From whales to a frog the size of a fingernail, always two eyes.
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My uneducated guess is that two is enough and three or four eyes do not provide much of an advantage over two eyes. Survival of the fittest, adaptation, … Is simply who can reproduce the most before their death.
Having one eye allows you to see two dimensions. Having a second eye allows you to perceive depth, a third dimension. Given there are no more spacial dimensions, there is not much advantage for most creatures to have more eyes.
One, you’re at risk of losing it, which’ll leave you blind.
Two, you get binocular vision, which helps a lot to judge distances to objects.
Three would be a minor improvement, but it would cost more in terms of space. Put one on the back of the head and you’d be safer, but…. And most animals start with a bilateral symmetry, so adding a third would be difficult, as an evolutionary step.
Four, you’re probably past the point of diminishing returns. Devoting twice as much space in your head to vision would mean giving something else up.
More, well, having many simple eyes spread around your head plus a brain set up to synthesize their inputs might work.
Evolution isn’t a process that produces “the best at xyz” rather it produces animals that are good at surviving in their environment; no more no less. Any extra adaptation without a major benefit would not survive. Extra eyes would require more brain to process. Extra brain would consume a ton of oxygen an energy. Two eyes allow for animals to have depth perception (ability to tell distance visually) where one would not allow that and more would consume energy without a true extra benefit.
Both whales and frogs (plus the vast majority of two-eyed animals you can think of) are descended from a common ancestor that had two eyes. The origin of the two-eyed body plan dates all the way back to the first primitive fishes that lived half a billion years ago, which had flat vertical bodies with one eye on each side to get a 360º view of their surroundings. The two-eyed setup has stuck around because 1) two eyes on opposite sides of the head is plenty for animals to see the entirety of their surroundings, and 2) evolution tends not to make radical leaps like a brand new set of eyes, but instead makes a long series of small tweaks to existing structures, like adjusting the size and placement of the two eyes.
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If you look at your whale flipper you will find that it contains 5 digits, and that the frog hand also has 5 digits. In fact almost everything does. I would bet alot of their internal structures are quite similar. Human embryos also have gil slits like a fish that close up before birth.
This is to say that these traits are because all life on earth is related and somewhere way down the line this blueprint of bilateral symmetry worked successfully enough to dominate the planet. This is why most things have two eyes and not because two eyes is more or less useful than three.
more eyes need more brain power to process, less eyes doesn’t give depth perception, my 2 cents
The real answer: common ancestry. Evolution tends to keep things that work, and animals with 2 eyes share a common ancestor with a species that also had 2.
The more nuanced question might then be: why did that species have two, and why was that aspect near-universally maintained in descendant species? That comes down to a few parameters:
– chance of evolution can’t be ruled out
– body plans tend to favor symmetry, and animals with bilateral symmetry will have most things in pairs
– two is the minimum necessary to get to what is roughly/nearly 360 degrees (for prey species) or stereoscopic/binocular vision (for predator species)
– eyes are sensitive, fragile organs -more of them means more vulnerable spots to protect, so minimizing them to only what you really need makes biological sense
How much each of these factors played is difficult to determine.
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