Answer: adaptation
natural selection
Natural selection refers to the differential survival of the organisms in the population. It describe the fact that individual that have favorable phenotypic characteristics have better chances of survival over others in the changing environment. Adaptation is a genetic change or mutation that helps organism to survive in it's ecological niche. Due to the helpful nature of these genetic changes they are passed down from one generation to the next generation and favors the survival and abundance of a species.
Adaptation and natural selection has resulted in the change in the population of the giraffes. The giraffes being in short neck adapted the increase in neck length with an effort to obtain leaves from tall tries. The long neck giraffe got the better chances of survival and reproduction, due to natural selection the trait was passed on to generation. This resulted in the evolution of long neck giraffe species.