Flatfish: solving Darwin's puzzle

How did a flatfish's eyes end up on one side of its head?

It's a question that puzzled Darwin when developing his theory of evolution but in a talk this week to the Society of Vertebrate Palaeontologists Matt Friedman, of Oxford's Department of Earth Sciences, presented fossil evidence showing how such a strange bodyplan gradually evolved.

Still at the conference, Matt gave some quickfire responses to OxSciBlog's questions:

OxSciBlog: What was it about flatfish evolution that so puzzled Darwin?
Matt Friedman: Flatfishes - like the gastronomically familiar halibut and plaice - are unusual in having both eyes on one side of the head. The tricky bit for Darwin was making sense of intermediates between normal symmetrical fishes and asymmetrical flatfishes. What good is it having an eye moved slightly over?

In fact, the origin of flatfishes was used as an early attack on natural selection, and actually led some scientists to propose that sometimes evolution operates in leaps and bounds rather in a more gradual fashion.

OSB: What evidence did you study to understand how flatfish evolved?
MF: I examined fossil fishes from the Eocene (about 50 million years ago) of Italy and France, combined with study of skeletal materials of living fishes.

OSB: What does this fossil evidence show?
MF:The fossil evidence clearly shows fishes with an arrangement of eyes intermediate between that of normal fishes and flatfishes. One eye is shifted toward the opposite side of the skull, but does not quite make it there. This is precisely the arrangement dismissed out of hand as 'unlikely' or 'not functional', which is what led to all the problems for Darwin.

OSB: How does this add to our knowledge of how evolution works?
MF:Well, each individual evolutionary story is unique, so care must be taken in not pushing this too far. However, in this particular instance, I am able to reject the scenario that the unusual bodyplan of flatfishes evolved suddenly (ie, went from symmetrical to the modern condition in a single step), but rather occurred in a more gradual, conventionally Darwinian, fashion.

Dr Matt Friedman recently moved to join Oxford University's Department of Earth Sciences from the University of Chicago.