DNA Code Barrier
Inside of every one of your cells is the blueprint of who you are. You have inherited all of your characteristics from your parents including your ability to have children (if your parents could not have children, you won't be able to either). This information is passed down through DNA. If you and your spouse were able to have children every day for the rest of your lives you would never have two children the same because every one of you germ cells have different traits imprinted into each cell.
Even though every germ cell is different no new information is ever added. Some information is recessive and some is dominant but all the information needed to make you and all future generations is contained in every germ cell. Sometimes this information gets shifted around by a coping error. This is called a mutation. Almost all mutations are harmful and they never result in new, more complex, information. For one kind of animal to turn into another kind of animal it would require billions of beneficial mutations. Not only has this never been observed, but it can not happen because of the error checking mechanisms build into every cell.
Darwin experimented with birds and lots of scientists today have spent decades doing observational science by breading fruit flies and other kinds of animals yet all the results have shown it is not possible to brake the DNA code barrier and create a new kind of animal.
After breading fruit flies for over 100 years:
Some get larger,
Some get smaller,
Some get weaker,
Some get stronger,
Some change color,
But all are still fruit flies.
The typical size of a fruit fly is 1/8 inch and you may get a fly as large as 1/4 inch but you will never get one that is the size of a horse. This is because the DNA code barrier prevents anything more than superficial changes. Even if you did get a 1/4 inch fruit fly and left it in its natural environment the population will revert back to its normal size within a few generations. Un-natural selection (human intervention) can accentuate many different characteristics but that selection is limited to the genetic information that already exists.
A crude example:
Lets say you have a gum ball machine with red, blue, white, green, and pink gum balls. First take out the the pink, then the green, then the white gum balls. Now you are left with red and blue. How long would it take before one of those gum balls turn purple? It may take some time but it could happen given the right set of circumstances and loss of color from surrounding gum balls. How long would it take before one of those remaining gum balls turned into a piece of liquorice? It won't happen. Why? Because there is no liquorice in your original container.
The same applies to DNA. If your container full of DNA does not contain the information to create feathers then you and all of your descendants will never have feathers. No amount of selection can create new genetic information.
