Phylogeny (College Board AP® Biology): Study Guide
Phylogenetic trees and cladograms
Phylogenetic trees and cladograms are diagrams that show hypothetical evolutionary relationships among lineages
They represent testable hypotheses that are continually revised as new evidence emerges
Cladograms show branching patterns only (no timescale).
Phylogenetic trees also show the amount of change over time, calibrated using fossils or a molecular clock.
Constructing phylogenetic trees and cladograms
Traits that are either gained or lost during evolution can be used to construct phylogenetic trees and cladograms
Construction can be based on:
Morphological similarities in living species
Fossil evidence
Molecular data (DNA and protein sequence similarities)
Molecular data are typically more accurate and reliable than morphological traits, which can be misleading due to convergent evolution.
The following information should be considered:
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
Splitting at a node represents a speciation event.
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:
the node representing the common ancestor of all vertebrates
five nodes need to be traced back to here to link placental mammals with coelacanths, indicating a more distant relationship
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
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
the node representing the common ancestor of all mammals
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

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