January 28, 2010

Vestigial Organs

Vestigial organs are evolutionary souvenirs.  They are not necessarily "useless organs," which is how some people interpret the term.

A structure is vestigial if it doesn't have the same purpose as it had in the past.  Consider whale bones.  They have bones in their fins that are shaped much like our fingers (well, all mammal's fingers, not just ours).  They're not useless, they just have a different purpose now.  What's important is that it shows that whales descended from land mammals with fully developed fingers.

Discussions of vestigial organs usually only concern vertebrates.  All vertebrates have essentially the same body plan, so comparing and contrasting is very easy to do.  Comparing a whale and a cheetah is more interesting than comparing a rabbit and a clam.  There isn't much to say other than, "Yup, different. Anyone want to go to Chipotle?"

The general idea is that body plans don't shed things we don't need as soon as they become obsolete.  I could be perfectly fine without my appendix, pinky toes, and tonsils.  But even though I don't need them, I can't get rid of them (without surgery), and I can't help but have a child with an appendix, pinky toes, and tonsils (and that's only if all goes well).  The process of evolution doesn't care what you want.  It doesn't work on an individual level that way.  Traits are only lost if individuals that have them die before reproducing.  In other words, traits have to be a huge problem to the organism for them to be weeded out.  If they're just neutral--not super helpful, but not harmful either--there is little pressure to get rid of them.

Vestigial organs are great for science because, essentially, every organism is dragging its evolutionary past around with it all the time.  I tried doing that with travel souvenirs once, but I got some awkward stares for carrying around snow globes, spoons, and keychains all the time.  But at least everyone could see instantly where I'd been, right?  Right?