Final answer:
The decrease in the proportion of mice that only eat grass seed indicates a form of natural selection known as directional selection, where environmental changes lead to certain phenotypes being favored over others, altering the population's characteristics.
Step-by-step explanation:
The population increment of mice that primarily eat grass seed and the decline of those that do not suggests a form of natural selection called directional selection. This type of selection occurs when environmental factors change, benefiting certain phenotypes over others and shifting the overall characteristics of a population in a consistent direction. An example of directional selection in nature is the evolution of the peppered moth during the Industrial Revolution in England.
Directional selection leads to a reduction in genetic variance for traits that are not favorable while increasing the presence of advantageous traits. For the group of mice eating grass seeds, it is likely that this food source became more advantageous due to changes in the environment, such as availability of food or predation patterns. Over time, the proportion of mice that have traits favoring grass seed consumption is predicted to increase.