There are more duck offspring produced than there are offspring who will survive. Only some of the ducks will make it to adulthood before they get eaten.
For the variation between the ducks it can either be beneficial or harmful depending on the mutation caused by a change in the DNA. In this case the mutation is that the the tops of the ducks heads are different colors. One is blue and one is green.
There are many different animals living in the same swap area as the ducks such as alligators who are predators, and fish as a food supply and lastly hawks.
There are many different struggles that the ducks have. First they have to compete with the hawks to get the fish for food, and they also have to try not to get eaten by the alligators.
The variations between the color of the ducks head are selected for and selected against. because the ducks with the blue head mutation can blend in with the water it makes it easier for them to blend in causing them to be more likely to survive compared to the ducks with the yellow head adaption.
Over a very long period of time there will be none or close to no yellow headed ducks left this is because of how natural selection effected how the ducks evolved.