Understanding Evolution

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Understanding Evolution

Homology & Convergent Evolution (HS)

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Homologies are everywhere

Once you know what a homology is, you can find them anywhere.

The leaves of an oak (left) and the leaves of a gingko (right)
The leaves of an oak (left) and the leaves of a gingko (right) are homologous — they were both inherited from a common ancestor with leaves. Oak photo © Br. Alfred Brousseau, Saint Mary’s College; Gingko photo provided by Mike Clayton and the University of Wisconsin, Madison Virtual Foliage page.
wing of a dragonfly (left) and the wing of a butterfly (right)
The wing of a dragonfly (left) and the wing of a butterfly (right) are homologous — they were both inherited from an ancient flying insect. Dragonfly photo provided by Gerald and Buff Corsi © California Academy of Sciences; Butterfly photo provided by T. W. Davies © California Academy of Sciences.

Since all life shares a common ancestor, we find homologies all over the tree of life.

Beyond the obvious

Not all homologies are obvious. If two homologous structures have been adapted for different roles, they may not look very much alike. For example, at first glance, the bright red “petals” of a poinsettia plant look quite different from the leaves of a beech tree.  Yet they are homologous structures descended from an ancestral leaf.

left, poinsettia; right, american beach tree.
Poinsettia “petals” are homologous to leaves. American Beech tree photo from Brock under CC-BY-NC.

If you examine these structures closely, you will see that each is a modification of the basic leaf structure. Over time, evolution adapted poinsettia so that some of its leaves could help attract pollinators, while oaks use their green leaves for photosynthesis. Careful study reveals homologies where they might not be obvious.

 

Homologies can be identified in many places; it's not just anatomy.

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Inheriting homologies

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Not all similarity is homology