In the following video, professor George Wolfe explains us what homologous structures are. He starts by mentioning Vestigial organs (organs in our bodies that have no aparent funtcion), an after that, he begins with what we know as homologous structures. These are structures in animal bodies that may look different in several species(e.g. legs in cats and arms in humans), but in fact, they are composed by the same physical structure.

This example is clearly seen with the structure of a human arm and a dolphin flipper. They both have  one bone at the beginning of the arm (humerus), followed by two other bones (radius and ulna), a wisrt and the five fingers.

Homologous structures may have different function, but will definitively have the same structure. And as we know, homologous structures are determined by our genes.

After this, the man in the video explains analogous structures or convergent evolution. This means that different animals with an aparent same adaptation (e.g. birds with wings and moths with wings aswel) may have evolved from completely different ancestors, and in order to determine this, we need to look at the DNA of both examples, not just stay on the surface.

At the end of the video, the man shows us that when we are embryos, we have a lot more homology than when are adults. We can compair embryos  from different species (reptile, human, pig and bird) and we can perfectly see the homologous organs and structures they have in common (e.g. tail and gills), what leads us to think that we all come from a similar origin (divergent evolution).

Sources: (pictures: http://www.sciencewithmrmilstid.com/media/comparativestructure.jpg http://www.peabody.yale.edu/exhibits/treeoflife/images/convergence4.jpg http://www.carolguze.com/images/embryos/comp_embryosHaeckel.ipg.jpg) (links: http://en.wikipedia.org/wiki/Vestigiality , http://en.wikipedia.org/wiki/Convergent_evolution , http://en.wikipedia.org/wiki/Divergent_evolution)

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