Similarities among two or more organisms that are a consequence of convergent evolution rather than representing homologies.
Analogies result when different lineages hit upon similar solutions to similar ecological challenges. That is, many organisms have wings or legs or eyes, but that does not mean that all of these structures are identical by descent.
One especially can look to particulars of underlying anatomy and/or embryonic development to discern structures and functions that exist because they were also an aspect of a common ancestor (homologies) or instead were absent from the most recent common ancestor and therefore evolved after the separation of lineages (and therefore, if similar, are products of convergent evolution, i.e., analogies).
For example, the common ancestor to birds and bats was a reptile that lacked wings. The wings of birds and bats therefore are analogous rather than representing homologous structures, though with the caveat that in this case both are built upon a homologous structure: The tetrapod forelimb. Similarly, the body shape of Ichthyosaurs, a kind of prehistoric swimming lizard, and that of modern dolphins is quite similar. The most recent common ancestor to both, however, was land-dwelling quadruped that better resembled a lizard than a whale.
Analogies are problematic in terms of organism classification for two reasons. First is that analogies can fool researchers into suspecting that organisms are more closely related than is actually the case, resulting in the formation of what are known as polyphyletic taxa. This issue has become less relevant as DNA sequencing has become increasingly employed as a means of phylogenetic analysis.
The second problem is seen particularly among microorganisms and is that seeming convergent evolution can instead be a consequence of horizontal gene transfer. That is, new DNA along with new traits in many cases ia readily obtained directly from other organisms, resulting in a convergence of phenotypes that actually represent homologies rather than analogies.
Analogies tell us not only that evolution can be quite versatile, but also that the number of effective solutions that may be arrived at via a combination of mutation and natural selection, i.e., adaptation, may not be infinite. Rather, similar adaptations can appear among diverse lineages and even reappear within specific lineages. What is important to keep in mind, however, is that what appears in the course of convergent evolution are adaptations that are similar, i.e., analogies, rather than effectively identical, i.e., which instead are homologies.
See also homoplasy.