Data for: Experimental test of the influence of light availability on the evolution of eye size and behavior in Daphnia

There exists extensive variation in eye size. Much work has provided a
connection between light availability and differences in eye size across
taxa. Experimental tests of the role of the light environment on the
evolution of eye size are lacking. Here we performed a selection
experiment that examined the influence of light availability on shifts in
eye size and the connection between eye size and phototactic
(anti-predator) behavior in Daphnia. We set up replicate experimental
populations of Daphnia, repeatedly evaluated phenotypic shifts in eye size
during the ~50-day experiment and performed a common garden experiment at
the end of the experiment to test for evolutionary shifts in eye size and
behavior. Our phenotypic analyses showed that eye size rapidly diverged
between the light treatments; relative eye size was consistently larger in
the low versus high light treatments. Selection on eye size was also
modified by variation in density as increases in Daphnia density favored a
larger eye. However, we did not observe differences in eye size between
the light treatments following two generations of common garden rearing at
the end of the experiment. We instead observed strong shifts in
anti-predator behavior. Daphnia from the low light treatment exhibited
decreased phototactic responses to light. Our results show that decreased
light relaxes selection on anti-predator behavior. Such trends provide new
insights into selection on eye size and behavior.