The Theme of Unity and Diversity in Genetics
A large part of what makes species look and act differently is coded in their genetic material. What genes make species look different? A possibility is to have unique genes that make a fly, a mouse or a human look like a fly, a mouse, or a human, respectively. As it turns out, you have a lot more in common with a fly and a mouse than you think…
Research has shown that genes determining how major body parts develop are almost the same across very different species. A really cool example is a gene involved in eye development known across species as Pax-6. In humans, this gene is called aniridia (an for short), in mice its Small Eye (or Sey) and eyeless (Ey) in the fruit fly Drosophila. Despite the difference in its name, it is the same "master control" gene for eye development across these widely disparate organisms. In humans, defects in this gene cause a disorder called aniridia (absence of the iris and other eye problems), and in flies it can cause a range of eye mutations including lack of eyes. The fly, mouse, and other species' versions of this gene are so similar that they are virtually interchangeable: in some cases over 90% of its sequence is identical! Expression of either the fly or the mouse Pax-6 can make eyes anywhere in the fly: in its legs, for example (Halder, Callaerts, & Gehring, 1995).
So you have at least one gene that is the almost the same as in a mouse and in a fly, yet your eyes do not look like mouse or fly eyes. That is because Pax-6 is one of many genes in a complex pathway that leads to the development of eyes. How is it possible that such disparate species can be so similar at the genetic level? Millions of years ago, Pax-6 evolved in an ancestor shared by flies, mice and humans. Research on Pax-6 is part of a body of work suggesting that eyes, despite their enormous diversity, evolved only once in animals (Gehring, 2005; Gehring & Ikeo, 1999).
And the Pax genes aren't the only ones you share with other creatures: did you know you also have one named after Sonic the Hedgehog (Shh), that's involved in regulating body development in embryos? And why Sonic? Because a mutation in the Drosophila version of the gene makes the fruit flies look all prickly, just like little hedgehogs (Arthur, 2011).