TeaSea

It's a semantic issue. When you say "giraffes adapted to their environment by developing longer necks" - of course no one means that in the Lamarcist sense anymore. No individual animal ever made its neck longer by stretching and then passed that on to its offspring. But, especially in animals which reproduce sexually, there has always been variation in all inheritable traits. Think of any litter of puppies. They had the same mom and dad, but no two of them are identical. I'm sure that proto-giraffes varied a bit in the length of their necks. In harsh times when food was hard to get, the taller ones lived longer and begat more offspring. If harsh times continued, after a lot of generations, they would look different. It's not a matter (usually) of change in a gene. Everybody knows that genes can be changed by mutation. That is usually fatal. What is not so widely known is that changes can occur in the ways genes are used and that a lot of our genetic material is no longer used at all - it was long ago, but as we evolved, so much became useless - same with all other life-forms. Usually, a trait like neck length or height is governed by the interplay of numerous genes. It is in DNA's nature to change as much as it is in the nature of water to run downhill.
Think of how we speed up evolution in dog breeding. I had an Italian greyhound once. This breed was developed from the older version of regular sized greyhounds, and in conformation they look just like a full-sized greyhound. Yet Italian greyhounds only weigh 8 - 12 pounds full grown. When breeders decided they wanted tiny greyhounds, they started with normal sized greyhounds and simply disallowed any except the smallest individuals to reproduce. Sort of an artificial harsh environment for that breed. Natural environments, given enough time, do the same thing, albeit without any plan or intent.