What evidence would convince a scientist that a well established theory is incorrect?

Dawkins touches on this issue in The Greatest Show on Earth, his latest book.  It sets out the evidence against Creationism/Intelligent Design and for Evolution.  At one point he asks whether there is any finding that could potentially disprove the Theory of Evolution.

Dawkins answers this question with Haldane’s famous quote “fossil rabbits in the precambrian”.  In other words finding an animal fossilised in rocks dating from an era long before the animal supposedly evolved would pose a big problem for evolutionary theory.

However this looks like a rather large simplification of how things would really work.  Since this isn’t the main point of Dawkins’ book it is unsurprising he didn’t decide to go into too much detail, but it does give me a chance to explain some of the philosophy of science.

For a theory with so much evidence supporting it, it is unlikely that one fossil found and dated to the wrong period of history would be enough to disprove it. Instead scientists would probably be asking some searching questions about the fossil.

Fossils can’t be dated directly and their age has to be inferred from the age of surrounding rocks.  Questions scientists would ask is whether the dating has been done correctly,  whether some quirk of geology had caused a young fossil to end up with older surrounding rocks and whether the fossil was a forgery like Piltdown Man.

If these questions were answered satisfactorily then rather than throwing the entire Theory of Evolution in the bin I suspect biologists would attempt to modify only a small part of it to accomodate the new fossil.  In fact Dawkins mentions an example of just this type of modification later on in the book.

At the end of the 19th century many geologists noted that similar plants and fossils could be found on the east coast of South America and the west coast of Africa.  If a plant or animal had evolved in Africa how had it crossed the oceans to South America?

As you no doubt guessed, the answer was a modification of our understanding of the Earth’s geology.  Scientists accepted that the continents were not stationary and that in the past South America and Africa had been joined together.

This is the way science really works, as ideas are modified to take account of new evidence.   What persuades scientists that an old idea is wrong is when a theory needs to be modified for each new bit of evidence that arrives and no longer is able to predict new results.  It also helps if there is an alternative that does a better job.

This is what happened to the Ptolemaic view of the universe with the Earth at the centre of the universe orbited by the Sun, stars and planets.  Eventually it was replaced as more accurate measurements kept introducing modifications while the heliocentric model of Copernicus and Kepler gave much more accurate predictions.

Real science is messier than the simple view that just one piece of contradictory evidence is enough to overthrow a scientific theory.