A person may say the environment changes, then a mutation takes place to make sure the organism can survive in the new environment. Simple right? Yes, but this isn't what happens.
In reality what happens is this: random mutations are happening inside the animal all the time, then the environment changes, then all the animals which just so happen to have the right features at the right time survive.
So how do these random mutations happen? Is there some cosmological mandate which tells the DNA to change every so often? No, the answer is much simpler.
You may think your body is a finely tuned orchestra - with every instrument playing the right note at the right time, with the right dynamics etc. The real truth is your body is pretty bad at what it does, especially when it comes to replicating your DNA. I won't go into how it gets replicated but basically 80% of the time it does it wrong and has to start over again. All this incorrect DNA is just then stored in the cell, doing nothing.
However, from time to time, the incorrect DNA is still readable. Since your cells don't have a conscience to say 'I'll only use the DNA I know is correct for this body,' it takes any DNA that is going, including the incorrect ones. So you're cell takes the incorrect DNA and processes it, and follows the instruction encoded in it. This is what leads to the mutation. It can be good, bad, or most of the time it just goes unnoticed. Simple.
These mutations are then passed onto the animal's young and their young and so on. Then comes a change in the environment. Only the descendants with the certain mutation can survive. The rest die out. That is natural selection.
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