1. Read/explore the information about "The Family Tree" provided here
(Read up to the "Quick Quiz") - this will help you get a good start on the worksheets we passed out in class today.
2. Watch video Discovering the Great Tree of Life - this is a great overview of what we are currently discussing and how it ties in with genetics. It's about 10 minutes long and well worth the time!!
If you need additional information to help you do the worksheet, you can also use your text - pp 340-348.
Some terms that you might find useful:
A clade consists of all of the groups sharing a common ancestor, plus that ancestor.
A clade is also called a monophyletic (see below) group.
A synapomorphy is a shared derived character. It is a specific feature shared by all of the descendants of a particular common ancestor. A clade is defined by one or more shared derived characters that are unique to that group. For example, the presence of mammary glands and hair/fur are the shared derived characters, or synapomorphies, that define the mammals, which all belong to the same clade. The mammal clade excludes any group that does not have those two traits, and are presumed to all be descendants of some common ancestor who did have hair and mammaries. Thus the mammal clade is distinct from the reptilian clade (which you now know includes birds), the amphibian clade, and the fish clade. All four of these groups together, however, can be combined to form a bigger clade - the vertebrates.
A plesiomorphic trait is an ancient trait that all members of a specific group share, so it's not helpful in distinguishing among them. For example, the presence of a backbone is plesiomorphic for all of the vertebrate clades.
* A monophyletic grouping is one in which all species share a common ancestor and all species derived from that common ancestor are included. This is the only form of grouping accepted as valid by cladists. (For example, turtles, lizards, crocodilians and birds are all derived from a shared common ancestor. Thus a monophyletic grouping would place all of these together, rather than placing birds into a separate group.)
* A paraphyletic grouping is one in which all species share a common ancestor, but not all species derived from that common ancestor are included (for example, grouping turtles, lizards and crocodiles as "reptiles" and separating that grouping from the birds).
* A polyphyletic grouping is one in which species that do not share an immediate common ancestor are lumped together, while excluding other members that would link them (for example, a hypothetical group the "lizmams" made by grouping together the lizards and the mammals).