|MadSci Network: Evolution|
This is an interesting question, but your confusion is based on some misunderstandings. Let me clarify. Remember, traits are selected for when they increase the fitness of the individuals with that trait. So, you are right: animals in the water are better suited for catching food or escaping predators if they have flippers; on land, things with flippers are easy prey, thus we see legs, etc... But, how does the horn increase fitness, and thus evolve?
I must correct a few of your points: a horn is not a bone (antlers are bone) and it is not used for hunting (and rarely for protection). The horn is a wonderful example of sexual selection. Notice only males have horns! That is because they have evolved as structures to demonstrate a male's health, age, and stature. Horns are used by males to compete for females, and females may chose mates based on horn size or other features. So, the male with the biggest horn will breed the most, producing sons with big horns who breed more, and so on and so on... Thus, there is a positive selection for large horns. Obviously this isn't the whole story, or horns and antlers would grow HUGE! There are costs associated with having large horns, such as nutrients to grow them, the added weight which could slow your escape from predators, and others. Because of this, there is an upper limit to horn size. This is exactly why females like big horns: if a male can survive and maintain healthy horns, he is obviously a suitable mate.
I hope this helps clarify some of your concern. Feel free to get back with more questions!
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