The downward arrows point to those phenotypes against which selection acts.
Stabilizing selection (left column) acts against phenotypes at both extremes of the distribution, favouring the multiplication of intermediate phenotypes.
Directional selection (centre column) acts against only one extreme of phenotypes, causing a shift in distribution toward the other extreme.
Diversifying selection (right column) acts against intermediate phenotypes, creating a split in distribution toward each extreme.
Included at that level are studies of the roles that specific kinds of plants and animals play in the complex and self-perpetuating interrelationships that exist between the living and the nonliving world, as well as studies of the built-in controls that maintain those relationships naturally.Those broadly based levels—molecules, cells, whole organisms, and populations—may be further subdivided for study, giving rise to specializations such as morphology, taxonomy, biophysics, biochemistry, genetics, epigenetics, and ecology.A field of biology may be especially concerned with the investigation of one kind of living thing—for example, the study of birds in ornithology, the study of fishes in ichthyology, or the study of microorganisms in microbiology.Evolution itself is a biological phenomenon common to all living things, even though it has led to their differences.Evidence to support the theory of evolution has come primarily from the Three types of natural selection, showing the effects of each on the distribution of phenotypes within a population.