Question

a change in DNA that is useful for one property to start with can become useful later for a different functions explain it

Answer 1

If there are changes in the DNA, then such a change can cause a change in the number of chromosomes too. This may lead to a new variation. Over the generations, genetic drift will accumulate different changes in each sub-population.

The frequency of an inherited trait tends to change over generations. Since genes in the DNA control the traits, the frequency of certain genes in a population can be changed over generations. This is the essence of the idea of evolution.

Example:  In the population, there are a few blue beetles and many are red. However, due to situations, most of the beetles are killed. By chance, the few beetles that have survived are mostly blue. The beetle population slowly expands again, but now, the beetles in the population are mostly blue. This helps in the evolution of Blue colored beetles.

If the DNA changes are severe enough, then there will be a huge change in the chromosomes and traits also. If in a population there are red and blue beetles, a new variation emerges due to severe DNA changes, then the germ cells of the two groups cannot fuse with each other. A new variation emerges in which blue females will not mate with red males, but only with blue males. This allows very strong natural selection for blueness. Now, if such a blue female beetle meets a new green male from the other group, there is a chance of reproduction between them. Effectively, new species of beetles are being generated with a new DNA change.

Answer 2

Earlier the feathers of dinosaurs protected them from cold but nowadays these feathers are present in birds which are helping them for flight/ to fly.