A homoplasy is a character shared by a   set of species but not present in their common ancestor. A good example is   the evolution of the eye which has originated independently in many different   species. When this happens it is sometimes called a convergence. Homoplasies   can be compared with homologies, which are characters shared by a set of   species and are present in their common ancestor.

Note that the above definition of   homoplasy is its technical definition used by biologists. A homoplasy has an   older, pre-Darwinian meaning of similarity explained by a shared way of life.   For example, the wings of insects, birds and bats are all needed for flying:   they are homoplasious structures in the non-evolutionary, as well as the   evolutionary, sense.

异源同形（非同源相似）是一组物种共有的特征，但该特征不存在于它们的共同祖先中。一个很好的例子是眼睛的进化，它独立起源于许多不同的物种。这种情况发生时，有时也被称为趋同。异源同形可以与同源进行比较，同源是一组物种共享的特征，且该特征存在于它们的共同祖先中。

请注意，上述异源同形的定义是生物学家为便于操作给出的技术定义。它还有一个古老的，可追溯到达尔文之前的意义，即因为生活方式相似而产生的相似性。例如，昆虫、鸟类和蝙蝠的翅膀都是飞行所必需的：无论从进化的角度还是非进化的角度来看，它们都是同形结构。