Taxon A Taxon BTaxon C What is a Cladogram?

A Character: any characteristic that scientists can observe or measure. These characters are useful to show which taxa are closely related. Cladograms: a family tree for TAXA based on Shared Derived Characters EXAMPLES:  Milk production  the use of a specific enzyme to digest food  the number of petals on a flower

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.

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. 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!

A clade is a group of taxa that are all descendants of a common ancestor. 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. It is considered a natural group because it shows the actual evolutionary relationships between taxa as a result of their common history.

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.