This early example of
using the tree metaphor
implies that branches
that are lower on the
tree of life represent
This is incorrect.
Basic tree notation.
• What are phylogenetic trees?
Nested hierarchies of sister taxon
node: an ancestral species,
common to two sister taxa
polytomy: could be either an unresolved node, or that
three species were derived from a single ancestor
What are phylogenetic trees?
Although phylogenetic trees are
usually read left to right, each
node can be rotated
Therefore, there is no
evolutionary sequence to be
read from the tree.
Because trees are made from
characteristics of organisms, and because
almost all taxa on trees are extant, basal
branches does not equal basal taxa, or
The most common error in tree
reading is to assume that the early
diverging taxa (i.e. at the base of the
tree are more “primitive” than those
at the tips).
Taxonomic sampling bias creates
the illusion of ancestral vs. derived
Krell and Cranston, 2004
Tree shape depends on what
geological period sampling is done
and whether extinct taxa are
included or not.
Basal branching does not mean primitive
we would not call humans primitive: we
would refer to humans in this case as
the sister taxon of the proteobacteria
Another example of how
taxonomic sampling and tree
presentation can be misleading
which taxon is basal here?
Natural Selection: easy to
understand, difficult to
• Selection is ademonstrate. to
denote the way the abiotic and biotic
environment creates differential
• reproductive success.
differential reproductive success is the
definition of fitness. Individuals with greater
fitness pass along those features to
• If those features become fixed in the
population, evolution has occurred.
• How can we demonstrate which features of
organisms have responded to natural