1. There exists a variation in genetic traits, between long necks and short necks.2. The genetic trait of neck length is heritable.3. Variation leads some giraffes to be more successful than others, with long neck giraffes reaching food that short neck giraffes cannot reach.4. The difference in suitability to the environment means that long neck giraffes have better reproductive success5. Over time, the advantageous trait becomes fixed due to continued reproductive success.