Final answer:
Natural selection is the force of evolution that affects populations and causes changes in traits. It acts on the phenotype of individuals, selecting for variations that provide a competitive advantage. For example, in a population of birds, individuals with larger beaks may be selected for if the environment favours larger seeds.
Step-by-step explanation:
Natural selection is the force of evolution that affected this population and caused a change in the expression of the trait mentioned. Natural selection is the process by which beneficial traits that enhance an organism's ability to survive and reproduce become more common in a population over time. It acts on the phenotype (the observable traits) of individuals, selecting for variations that confer a competitive advantage. For example, if a population of birds has variations in beak size and the environment changes so that small seeds are more abundant, individuals with larger beaks may have an advantage in cracking open and accessing the larger seeds. Over generations, the frequency of the gene(s) responsible for larger beak size would increase in the population due to natural selection.