A Small Species of Fly Lays Its Eggs in Plant

Essay

A small species of fly lays its eggs in plant stems. The developing fly larvae cause a gall, or large round structure, to form in the stem. Genetic differences among flies cause variation in the size of the galls they produce. Both birds and wasps feed on the larvae in the galls. The proportions of small, medium, and large galls able to survive attacks by either birds or wasps are shown in the table. If a population of flies lives in a field with equal abundance of birds and wasps, will gall size be by directional, stabilizing, or disruptive selection? Explain your answer.
$$\begin{array} { | l | l | l | } \hline \text { Gall size } ( \mathrm { mm } ) & \text { Gall survival with birds (\%) } & \text { Gall survival with wasps (\%) } \\\hline 15 & 100 & 25 \\\hline 20 & 100 & 100 \\\hline 25 & 25 & 100 \\\hline\end{array}$$