English for

 
5.1 Reading Resource #2
:
Classification and Phylogeny
 
A phylogenetic tree is a tool that scientists use to depict the relationships and evolutionary 
pathways among organisms. A 
phylogenetic tree
is a diagram used to reflect evolutionary 
relationships among organisms or groups of organisms. Diagrams reflect the hierarchical 
classification of groups nestled within more inclusive groups. Because it is impossible to go back in 
time and verify the suggested relationships, scientists view phylogenetic trees as a hypothesis of the 
evolutionary past. 
A phylogenetic tree, in contrast to a taxonomic classification, can be interpreted as an 
evolutionary history map, as shown in Figure 71. Phylogenetic trees are built using shared 
characteristics. The point where a split occurs in a tree, called a 
branch point
, represents where a 
single lineage evolved into distinct new ones. A common ancestor of all the branches in many 
phylogenetic trees is represented by a single branch point at the base of the tree. Such trees are what 
biologists refer to as "rooted," meaning that they all descended from a single ancestral taxon at the 
root of the phylogenetic tree, from which all of the organisms depicted in the diagram. When two 
lineages stem from the same branch point, they are called 
sister taxa
, for example the two species of 
orangutans. A branch point with more than two groups illustrates a situation for which scientists have 
not definitively determined relationships. An example is illustrated by the three branches leading to 
the gorilla subspecies; their exact relationships are not yet understood. It is important to note that 
sister taxa share an ancestor, which does not mean that one taxon evolved from the other. The branch 
point, or split, represents a common ancestor that existed in the past, but that no longer exists. Humans 
did not evolve from chimpanzees (nor did chimpanzees evolve from humans) although they are our 
closest living relatives. Both humans and chimpanzees evolved from a common ancestor that lived, 
scientists believe, six million years ago and looked different from both modern chimpanzees and 
modern humans.