email - December 2016

Blind Fish

Have blind fish evolved or devolved?

Joseph asks,

Frankly, I'm still trying to wrap my head around this "blind fish" thing. There is no advantage to being a blind fish; moreover, only a percentage of a blind fish's offspring will also be blind, and if it continues to mate with other normal fish, the blind mutation will be weeded out.

So how did the blindness remain in the gene pool? Don't we normally need to do inbreeding just to get variations of dogs? (Hence problems such as German Shepards having hip problems.) How could a fish, in the wild, with a disadvantage (as slight as it may be) mate in such a way that it preserved the blind mutation?


There is no advantage or disadvantage to being blind in total darkness, so natural selection (as it applies to sight) does not come into play.

The association of hip problems with German Shepherds is what Darwin called, “correlation of growth.” That is, people bred German Shepherds for particular traits, and genetic hip problems just came along for the ride. That is, the breeding pairs that were selected for the desired genetic German Shepherd traits also happened to have genetic hip problems. Breeding for one trait turned out to be breeding for both.

The first blind fish might also have had another mutation that did provide some sort of survival advantage that had nothing to do with blindness. That other mutation could have caused blind fish to predominate.

Regardless, loss of sight has nothing to do with how vision in any living thing originated.

If a bumper accidentally falls off a dirt-track stock car, it will make it lighter, so it might go faster. That’s an advantage in a race (as long as the car doesn’t crash). But the fact that a bumper can fall off a stock car accidentally doesn’t prove a bumper could accidentally fall onto a stock car. Nor would it prove that bumpers spontaneously come into existence.

Yes, creatures can have accidental mutations which cause a loss of functionality—but that doesn’t prove that accidental mutations can create new functionality and insert that new functionality into the proper place in the genome.

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