Keep Your Eyes Open

Natural selection favors a larger eye in response to increased competition in natural populations of a vertebrate (2019) Beston & Walsh, Functional Ecology, doi: 10.1111/1365-2435.13334

The Crux

Studying the evolution of traits in response to selection pressure often helps us understand why species look and act the way they do. Selection pressure can include the need to find food before other members of your species, or the need to escape predation.

But what happens when improving your ability to obtain resources also means you’re more vulnerable to predation? Which will win out? This paper looks at a small species of freshwater fish, Rivulus hartii, and determines which of the two pressures contributes most to the evolution of the size of their eye.

What They Did

Larger eye size in Rivulus allows more light into their vision, and enables them to locate and acquire more food. However larger eyes also attract more predators. To see which of these drivers influenced Rivulus development, the researchers collected fish from approximately 20 pools in Trinidad, roughly half with predators and half with only Rivulus. They used a mark-recapture experiment, marking and releasing the fish and then attempting to find them again later on, a procedure which gives a reasonable estimate of fish survival, and also allowed them to measure growth rate in the recaptured specimens.

Levels of survival and growth were then tested against the eye size to determine what impact it had on the two variables.

Did You Know: Freshwater Connectivity

We’ve spoken before about the effects of dispersal barriers on species. Freshwater species are especially vulnerable to the effects of barriers, as there is often only one pathway between two locations. This effect of fragmentation can also increase biodiversity though, with many new species forming in isolation from larger systems. Recently a new fish was discovered in northern Australia (the Running River Rainbowfish), having evolved in a 13km stretch of river fenced in by a waterfall at each end.

What They Found

Survival was (as expected) lower in populations which contained predators. However the effect of eye size on survival did not differ significantly between the two populations, and larger eye size actually increased survivorship rates, presumably because they were able to gain more access to resources.

Growth rate’s relationship to eye size differed between the two populations, with eye size positively affecting growth rate where predators were absent, whilst there was no strong relationship between the two factors when predators were present.

Waterfalls like this one can prevent predators from dispersing upstream, which relieves predation pressure and instead leads to competition within a species selecting for larger eye size

Waterfalls like this one can prevent predators from dispersing upstream, which relieves predation pressure and instead leads to competition within a species selecting for larger eye size


With a study that relies so heavily on visibility, I was surprised to see that turbidity wasn’t included as a variable in the models here, as underwater visibility would surely play a role in influencing predation/resource acquisition. However it may simply be that turbidity was so low in these pools as not to matter.

So What?

It seems here that predation has very little impact on the evolution of eye size in this species of fish, and that the competitive advantage in acquiring resources is the driver instead. The fish do not have as much trouble acquiring resources when predators are present, which decreases selection pressure on eye size.

I really appreciate experiments like this, whereby there are two clear competing hypotheses, as opposed to one hypothesis, which can suggest use of a null hypothesis. Science has been moving away from the “well if we can’t prove this then the reverse must be true” approach for a while, and it’s a good direction.

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