The process of natural selection results in organisms that are optimized for all the design features favored in a particular environment. All mutations are equally probable. Any mutation is equally likely to improve or not improve the organism. Mutation is the primary source of new genetic variation. Most phenotypic traits vary continuously and are determined by single loci.
Many organisms modify or even create their own environments.
The environment of an organism includes others of the same species. Natural selection would be expected to affect populations differently in different environments. Whether the environment is uniform or heterogeneous can affect the likelihood of speciation. The organism is not part of its own environment.