amphibians so we know that wings is not an ancestral amniote character. The question then is: does birds + bats constitute a monophyletic group defined by the derived character "wings" or were wings independently (convergently) derived in these two lineages? We find, upon further morphological investigation, that birds and bats do indeed share wings, but that bats and all other mammals share hair, live birth, mammary glands, and a great many other derived characters. The parsimony principle says that we should accept the hypothesis that bats + other mammals is a monophyletic group, and reject the hypothesis that bats + birds is a monophyletic group, because it is simpler to assume that the single character "wings" evolved independently in birds and bats than it is to assume that hair, and live birth, and mammary glands, and all the other derived characters which bats and other mammals share all arose independently. The parsimony principle does not imply that evolution necessarily operates parsimoniously or "optimally" or "efficiently." It is just a logical principal for choosing among different hypotheses which attempt to explain the same data. In a large study with many taxa and characters it is often very difficult to find the most parsimonious tree (the tree with the least number of character state changes going from the ancestral root to the tips). In such cases, computerized algorithms are often used to search among the possible trees to find the shortest. The development of such methods is a very active area of systematic research.
A few questions to apply these concepts
Most mammals give birth to live young, but a small number of mammals (the Prototheria) lay eggs. Answer the following set of questions, referring to Figure 1 if you must, to make sure you are comfortable applying these concepts.
Is egg-laying a derived or primitive feature among mammals? How do you know?
Is the shared trait of egg-laying, by itself, an indication that egg-laying mammals are
more closely related to each other than either is to other mammals?
For what group of amniotes is live birth a shared derived characteristic?
For what group of amniotes are shelled eggs a shared derived characteristic?
RECONSTRUCTING PHYLOGENY (in three steps)
1) Specifying the outgroup
As mentioned above, an outgroup allows you to determine which character states are primitive for your group of organisms, and which are derived (and therefore useful indicators of relationship). Suppose you are trying to reconstruct the phylogeny of a group of organisms, some of which have hair and some of which don't. If your outgroup has