Final answer:
The statement about the variety of apples indicates the natural selection factor of 'All populations have genetic variation', showing that diversity is crucial for the selection of traits that improve an organism's survival and reproduction chances.
Step-by-step explanation:
The statement that apples at the grocery store often come in different sizes, colors, and shapes illustrates the factor of genetic variation within a population, which is essential for the process of natural selection. This diversity among individuals provides a basis for selection when environmental pressures arise. In the workings of natural selection, those individuals with traits better suited for survival in their particular environment are more likely to survive and reproduce, passing on their advantageous characteristics to the next generation. Consequently, over time, these favorable traits become more common within the population, leading to adaptation and evolution.
Darwin's theory of natural selection is underpinned by three principles: inheritance of traits, overproduction of offspring leading to competition for resources, and variation in characteristics which are inherited. The variety in apple characteristics is an example of this variation. It is based on this genetic variation that natural selection can operate, leading to the adaptation of a population to its local environment over generations.