A out of large number of totaly random mutation ...myou may have some that just by happen stance offer serendipitous advantage to the animal with a mutation ....and because of that serendipitous advantage that animals more likely to survive and more likely to reproduce so that mutation is more likely to get propagated
..IE random mutations produce some animals that juse happened to have longer necks...amd if they eat leaves in trees.. because they have longer necks they can reach higher in the trees to get leaves that other can not ... so they have serendipitously found new unexploded niche for food...and the giraffe is created
(Blood vessels keeping blood at high enough pressure to climb the gravity well; veins preventing the blood from "falling" too quickly down the neck; structural changes to the esophagus and lungs to pull air in from so many feet away; changes to the frame/chassis of the beast to balance the long neck, including the front legs being so much higher than the hind legs; adaptations to the cervical vertebrae and spine; and those cute little joystick-looking horns on the head.)
What is the rate at which mutations occur within that portion of the genome?
What is the expected time for the proper set of mutations to occur -- compared to the time over which the environment changes?
Are there any trade-offs to having such a long neck (not being able to scurry into the foliage to escape predators)?
What corresponding genes lengthened the legs?
When and how did the male challenge / courtship behavior change (fighting by slamming the other male with one's neck)?
These must all be accounted for by random mutations occurring in tandem.
Where are the California giraffes; that munch on the top of redwoods?