Homology (biology)

In evolutionary biology, homology refers to any similarity between characteristics that is due to their shared ancestry. The word homologous derives from the ancient Greek ομολογειν, 'to agree'. There are examples in different branches of biology. Anatomical structures that perform the same function in different biological species and evolved from the same structure in some ancestor species are homologous. In genetics, homology can be observed in DNA sequences that code for proteins (genes) and in noncoding DNA. For protein coding genes, one can compare translated amino-acid sequences of different genes. Sequence homology may also indicate common function. Homologous chromosomes are chromosomes with the same genes, but non-identical nucleotide sequences that can pair (synapse) during meiosis, and are believed to share common ancestry.

Homology of structures in evolution
Shared ancestry can be evolutionary or developmental. Evolutionary ancestry means that structures evolved from some structure in a common ancestor; for example, the wings of bats and the arms of primates are homologous in this sense. Developmental ancestry means that structures arose from the same tissue in embryonal development; the ovaries of female humans and the testicles of male humans are homologous in this sense.

Homology is different from analogy. The wings of a maple seed and the wings of an albatross are analogous but not homologous (they both allow the organism to travel on the wind, but they didn't both develop from the same structure). This is called homoplasy. But structures can be homologous and analogous. The wings of a bat and a bird are homologous, in that they both developed from the pectoral fins of fish. They are also analogous, in that the forelimbs of the ancestors of birds and of bats developed into organs of a similar new function independently. Thus evolution can be initially divergent, giving rise to homologous structures, and subsequently convergent, causing the structures to become analogous again.