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by **Sotos** » Mon Jun 04, 2012 12:06 pm

Do similar organs in different organisms exist because those organisms share a common ancestor who had that organ in some primitive form OR similar organs can evolve in unrelated organisms due to environment factors? Example: Many animals and insects have eyes. And most (all?) of those seem to have exactly 2 eyes. The eyes that different animals have are not exactly the same but they all seem to be organs that can detect light. Is this because we all share a common ancestor which had some primitive form of light detecting organs? So would a human and a fly have a common ancestor that had such organ? Or this is not necessary?

by **SKI-preo** » Mon Jun 04, 2012 1:39 pm

This is a year 7 science question.
convergent v divergent evolution:

Convergent
Convergent evolution wherein organisms evolve structures that have similar (analogous) structures or functions in spite of their evolutionary ancestors being very dissimilar or unrelated.

Supplement

Examples are:

the wings of bats, birds, and insects evolved independently from each other but all are used to perform the function of flying
the complex eyes of vertebrates, cephalopods (squid and octopus), cubozoan jellyfish, and arthropods (insects, spiders, crustaceans) evolved separately, but all perform the function of vision
the smelling organs of the terrestrial coconut crab are similar to those of insects
the very similar shells of brachiopods and bivalve molluscs
prickles, thorns and spines have evolved independently to prevent or limit herbivory
plant hormones such as gibberellin and abscisic acid of plants and fungi

Divergent Evolution
(1) The process by which an interbreeding population or species diverges into two or more descendant species, resulting in once similar or related species to become more and more dissimilar.

(2) The process of tracing back two or more species to their common ancestor that indicate how they have diversified and diverged.

Supplement

For example, a flock of bird in migration gets divided as half of them settled to a new island while the other half continued migrating to a farther land. Over time, the two groups become more adapted by developing new characteristics to enable them to survive to their new habitats. Thus, they become a different species to their ancestors and what was once one species has diverged into two.

by **Sotos** » Mon Jun 04, 2012 2:41 pm

Thanks.

Whether one considers the eye to have evolved once or multiple times depends somewhat on the definition of an eye. Much of the genetic machinery employed in eye development is common to all eyed organisms, which may suggest that their ancestor utilized some form of light-sensitive machinery – even if it lacked a dedicated optical organ. However, even photoreceptor cells may have evolved more than once from molecularly similar chemoreceptors, and photosensitive cells probably existed long before the Cambrian explosion.[10] Higher-level similarities – such as the use of the protein crystalin in the independently derived cephalopod and vertebrate lenses[11] – reflect the co-option of a protein from a more fundamental role to a new function within the eye.[12]

Shared traits common to all light-sensitive organs include the family of photo-receptive proteins called opsins. All seven sub-families of opsin were already present in the last common ancestor of animals. In addition, the genetic toolkit for positioning eyes is common to all animals: the PAX6 gene controls where the eye develops in organisms ranging from mice to humans to fruit flies.[13][14][15] These high-level genes are, by implication, much older than many of the structures that they are today seen to control; they must originally have served a different purpose, before being co-opted for a new role in eye development.