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Homologies are not just anatomy

We tend to think of homologies in terms of anatomy: the tetrapod limb, insect wings, vertebrate eyes. However, anatomical homologies are just one of many sorts of homologies. Any heritable trait — anything that can be directly or indirectly encoded in DNA — can be a homology:
  • A simple trait
    Among milkweed species, having milky sap is a homology.

  • A complex structure
    The antennae of beetles and moths are homologous, for example.

    beetle antennae moth antennae
  • A gene sequence
    The gene that codes for the protein insulin is homologous in humans and cows; both sequences descended from the same ancestral gene sequence.

  • A behavior
    Birds and crocodiles both provide parental care to their offspring; this behavior was inherited from a common ancestor and is homologous.

    robin with young crocodile with young

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Beetle photo © 2003 Frank T. Hovore; Moth photo © 2005 Dr. Amadej Trnkoczy; Robin photo provided by James C. Leupold and the U.S. Fish and Wildlife Service; Crocodile photo provided by Dr. Grahame Webb at Wildlife Management International