During the course of evolutionary history, certain species develop adaptations that enhance their survival and reproduction in specific environments. Researchers discovered a population of a hypothetical bird species, known as the 'Aviatorius variabilis', that possesses a variable beak shape depending on the availability of food sources in their habitat. In a specific island ecosystem, the population has predominantly longer and more robust beaks due to the presence of larger fruits and seeds, while another island with softer insects has resulted in predominantly shorter and finer beaks. This phenomenon is best explained by: