is considered during the construction of a cladogram or phylogenetic tree includes:

  • shared characteristics

    • Traits present in more than one lineage indicate a shared evolutionary history

  • shared derived characteristics

    • Homologous traits within a specific lineage indicate common ancestry

    • These are shown in a group called a clade which is set apart from other organisms in the cladogram

  • common ancestry

    • Nodes on the cladogram represent common ancestors

      • A more recent common ancestor indicates that two species are more closely related

    • The root of the cladogram represents the common ancestor of all members of the tree

    • The out-group represents the lineage that is least closely related to the other organisms in the diagram

Cladograms

• Cladograms show the relationships between organisms based on shared derived characteristics (synapomorphies)

• They are primarily concerned with branching patterns, not time or genetic distance

Interpreting cladograms

• The cladogram above can be interpreted as follows:

  1. the node representing the common ancestor of all vertebrates

  2. five nodes need to be traced back to here to link placental mammals with coelacanths, indicating a more distant relationship

  3. three clades are directly linked to one common ancestor, indicating that this cladogram was built without enough data to determine which of these three clades are more closely related

  4. only one node needs to be traced back to here link crocodilians and birds, so they are more closely related to each other than to any other clade

  5. the node representing the common ancestor of all mammals

  6. note that the reptiles are not a true clade, as they do not include the birds, with which they share a common ancestor

Phylogenetic trees

• Phylogenetic trees show evolutionary relationships along with the evolutionary time or genetic divergence between organisms

• These timescales are calibrated using the fossil record or molecular clocks

The molecular clock

• Molecular data about DNA and protein sequences can be used to determine an evolutionary timescale

  • Differences in sequence data arise from DNA mutations, which occur at a fairly constant rate

  • The number of differences between sets of sequence data provides information on how closely related two species are

    • The more differences there are between the sequences, the longer ago the species diverged

  • Molecular comparisons, e.g. after DNA sequencing, are typically more accurate and reliable than morphological traits for constructing phylogenetic trees and cladograms

    • Morphological traits may also occur as a result of convergent evolution rather than as a result of any evolutionary link, e.g. the independent evolution of wings in unrelated animals such as bats, birds and insects