If you are talking about humans, it it genetics, if you are talking about animals, it is the evolution of genetics, and I guess you could say their feet became webbed for swimming.
Webbed feet perhaps
The swan's WEBBED FEET used for swimming is an ADAPTATION which is considered a biotic factor.
Webbed feet enable them to paddle with less effort.
"Webbed Feet" are feet that have a very thin and tiny "phalanges" They are like a duck or a chicken feet.it means you have feet like a duck where in between each toe theres this little web like thing there
swimming
The majority of dog breeds do not have webbed feet. Only dogs breed for swimming, like the Newfoundland, have webbed feet.
Webbed feet to help swim
A duck does..
Swimming
Two creatures come to mind that have webbed feet for swimming, One is the platypus. And the second is the Beaver. This is actually a common adaptation for any creatures that live on the water; however it is far more common is birds then in mammals.
Bats have webbed feet as one of their adaptations. This will help them to float on water when they are swimming.
Mainly webbed feet are seen on swimming birds