Natural selection is fairly lazy, in my opinion, because it doesn't want to put energy into developing something new. If it absolutely has to create something new, though, it will do so at the expense of something else. Never will it just decide to make changes for the fun of it. For instance, we humans have five senses (sight, hearing, touch, taste, smell) and at one point during human evolution (approx. 23 mya), natural selection decided that trichromatic color vision would be very beneficial to humans. But because this would be such a huge expense (in time and energy spent), something else would have to be diminished.
In one study, they found that the deterioration of the human olfactory repertoire was probably due to the development of trichromatic color vision. Olfactory receptor (OR) genes provide a basis for our sense of smell and human genomes possess more than 1,000 of these genes. But ~60% of these genes are pseudogenes, meaning that they are "turned off" and don't contribute to the phenotype. This is why we can't smell things as well as a dog. Most of dogs' OR genes are "turned on", allowing them to smell more things than we can.Is there a reason why most OR genes are turned off in humans? The answer is yes, because natural selection does things for specific reasons, and for humans it seems to have deemed trichromatic color vision as more important than a great sense of smell.