Genetics and evolutionWatch
Back to basics; Giraffes with short necks evolved to having long necks - this was so they could reach the leaves on taller trees. The giraffes would have been competing with each other, the ones that adapted to have taller necks were at an advantage as they could reach more available food source.
> Selective pressures determine natural selection - Some members of the population will not survive and reproduce. (short neck giraffes) will not pass on their genes into the next generation. Gradually, the population changes, and genes that improve survival and reproduction will become more common, while genes that are disadvantageous to survival and reproduction will become more rare. This change in the genetic makeup of a population is called evolution.
isob08 gave a good example above.