How are these eyes related?Though the eyes we've seen so far differ in many ways, they all share the ability to sense light. They all have light-sensing cells (called photoreceptor cells) that relay information to the brain or nerve mass and some of the eyes are laid out in similar ways. But why are they similar? In terms of evolution, there are two basic explanations for similar structures:
Homologous structures reflect the common ancestry of living things. Because doves and ostriches share a common ancestor, their wings are deeply similar on a structural level. At the same time, the differences between dove wings and ostrich wings (in color, size, shape, etc.) reflect the diversity of life and the unique evolutionary path that each of these two lineages has taken since the time of that common ancestor.
To figure out if two anatomical structures are homologous, scientists evaluate many different lines of evidence. Here are some key criteria used to judge whether a trait is homologous:
Dove and ostrich wings are homologous and match on all three criteria. Butterfly and bird wings, on the other hand, are analogous and do not meet any of the criteria. Analogous structures may look similar on the surface, but since their similarity is due to convergence rather than common ancestry, analogous structures generally don't meet all of the criteria above.
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Ostrich photo provided by H. Vannoy Davis © California Academy of Sciences; dove photo provided by Joyce Gross; butterfly photo provided by T. W. Davies © California Academy of Sciences; feather photo provided by the Bozeman Kiwanis; butterfly wing scales photo provided by Stennett S. Heaton © California Academy of Sciences
Understanding Evolution © 2016 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California