Synapomorphy - Comparisons With Other Shared Traits

Comparisons With Other Shared Traits

A synapomorphy should not be confused with other types of shared traits:

• A synapomorphy is a shared trait found among two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. An example is the dipteran halteres, the uniquely modified hind wings found in all families of winged flies. No other group of insects possesses similar structures (in place of hind wings—insects in the order Strepsiptera have convergently-evolved halteres in place of fore wings). However, the fact that the trait is found exclusively in Diptera, to the exclusion of all other groups, is not essential in identifying the trait as a synapomorphy; rather, this fact makes its determination easier.

• A symplesiomorphy is a shared trait found among two or more taxa, but which is also found in taxa with an earlier common ancestor. An example of this is the five toes seen on the hind legs of rats and apes. This character-state originated very early in Tetrapoda and occurs in other tetrapod groups, e.g. in lizards. There is thus no indication that the group formed of rats and apes is a clade to the exclusion of these other groups.

• A homoplasy is a shared trait found among different taxa but not in their common ancestor (i.e., the same trait emerged in different taxa independently of each other). An example of this is homeothermy in birds and mammals. This trait is a derived character-state (in relation to poikilothermy, the character-state of the last common ancestor of both groups), which evolved independently in these two groups (or at least in the larger clades to which these groups belong).