Falcon Stormvoice said:
Sure, I'll bite.
So, let's use your idea. A weird bump which females find attractive for some reason. In most species, females pursue a strong, healthy male. This is because a strong and healthy male provides better children than a weak and sickly one. Often times, males will demonstrate how they are stronger than other males by clashing heads, such as in deer and sheep.
Assume that, initially, they are all flatheaded, until one of them is born with a skeletal protrusion in his skull. It's not a full horn, or even a partial one. Just a slight thickness in the top front of the skull. This is enough to create a bump. It hits harder than flatheads (very slightly), giving it an advantage. That is, it increases its probability of winning a competition for a female.
This unique individual will have more matings than other males. Since a male can mate with multiple females over its lifespan, let's say it has 11 offspring total (11 matings), in comparison to the normal 10. A very small difference. However, it is a difference. And over time, differences become highly pronounced. A car driving 35 mph vs a car driving 36mph is hardly any difference in speed at all. But after 1,000 hours of driving (or 1,000 generations) the distance between the two is very large. Likewise with species. So, bigger protrusion= more mating. This continues, with the species gaining larger skull protrusions. Eventually, it would lead to a horn (of course there are many more generations, with selection pressures leading to not having skin over the protrusion, a sharper protrusion over a broad one, etc).
You're right in that it is very unlikely that a specific horn will evolve. However, evolution uses what works, not what is specific, optimal, or best for the species. Thus, some species got horns out of it, while others just got thicker heads, or some other mutation that grants benefits.
And yes, the majority of mutations are bad. The few that are good, however, allow the entire species to advance. This is especially visible in viruses. We create antibodies and flu shots to kill the viruses. However, the ones with mutations for immunity are not killed. They reproduce, and as they are the only survivors, the entire species will have that immunity (requiring a new flu shot the next year).
Furthermore, bad mutations can be very beneficial in the right circumstance. Sickle Cell Anemia is a genetic disease that will kill a human early in their life (teens or twenties if I recall correctly). In Africa, Malaria will kill a human while it is still a child. However, Sickle Cell Anemia grants immunity to Malaria. Sure, the person will die young, but they are able to reproduce (not able to reproduce if they die of malaria). This is why Sickle Cell Anemia is is much more prevalent in people with African heritage than it is in all other people. Despite it being a very bad mutation, it was beneficial to the population.