Appearance of similar but not identical adaptations that are a response to similar selective pressures.
Convergent evolution is a consequence of natural selection and gives rise to what are known as analogies. For example, from that entry, the common ancestor to birds and bats was a reptile that lacked wings. The wings of birds and bats therefore, as wings, are analogous structures rather than homologous structures.
Convergent evolution also can be viewed as a form of directional selection, i.e., which results in change. With convergent evolution, species thus, seemingly paradoxically, come to diverge from their ancestral species.
In other words – and read this slowly/carefully – convergent evolution gives rise (in species B) to a change from the ancestral state (species A). This is a change in particular that results in a descendant (species B) coming to resemble another species (species C), one to which it (species B) is less closely related.
Homologies, by contrast, are not products of convergent evolution. Homologies, instead, are maintained by stabilizing selection. Homologous structures represent a relative lack of evolutionary divergence from ancestral species. See, also by contrast, divergent evolution.
Convergent evolution can give rise to phylogenetic errors, that is, in classifying organisms, specifically resulting in polyphyletic taxa.
At the molecular level, convergent evolution can be even more misleading since the result is structures that literally are not distinguishable. This issue is further complicated by horizontal gene transfer, which can result in two organisms sharing molecular structures that are not a consequence of convergent evolution but which instead literally are homologies, though without a recent sharing of a common ancestor, at least in the sense of vertical inheritance (that is, vertical gene transfer).