Question

A population of tree-dwelling rodents (think squirrels) lives in a species of pine tree. The pine trees have evenly spaced branches of relatively the same thickness. Rodents initially ranged in size with a normal distribution of small, medium, and large body sizes (i.e., typical bell curve). Rodents that are too small cannot reach between the branches to move around easily in the tree. Rodents that are too big are heavy and break through the branches and fall. Currently, the rodent population currently contains adults that are all relatively the same size. There is not currently much diversity in size. What is the reason for the lack of diversity in size among the adult rodent population?

1) The small rodents have a higher survival rate due to their ability to easily move between the branches.
2) The large rodents have a higher survival rate due to their ability to break through the branches and fall.
3) The medium-sized rodents have a higher survival rate due to their ability to navigate between the branches without breaking them.
4) The lack of diversity in size is due to genetic factors that have led to a decrease in variation among the rodent population.

Answer 1

The adult rodent population lacks size diversity because of directional selection, favoring medium-sized rodents that can easily navigate pine branches without breaking them. Over time, this has led to a narrower distribution of body sizes focused on medium-sized individuals. Population structure and optimal survival and reproduction conditions have contributed to this uniform population size. Option C is the correct answer.

Step-by-step explanation:

The reason for the lack of diversity in size among the adult rodent population is likely due to natural selection favoring the medium-sized rodents. Initially, the population had a normal distribution of small, medium, and large individuals. However, the small rodents were unable to move easily between branches, while the large rodents would break the branches and fall. Consequently, the medium-sized rodents, which could navigate safely between branches, had higher survival rates. This situation led to directional selection, where selective pressures favor one extreme phenotype over the others, resulting in the population evolving towards medium body sizes.

Creating histogram plots would show the shift in body sizes over generations. Initially, the histogram would display a normal distribution of rodent sizes, with the highest frequency at the medium size. Over time, as natural selection acts on the population, a new histogram would show a narrower distribution centered on the medium size, indicating that the extreme sizes (very small or very large) have been selected against.

Another factor that could influence population size and density is the population structure and the ability of individuals within that population to survive and reproduce. Medium-sized rodents likely have the most advantageous body size for the specific environmental conditions posed by the pine trees they inhabit. This optimal size for survival and reproduction would lead to a more uniform population size over time.