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Facts
The word "giraffe" comes from the Arabic word "zarafah," which means "one who walks swiftly." Male giraffes can be up to 18 feet tall, and females can be as tall as 15 feet. Full-grown males weigh between 1,764 and 4,255 lb., and full-grown females weigh between 1,212 and 2,601 lb. Giraffes typically inhabit the savannahs and semi-arid regions of sub-Saharan Africa, forming loosely associated herds of up to 50 individuals.
Darwin's Evolutionary Theory of Development
Charles Darwin, pioneer of the theory of evolution, initially thought that giraffes' necks evolved to great lengths in order to enable them to eat leaves located in tall trees. However, this theory has fallen out of favor because it fails to explain why females consistently have had shorter necks than males throughout history; if females are able to get enough food with shorter necks, it makes no sense for males to have increasingly longer necks. Also, giraffes typically graze in wide open spaces without trees, which is not consistent with Darwin's idea of giraffes feeding from tall trees.
Sexual Selection Theory of Development
More recent theories point to sexual selection as the impetus for the unusual neck lengths of giraffes. Male giraffes use their necks to attack each other when competing for mates. The theory of sexual selection in giraffes' evolution states that the giraffes with longer necks defeated their shorter-necked counterparts in competing for females, thus leading to the increased likelihood of offspring having longer and longer necks as they evolved. However, this theory fails to explain why the necks of female giraffes are also long: they never use them to compete with each other, so having a long neck seems unnecessary.
Other Problems With Evolutionary Theories
Several other inconsistencies must be clarified before any theory of giraffe development can gain traction. Some scientists oppose the idea of isolating a certain part of the giraffe -- like its neck -- and explaining its evolution separately from the entire animal, and propose that the best approach is to look for explanations that are more holistic. Other scientists point out that the current fossil record of giraffes remains remarkably consistent over thousands of years, without any instances of "transitional species" -- that is, species of giraffe that have only partially evolved into the modern giraffe -- which is inconsistent with the theory of evolution itself.