Wings serve most birds for flying, or (as in penguins) for swimming. But ostriches, which exclusively use only their legs for locomotion, still have wings. Why?
There is a common misconception that a selective process is needed to remove a feature from a population. However, the correct approach is quite the opposite: selective processes normally maintain a feature in a population: if a given feature stops promoting selective advantage, its corresponding genes start to "erode" and the feature disappears from the population (and that can happen in different rates, depending on several factors: have in mind that maladaptative features do exist).
That being said, the maintenance of wings in ostriches indicates us that there must be a reason for it.
It has been hypothesised that wings:
- Improve balance when running;
- Helps regulating temperature;
- Serve as mating display.
(source: University of Washington)
They are vestigial structures. Vestigiality refers to genetically determined structures or attributes that have lost some or all of their ancestral function in a given species, but have been retained during the process of evolution.
The emergence of vestigiality occurs by normal evolutionary processes, typically by loss of function of a feature that is no longer subject to positive selection pressures when it loses its value in a changing environment. The feature may be selected against more urgently when its function becomes definitively harmful. Typical examples of both types occur in the loss of flying capability in island-dwelling species.
In the case of an ostrich, its ancestors used wings to fly, but the development of faster running abilities, keener eyesight and a paucity of capable predators allowed the bird to no longer receive positive selective pressure to have " functional" wings.
It's late. I'll add more detail in the am.