Homology is a term used in biology to describe similarity due to shared ancestry between a pair of structures or genes in different taxa). Homologous structures are those that have a similar structure, but may have different functions, and are thought to have evolved from a common ancestor. For example, the forelimbs of vertebrates are homologous structures, despite their different functions in different species).

Homologies can be identified by comparing the anatomies of different living things, looking at cellular similarities, studying embryological development, and studying vestigial structures within individual organisms. Scientists test a claim of homology by making a prediction based on that claim, and then seeing whether the evidence supports that claim or refutes it. If repeated tests continue to confirm a claim of homology, scientists can be increasingly confident that they are right.

It is important to distinguish homology from analogy, which is a functional similarity of structure based not upon common evolutionary origins but upon mere similarity of use. For example, the wings of birds and insects are analogous structures, as they have different structures and evolved independently, but have a similar function.

In summary, homology is a concept in biology that describes similarity due to shared ancestry between structures or genes in different taxa. Homologous structures are those that have a similar structure, but may have different functions, and are thought to have evolved from a common ancestor. Homologies can be identified by comparing the anatomies of different living things, looking at cellular similarities, studying embryological development, and studying vestigial structures within individual organisms.