What is the purpose of having brown eyes, since most animals have brown eyes, and does it effect your vision?

I don't really know why most animals have brown eyes, but I believe it is a dominant characteristic. In any event, here is an interesting site:

Children look like their parents, but not exactly. Children inherit characteristics from their parents. Individuals have an appearance (say brown eye color). Individuals also have genes. Each individual carries two copies of each of their genes, one from each parent. A gene often comes in two flavors (such as a gene for eye color that makes eyes brown or blue). An individual can thus have two copies of the gene with the same flavor (brown-brown), or two copies of different flavors (brown-blue). Human eye color is controlled by at least three genes, though we only understand two of them well.

Let's pretend that two genes (with two flavors each) explain the inheritance of human eye color (though we know that the situation is more complex than this). These two genes are a brown-blue gene and a green-blue gene. We can ask what children different combinations of parents would produce. For example, what happens when blue eyed and brown eyed parents have children?

Note that this two gene model does not explain most human eye color inheritance. We know that there are black eyes, grey eyes, hazel eyes, and differing shades of brown, blue, and green eyes that are not explained by these two genes. Human eye color inheritance is a complicated polygenic system than we are pretending in this simplified two gene example

http://www.athro.com/evo/inherit.html

First of all, cats don't generally have brown eyes, but rather green or yellow or somewhere in between, and my sister has a cat with blue eyes. The cat is also deaf, which is a common trait among cats with blue eyes.

Most ferrets have brown or ruby eyes. Albino ferrets have red eyes, like albino rats and rabbits, but many ferrets who are not albinos have a dark red-ruby eye color that is definitely not an albino eye color.

I've met more than one dog with blue eyes, but that is also not common, and one of those dogs had only one blue eye and one green eye, and it was also deaf.

Brown eyes are a dominant trait, and it seems that often the recessive trait is or can be linked with developmental problems. In the wild, this of course would lead to the death of the animal, often before it had a chance to breed, which explains why it is so uncommon.

One such example of this linking of recessive traits with developmental disorders is Waardenburg's Syndrome. I'm sure this is not the only such link, but if this is a subject of interest to you, it is something I recommend researching.

Eye colour is determined by the amount of melanin present in the iris. As melanin is brown, large amounts mean brown eyes and small amounts mean blue eyes. In between are green, hazel, etc. Albinos have none so the blood vessels at the back of the eye give them their pink colour. All newborn human babies have blue eyes as they have not started to produce melanin at the time of birth.

The "purpose" of a trait like eye color could be translated to the evolutionary advantage of eye color. I'm not 100% sure we've determined an evolutionary advantage of brown eyes, though some have suggested that the darker the eyes, the more resistant to sunlight they are. This is to some extent supported by the idea that albinos are more photosensitive than non-albinos. This might be true in humans, but not in all mammals. It has been proven, for instance, that albino ferrets are not more photosensitive than non-albino ferrets. So there may or may not be anything to the idea that dark eyes make you less photosensitive.

In any case, it is possible that, in addition to it being a recessive trait, lighter eyes have an evolutionary disadvantage, rather than brown eyes having an evolutionary advantage. As with Waardenburg's Syndrome, which is common in many types of mammals, the possibility exists that mammals with lighter eyes are less likely to also possess certain other survival traits, like the ability to hear. Among humans and other domestic mammals, this becomes less of an issue, because there is protection from predators, but in the wild, such animals simply would not survive.

There are probably lots of theories, but I'm not sure any one really knows what the "purpose" of eye color is, let alone the reason why most mammals have brown eyes.

I'm not sure about animals, but I know why brown eyes are dominant in humans. Brown eyes are like a 'default setting' for humans.

When we are born, many of us are born with blue eyes. However, as our eyes are exposed to light, the chemical melanin, the dark coloured pigment, is produced. This may suggest why people who originate from hot countries tend to have dark hair, skin and eyes.

It is the lack of melanin which causes eyes to be of lighter shades, although eye colours can also be hereditary. For example, if blue-eyed is dominant in the genes of parents, the next generation will also produce blue-eyed children.

Most animals rely very heavily on melanin for survival - and melanin means, among other things, darker eyes. In most species, albinos often stand out as prey, have developmental problems related to their skins and senses, and generally do not survive for very long. The main exception is in the polar icecaps, where being melanin impoverished would be an advantage to say, a polar bear, or to the albino rabbits it would often hunt. So in most temporate and tropical species, the animals that live to adulthood are generally those that have plenty of melanin.

It is more common to see blue-eyed humans because humans are better at taking care of their young and raising their children to adulthood regardless of something so seemingly trivial as eye color. In fact, in some human cultures blue eyes are seen as superior. But in fact there is a difference. Blue eyes are more sensitive to light, and brown eyes are more sensitive to color, at least in humans.