It has been suggested that sexual reproduction became the dominant type of reproduction among organisms because of its inherent advantage of variability, which is the mechanism that enables a species to adjust to changing conditions.
Diversifying selection (right column) acts against intermediate phenotypes, creating a split in distribution toward each extreme.
Despite the basic biological, chemical, and physical similarities found in all living things, a diversity of life exists not only among and between species but also within every natural population.
The phenomenon of diversity has had a long history of study because so many of the variations that exist in nature are visible to the eye.
The fact that organisms changed during prehistoric times and that new variations are constantly evolving can be verified by paleontological records as well as by breeding experiments in the laboratory.
Biology is often approached on the basis of levels that deal with fundamental units of life.
At the level of molecular biology, for example, life is regarded as a manifestation of chemical and energy transformations that occur among the many chemical constituents that compose an organism.Biology, study of living things and their vital processes.The field deals with all the physicochemical aspects of life.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.Biology is subdivided into separate branches for convenience of study, though all the subdivisions are interrelated by basic principles.