Evolution is heritable change. When the environment does not change, then natural selection would in most cases eliminate mutations. Mutatoins cause changes.If mutations are eliminated by natural selection because they are harmful, then there is little or no change even though natural selection continues to operate. Scientists call this punctuated equilibrium because they observe that during a species' life time, they often see little change and then it may become extinct. In fact species that change little over vast stretches of time are known, and they are referred to as living fossils. A good example of a living fossil is the coelacanth. They were thought to have become extinct, and the living ones that have been discovered are very similar to fossils that lived during the age of the dinosaurs. The type of natural selection that eliminates mutations is known as negative selection or normalizing selection. Individuals that have a particular trait as the result of mutation are eliminated. For example, a falcon that has a mutation making it unable to fly will die when it tries to leave the nest since it will crash to the ground or water below.
When the environment changes, what were once good genes are not as good and they may even become bad genes. When that happens, some mutations that were normally eliminated may actually be by chance better as they may fit the new environment better. If so, then natural selection will result in the retention and even increase in the percentage of individuals that have the mutation. And if most individuals now have the new mutations but not the old genes, then evolutionary change has happened. The type of natural selection that results in change is referred to as directional selection or positive selection. Individuals that have a particular trait are more likely to survive than those that do not. They are positively selected, instead of being eliminated by negative selection. For example, when ostriches evolved from a flying bird, the first ostriches that evolved the loss of flight are more likely to survive because the flight muscles use a lot of energy. Losing the flight muscles means a bird can survive without eating as much food, or it can grow bigger on the same amount of food, since it is not wasting energy feeding the flight muscles. So, even though the mutation may be similar (loss of flight muscle), it can be either adaptive or deleterious, depending on the environment. Loss of flight can be devastating to some birds that rely on flight, but it can be very helpful to a a bird that finds it unnecessary and even wasteful of energy to have flight muslces because it does not need to fly or if it had evolved to be so heavy that it can no longer fly.
Natural selection does not automatically favor one trait above other traits. The particular trait that is favored depends on the environment and the organism's lifestyle.