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Not all similarity is homology

In the game you played at the beginning of this module, you decided that some legs were "not like the others." The matching limbs in the game have bones, and you eliminated the limbs that don't have bones. Since the octopus, sea star and grasshopper limbs don't have bones, you concluded that they are probably not homologous to tetrapod limbs.

The lizard, bird, human and whale limbs are homologous.  The octopus, sea star and grasshopper limbs are not homologous with the others.

Homologies are inherited from common ancestors. The octopus limb could only be homologous to the lizard limb if they both inherited the limb from a common ancestor. This tree shows how the octopus is related to tetrapods, and the points in their evolutionary histories when their limbs evolved.

Independent evolution of tetrapod and octopus limbs means that they are not homologous

Download these (five and six of a series of six) graphics from the Image library.

Tetrapod and octopus limbs evolved independently after their point of common ancestry, so they were not inherited from a common ancestor. Therefore, they are not homologous. The same is true of the grasshopper leg and the sea star arm. Simple observation tells us that these limbs are probably not homologous to the tetrapod limb, because they have such different structure.

Similar structures that evolved independently are called analogous structures, or analogies.

Homologies are everywhere


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