Joking aside, mutations are often caused by copying errors in the genetic material during cell division and by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses. Some mutations can lead to changes in a whole species naturally over time.
Mutations create variation in the gene pool, and the less favorable (or deleterious) mutations are removed from the gene pool by natural selection, while more favorable (beneficial or advantageous) ones tend to accumulate, resulting in (gasp!) evolutionary change.(From Wikipedia...read more about mutations HERE.)
Article from Yahoo news:
LONDON - Webbed feet run in Stumpy's family, but a rare mutation has left the eight-day-old duckling with two nearly full-sized legs behind the two he runs on.
Nicky Janaway, a duck farmer in New Forest, Hampshire, 95 miles southwest of London, unveiled the duckling to reporters on Saturday.
"It was absolutely bizarre. I was thinking 'he's got too many legs' and I kept counting 'one, two, three, four,'" Janaway said.
Stumpy would probably not survive in the wild, but Janaway, who runs the Warrawee Duck Farm in New Forest says he is doing well.
"He's eating and surviving so far and he is running about with those extra legs acting like stabilizers," Janaway said.
The mutation is rare, but cases have been recorded across the world. One duckling named Jake was born in Queensland, Australia, in 2002 with four legs but died soon after.