The word phenotype refers to the observable characters or attributes of individual organisms, including their morphology, physiology, behavior, and other traits. The phenotype of an organism is limited by the boundaries of its specific genetic complement (genotype), but is also influenced by environmental factors that impact the expression of genetic potential.

All organisms have unique genetic information, which is embodied in the particular nucleotide sequences of their DNA (deoxyribonucleic acid), the genetic biochemical of almost all organisms, except for viruses and bacteria that utilize RNA as their genetic material. The genotype is fixed within an individual organism but is subject to change (mutations) from one generation to the next due to low rates of natural or spontaneous mutation. However, there is a certain degree of developmental flexibility in the phenotype, which is the actual or outward expression of the genetic information in terms of anatomy, behavior, and biochemistry. This flexibility can occur because the expression of genetic potential is affected by environmental conditions and other circumstances.

Consider, for example, genetically identical bacterial cells, with a fixed complement of genetic each plated on different gels. If one bacterium is colonized under ideal conditions, it can grow and colonize its full genetic potential. However, if a genetically identical bacterium is exposed to improper nutrients or is otherwise grown under adverse conditions, colony formation may be stunted. Such varying growth patterns of the same genotype are referred to as phenotypic plasticity. Some traits of organisms, however, are fixed genetically, and their expression is not affected by environmental conditions. Moreover, the ability of species to exhibit phenotypically plastic responses to environmental variations is itself, to a substantial degree, genetically determined. Therefore, phenotypic plasticity reflects both genetic capability and varying expression of that capability, depending on circumstances.

Phenotypic variation is essential for evolution. Without a discernable difference among individuals in a population there are no genetic selection pressures acting to alter the variety and types of alleles (forms of genes) present in a population. Accordingly, genetic mutations that do not result in phenotypic change are essentially masked from evolutionary mechanisms.

Phenetic similarity results when phenotypic differences among individuals are slight. In such cases, it may take a significant alteration in environmental conditions to produce significant selection pressure that results in more dramatic phenotypic differences. Phenotypic differences lead to differences in fitness and affect adaptation.