Expert: John Shea - 2/13/2004

Question
Hi,

I'm assuming that if a genetic trait helps an organism have reproductive success it will be selected for.  But if a trait is neutral, will it necessarily disappear?  I've heard that modern humans are in the process of losing the little toe for example.  Surely there can't be a big metabolic cost for a little toe.  Why would it disappear?  Is it because people with genetic mutations that cause no little toe would have more reproductive success?  I understand this concept, but I don't understand why this would drive out the population of people who doe possess a little toe.  Must there then be some reproductive advantage to having no little toe?

Answer
If a genetically-programmed trait is neither advantageous nor maladaptive, i.e., if it is "neutral" it will not disappear.  A basic principle of evolution is that when you relax selection, you get increased variability.  So, for example, visual acuity.  Among our ancestors, the ability to see a lion hiding in tall grass had very definite fitness consequences.  Early humans with genetic programming for poor eyesight probaby ended up as lion food more often than ones with better eyesight.  Nowadays, in most parts of the world, there are no lions and there are technological fixes for poor eyesight (glasses, lasic surgury).  Consequently, there is a wide range of variability in visual acuity.  Similarly, shoes compensate for much of the variation in small toe morphology that we see among humans today.
The small toe is not disappearing.  It's been relatively small since Pliocene times (>1.7-5 Million years ago), when the foot was re-shaped under the influence of selective factors relating to bipedalism.  It is vestigal, though, it really doesn't play much of a role in locomotion.  (When I was young I knew a fellow who lost his small toe to a lawnmower, and he nevertheless went on to become a varsity sprinter and hurdle jumper.)
You are exactly correct, that for the little toe to disappear, there would have to be selection against it, -people lacking it would have to have a reproductive advantage over those who do.
I hope this helps.
Cheers
John Shea