| A | B |
|---|
| So how many species are there? | 1. no one knows for sure, best guess is about 10 million, but only about 1.8 million have been described by humans |
| most species | are tropical |
| human activities (particularly in the tropics) | certainly destroying many species before they can even be described; we are undergoing the sixth mass extinction event in the history of life on earth (and the first one driven by the activities of man) |
| classification is largely based | on inferred evolutionary relationships between organisms; the two major approaches to this are cladistics and traditional taxonomy |
| phylogeny | evolutionary tree; explanation of evolutionary relationships among groups (what evolved from what, in what order, and when) |
| systematics | study and reconstruction of phylogenies |
| groups of organisms may be | monophyletic, paraphyletic, polyphyletic |
| monophyletic | includes most recent common ancestor and all descendants |
| paraphyletic | includes most recent common ancestor BUT not all descendants |
| polyphyletic | does not include most recent common ancestor |
| both cladistics and traditional taxonomy avoid | polyphyletic groups |
| cladistics also avoids | paraphyletic groups |
| cladistics groups organisms on the basis of | unique shared characters inherited from common ancestor, or derived character |
| clade | group of organisms related by descent |
| synapomorphy | a derived character that is unique to and thus defines a particular clade |
| cladogram | branching diagram based on cladistic analysis that represents a phylogeny |
| cladograms are based on comparative analysis, so each cladogram | must have an outgroup and ingroup |
| outgroup | organism that is different from all others in the cladogram (but not too different); it is expected to have split with the others from a common ancestor before any of the rest (the ingroup) split from each other |
| often different cladograms can be produced for | given set of organisms depending on how the analysis is done |
| usually a choice has to be made for which cladogram is the most likely | reflection of evolutionary history (usually the most parsimonious one, the one that requires the simplest explanation) |
| parsimonious one | , the one that requires the simplest explanation |
| cladograms are always open to | refinement as more date become available |
| naming based on cladograms only allows for | monophyletic groups |
