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The origins of life
Taxonomy or Systematics: The study of classification.
Why classify organisms?
The unit of taxonomy :The SPECIES.
It has biological significance:
The binomial system of classification
Species are grouped by levels of relationship called TAXA (sing. TAXON):
Kingdom; Phylum; Class; Order; Family; Genus; Species.
(Subdivisions of these may be necessary: e.g. sub phyla or superfamily)
Each species is given two names: The genus it belongs to and the name of the species (e.g. Homo sapiens). This is called the binomial system of nomenclature. Notice the species name does not carry a capital letter and it is printed in italics.
All taxa, other than species, are completely arbitrary in the way taxonomists decide which species fits into which group. As our knowledge improves a species may be moved from one taxon to another but it will always keep its species name.
How taxonomy works
The aim is to group organisms according to their evolutionary relationship (phylogeny). This is established by studying fossils or more recently from DNA sequencing. It uses characteristic features to group organisms together (e.g. all animals with feathers = Birds). It is the taxonomists who decide which are the most significant or "important" characteristic by the way it occurs in different groups of organisms.
Recently a method of systematics called cladistics has become an accepted way of classifying organisms. This method uses many different characteristics not just one to define a taxon. One character is not enough.
The characteristics are grouped in a hierarchy. So having four legs with five toes is common to all land vertebrates and their fish ancestor. This would be used to group the animals we call tetrapods. Having a nerve cord running down the back is a feature common to all the tetrapods but also all the rest of the vertebrates. So it can be used to group all the vertebrates but not the tetrapods alone.
This type of classification is expressed as a branching diagram called a cladogram. Each branch point or node indicates that above it all the organisms share a number of features.
Thus, birds are birds not just because they have feathers but because the have: hollow bones, flexible wrists, they are endothermic (warm-blooded), they have fused clavicles (the "wishbone"), a characteristic egg shell, three toes pointing forwards and one toe pointing back.
This method has lead to some surprising discoveries. All of the characteristics of birds listed above have been found in fossils of a group of dinosaurs called the theropods (which includes Tyrannosaurus rex). This led the taxonomists to the conclusion that birds are really dinosaurs.
In fact birds seem to possess only a few characteristics which are really their own:
In cladistics the absence of a characteristic is not considered relevant. It is often said that a characteristic of birds is that they lack teeth. Anteaters and tulips do not have teeth either and you would not call them birds.
Reference: Paladian & ChiappeThe Origin of Birds and Their Flight Sci Amer (1998) Feb p 28
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