The Tree of Life: A History of Descent with Modification With information about morphological, ontogenetic and molecular homologies, scientists can reconstruct relationships.

  • The evolutionary history of a species is its phylogeny.
  • A phylogeny can be represented with a diagram called a phylogenetic tree

    The metaphor of a branching tree is often used to represent origin and diversification from a common source. For example, one could draw such a tree to represent the origin and diversification of different branches of Christianity (redrawn from Kirk and Falk, 2010).

    So, too, can the relationships of living things be represented as a tree. A taxonomic group's evolutionary history is known as its phylogeny. A branching diagram like the above drawn to represent that phyogeny is known as a phylogenetic tree.

    Each branch point (node) represents the common ancestor of all taxa above that node on the tree. The endpoints of the tree's branches represent the taxonomic units included in that particular phylogeny.

  • each endpoint represents a taxon evolved from ancestral organisms
  • each branchpoint (= node) represents the common ancestor of the endpoints above it on the tree

  • A taxon is a group of organisms related by evolutionary descent from a common ancestor.
  • Examples: Domain, Kingdom, Phylum, Class, Order, Family, Genus, species

    A phylogenetic trees may be graphically presented any number of ways, including upright...


    As an emerging spiral:

    ....and several other ways. But they all say essentially the same thing:

    "This is how the groups on the endpoints of the tree are related by descent from common ancestors, going back through time."

    EDIT FROM THIS POINT: Systematics: Reconstructing Evolutionary Relationships A taxon has dimensions in

    The systematist is interested in both these aspects in reconstructing how different extant taxa are related to one another by common ancestry. Consider the apes (Family Pongidae):

    If the phylogenetic tree of the Apes above is correct, you can predict that humans and chimpanzees should share more derived homologies with each other than they do with other apes less closely related. Constructing Phylogenies That Reflect Common Ancestry In modern systematics, the goal is to reconstruct accurate phylogenies that indicate recency of descent from a common ancestor. This means that all members of any valid taxonomic group should share a common ancestor.

    The systematist does NOT want to accidentally group taxa that do not share a most recent common ancestor, such as this polyphyletic group (an artificial grouping) that is derived from more than one common ancestor.

    Phylogenetic trees constructed by comparing shared, derived characters (a technique in systematics known as cladistics) are called cladograms (from the Greek clad meaning "branch").

    A up-to-date cladogram of terrestrial vertebrate relationships can be seen here, at the Tree of Life. But these things can change every five minutes. So don't get too attached.