 Returning to the kinds of traits studied by biologists, some theorists make a distinction between the initial spread of a new phenotypic trait in a population and the more recent maintenance of traits in populations. Take a trait such as feathers, arising in a population by whatever means. Initially this trait may have spread because of a role in mating displays. Later, feathers may have contributed to improved thermoregulation. And still later, the trait may have become more widely distributed because feathers make good flight control surfaces. If display or thermoregulatory functions of feathers become less important in some niches, the trait may nonetheless be maintained in a population due to selection for its flight-control function. The shifting functional profile may also be correlated with differentiation in form, such as between downy feathers and flight feathers.