What is a Cladogram?
Taxon A
Taxon B
Taxon C
Cladograms: a family tree for TAXA
based on Shared Derived Characters
A Character: any characteristic that scientists can
observe or measure.
EXAMPLES:
Milk production
the use of a specific enzyme to
digest food
the number of petals on a
flower
These characters are useful to show
which taxa are closely related.
Shared Characters
vs. Similarity
Similarities exist between lots of living
things—how can we determine which exist
because of a SHARED history?
All mammals share certain characters – for example milk
production.
−Bats are mammals – they can fly and they produce milk
for their young.
−Pelicans are birds – they can fly, but cannot produce
milk. Their young develop in shelled eggs.
So what does it mean
to be DERIVED?
•Derived characteristics are characters that are NEW.
•How do scientists determine derived characteristics?
•Look at the Most Recent Common Ancestor!
If a character is shared between two taxa, and the
character is not shared by ANY OTHER ancestor
PRIOR TO the MRCA, it is derived.
A clade is a group of taxa that are all descendants of a
common ancestor.
It is considered a natural group because it shows the actual
evolutionary relationships between taxa as a result of their
common history.
Where are the clades
in the diagram?
Every 3-taxon statement shows
two clades: one that includes the
two most closely-related taxa and
one that includes all of the taxa.
Keeping it Simple…
Trout, zebra, and lizards all have a backbone. Zebra
and lizards both have four legs. Which cladogram
best represents the relationship between trout,
zebra, and lizards? Why?
Q: Dolphins and
sharks are grey,
have a large
dorsal fin, and live
in the ocean. Are
these shared
derived
characters?
A: No, because these characters are not due to
a MRCA. Dolphins and sharks developed these
characteristics independently.
Q: Mammals have four legs and a backbone. Are four
legs and a backbone shared derived characters of
mammals?
A: No, because these characters are not newly
derived; since all mammals share these with
reptiles and amphibians, they don’t tell us
anything about relationships among mammals.