I hope that helps with the beneficial mutations. I listed just a few, and maybe you can find the others I listed, but even a few are enough to show that there is nothing preventing beneficial mutations from happening.
But Paul brought up another interesting, and related, topic when he mentioned vestigal parts a few posts ago. Evolution tends to reuse or change existing parts rather than invent from scratch. (Though plenty has been invented from scratch!) Think of birds. They did not sprout wings from nothing and have their existing arms waste away over the eons. No, over time, the arms of the theropod dinosaurs was changed into functioning wings. And not with flight in "mind." It was hunting. The larger theropods, think T. rex, had small arms that do not appear to have been useful for much. But the smaller ones have bone structure that indicates that they were used for grasping prey. And as it turns out, the motion they used for grasping prey, according to the form of the bones, is exactly the same motion used for the powered upstroke of flight. The bone structures, such as the fused clavicle, and muscle structure that was later used in flight, was originally selected for based on a better ability to hunt. Only later was it coopted for flight.
I say all that as an introduction. We can look at the human body and see examples of where function is shared across species. I have been wanting to do this same post looking at how genes are reused for vastly different purposes, but I have not gotten around to it. Physical stuff is easier to see, anyway.
First, the familar. We are all familar with animals puffing up their fur. Cats can do it to make themselves look bigger when frightened. Sometimes you will see animals do it in the cold to puff up the fur for greater warmth. Now look at you own arm the next time you are cold and feel goose bumps coming up. Or when something frightens you with the same reaction. We have hardly any body hair. Raising the hair on end will not keep us warmer nor will it make us look bigger. Yet we retain this function from our harrier past.
Can you wiggle your ears? Why? It has no benefit. At least to us. Our distant ancestors could turn their hears to help them hear better. Watch a dog or cat. (Not that I am saying they are our ancestors!) Some of us have not lost this ability.
Most of us are sitting on our bottoms. These muscles are huge (I think they may be the largest in the body.) and are essential to upright walking like ours. The other apes have the same muscle, but it is much smaller. This is why when you see a chimp ambling around on two legs they have that funny look where their knees are sharply bent with the thigh bones much closer to horizontal than in a human. Humans have devoloped this into a large muscle for walking but it is the same muscle as in the other apes. For that matter, look at the whole subject of upright walking. Our bodies have many problems because the bodies of our ancestors were on all fours. When moved upright, problems insue. Look at how many people have lower back troubles.
While talking about four legged ancestors... There is a muscle, the subclavius, that goes from the first rib to the collarbone. In other animals this muscle is used in moving the front legs for walking. Humans have not completely lost this relic. Some people maintain both of these on each arm, some only one, and some people none. They serve us no purpose.
Another muscle we no longer use is the plantaris muscle. This is used by other primates to grasp with their feet. We have no use for it and it has shrunk to the size of a nerve fiber.
There is a similar muscle in the lower arm called the palmaris. It is used by primates for hanging and climbing. In humans it has no function and is often taken by surgeons in need of a muscle elsewhere for reconstructive surgery.
I have many others, but I think that gets the point across.
