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 1. Cladistics is classification based on common ancestry

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Join date : 2008-09-24

PostSubject: 1. Cladistics is classification based on common ancestry   04/02/09, 10:43 pm

• Shared traits can be from (a)sharing a common ancestor, or (b)evolving in the same environment
• To classify organisms, scientists must look at more than physical traits
• Phylogeny- the evolutionary history for a group of species
• Phylogenies can be shown as branching tree diagrams

Cladistics- classification based on common ancestry. They place species in order of when they descended from a common ancestor
• Cladogram- an evolutionary tree that proposes how species may be related to each other through common ancestors
• A clade is a group of species that share a common ancestor, shows evolutionary relationships
• Derived characters- trait that differs in structure or function from that found in the ancestral line for a group of species; used in constructing cladograms
• The more closely related species are, the more derived characteristics they have
• A group of species that shares no derived characteristics with the other groups being studied is called an outgroup


Derived Characters
• Species placed in order by the derived characters that have been added over time
• Hypothesized to be the order of decent
• Derived characteristics are the hash marks on the cladograms
• All specie above a hash mark share the derived character it represents

• Each place where the branch splits is a node
• The first node of the cladograms represents a common ancestor for the whole clade

Identifying Clades
• Whenever you take add a characteristic, some parts get cut off as you filter down
• To identify a clade, look at the level and everything above it. The smallest level will be in all clades

Tetrapoda clade- ( Diagram in pg. 527) (I will scan it in as soon as my scanner is working again…)

1. All of the organisms in this cladograms belong to the tetrapoda clade (brown). They all share the derived characteristic of four limbs.
2. An embryo protected by a fluid-filled sac is a derived character for all organisms in the amniotic clade (blue). Because amphibians do not produce an amniotic sac, the amphibian branch splits off from rest of the branches before the mark that represents this trait
3. Organisms in the retilia clade (yellow) have a common ancestor that had four legs, produced protected eggs, and had a skull with openings behind the eye. The third node in the cladograms represents this common ancestor. Because mammal skulls do not have these openings, they are not part of the reptillia clade.
4. Organisms in the diapsida clade (green) have openings in the side of the skull. The skulls of turtles and tortoises do not have these openings, so they are not part of the diapsida clade
5. Lizards and snakes branch off of the cladograms next. Their skulls do not have certain openings in the jaw that are found in crocodiles, alligators, and bids. This is the derived character shared by all organisms in the archosauria clade (pink). Feathers and toothless beaks separate crocodiles and alligators from birds within the archosauria clade.
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