Irreversible Evolution

Here's my candidate for an irreversible evolutionary step. In Adam's Curse, Bryan Sykes talks about the whiptail lizard, one species of which reproduces asexually - there are no male lizards of this type, only egg-laying females that produce genetic replicas of themselves.

This is an example of sexual reproduction disappearing as a species adapts. Since the ancestors of these lizards reproduced sexually, you would have to have a male one to step backward, as Dawkins wants to do. But there aren't any males left. To start putting those notches back on the bedpost, the whiptail needs to make two simultaneous backward steps - one being the production of a male hatchling, and the other the production of a female hatchling that can reproduce with it. But that's not the single step forward that was taken in the first place.

This doesn't mean that you can't evolve any species into any other - it simply means that argument from walking back up the evolutionary tree to the common ancestor doesn't quite cut it. Something more sophisticated is required (what, I don't know yet.)
- posted by Scott @ 5:20 PM