Homologies ­ whether of bones or of genes ­ are characteristics that organisms have in common because they inherited them from their common ancestor. For example, the wing of the bat, the leg of the horse, and the arm of the mole are all homologous: the same bones appear in the same relative positions, but with modifications for flying, running, and digging. Homologies are a beautiful proof of evolution: over time, lineages diverge and evolve, yet the hallmark of their common history remains clear. The more time passes, the fewer the homologies that organisms will have. In this way, homologies can be used to trace the evolutionary relationships between organisms. Nowadays, with the advent of molecular genetics, much of this work can be done at the level of DNA. For evolutionary geneticists are the modern-day taxonomists. More closely related organisms have more closely related genes and more closely related proteins than their more distant cousins.
