A species of bird has slight variations in its beak size and shape. Some individuals with bigger beaks are better at cracking seeds. Some individuals with narrower beaks are better at hunting for bugs. An invasive species is introduced that hunts all of the bug population, wiping out that food source for the birds, so all they are left with is seeds.
In this scenario, the invasive species has caused a significant change in the available food sources for the bird species. As a result, the natural advantage of having either a bigger or narrower beak is now influenced by the limited food options.
Initially, individuals with bigger beaks had an advantage in cracking seeds, while those with narrower beaks were more skilled in hunting bugs. However, with the bug population wiped out by the invasive species, only the birds with bigger beaks remain with an adequate food source - seeds. This situation creates a selective pressure favoring individuals with bigger beaks.
Over time, natural selection is likely to favor those birds that can effectively crack a wider range of seeds with their bigger beaks. As a result, the population may gradually shift towards an increased prevalence of individuals with bigger beaks, as they have a higher chance of survival and reproductive success due to the availability of seeds as their primary food source.
Through this process, the genetic composition of the population will change in response to the selective pressure, leading to an increased frequency of genes associated with bigger beaks. Eventually, the majority of the bird population may exhibit bigger beak sizes, as those individuals have a survival advantage in the given food environment.
It is important to note that this gradual change in beak size distribution may also depend on various factors, such as the rate of adaptation, genetic variation, and potential trade-offs associated with having a bigger beak (e.g., increased energy requirements or reduced maneuverability). Additionally, if the invasive species is eventually controlled or eliminated, and the bug population recovers, the selection pressures may shift again, allowing individuals with narrower beaks to regain their advantage in hunting bugs.