Life is organized as a pattern of a tree, with lineages diverging from common ancestors, and many branches ending (becoming extinct).

Phylogeny: a "family tree" of taxa.

Traditional taxa name different parts of the tree of life. Taxa represent different kinds of groupings:

• Monophyletic ("single branch"): all descendants of a common ancestor
  • Most recent common ancestor (concestor) of the members of a monophyletic group is also a member of that group
  • Represents a complete branch of the tree of life
• Paraphyletic ("nearly a branch"): some, but not all, descendants of a common ancestor
  • Concestor of the members of a paraphyletic group is also a member of that group
  • Represents a branch of the tree of life with one or more buds or stems clipped off
• Polyphyletic ("many branch"): two groups that do not share a direct common ancestor which is also part of the group
  • Concestor of the members of a polyphyletic group is not also a member of that group
  • Represents two or more separate branches of the tree of life

Monophyletic taxa prefered by most biologists today, because they represent whole historical parts of the tree.

Problem: the true shape of the Tree of Life is not known, because of missing data.

How to reconstruct the Tree?

Over time, the lineages acquire new adaptations that are passed on (sometimes with modification) to their descendants.

Example:

• Lions, tigers, bears, cows, platypi, and lizards all had a common ancestor with a backbone, limbs with digits, etc.: these adaptations (specializations) were passed down to all these descendant groups.
• Lions, tigers, bears, cows, and platypi (but not lizards) all had a common ancestor that had fur and gave milk.
• Lions, tigers, bears, and cows (but not platypi and lizards) all had a common ancestor that had a placenta.
• Lions, tigers, and bears (but not cows, platypi, and lizards) all had a common ancestor that had molar teeth modified for shearing.
• Lions and tigers (but not bears, cows, platypi, and lizards) both had a common ancestor that had retractable claws.

Or by a braching diagram:

In 1950s, German entomologist Willi Hennig realized that one could use this method to organize taxonomy (systematics) by reconstructing the phylogeny of life.

The branching diagram above is called a cladogram.

• A sort of map showing the simplest (most parsimonious) arrangement of organisms
• Inferred to best represent the evolutionary branching arrangements

Here is another, more comprehensive, cladogram of living terrestrial vertebrates:

The above arrangement can also be written in outline form:
Tetrapoda

I. Amphibia
II. Amniota
A. Mammalia
B. Reptilia
1. Testudines
2. Diapsida
a. Lepidosauria
b. Archosauria
i. Crocodylia
ii. Aves

So:

• Crocodiles are archosaurs, and diapsids, and reptiles, and amniotes, and tetrapods.
• Lepidosaurs are diapsids, and reptiles, and amniotes, and tetrapods.
• Etc.

For example, some evidence suggests that, unlike the cladogram above, turtles might be closer to lepidosaurs than lepidosaurs are to archosaurs, or that turtles might be closer to archosaurs than archosaurs are to lepidosaurs. This uncertainty can be shown by the following polytomy:

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Last modified: 14 July 2006