Phenotype

Phenotype is the set of effects through which a gene becomes consequential to natural selection. Genes are not favored because of what they are as molecules, but because their effects can help an embryo develop into an adult that survives and reproduces; in the extended account, those effects may reach beyond the body and alter the wider world.

Definition and Selection

DNA is enclosed in protein and membranes, shielded from the environment and invisible to natural selection. A selection process acting directly on DNA would have little basis for distinguishing one gene from another. Their differences matter through their consequences, usually effects on embryonic development and therefore on bodily form and behavior. The technical term Phenotype names a gene’s bodily manifestation: the effect it has, relative to its alleles, through development. Green eye color is one example; a gene may also have more than one phenotypic effect, such as influencing both eye color and hair texture.

Natural selection favors genes because of these consequences, not because of their molecular nature. A gene that improves a predator’s running speed may benefit the whole body: the animal is more likely to survive, reproduce, and pass on all its genes, including the one associated with speed. In such a case, what benefits one gene also benefits its companions in the body. The distinction becomes sharper when a gene’s effect benefits its own transmission while harming other genes sharing that body.

Development and Genetic Interaction

A phenotype is not a simple product of a gene acting alone. A survival machine contains thousands of genes, whose contributions to development are interdependent: one gene may affect several parts of the body, and any one bodily part may be influenced by many genes. Some genes act as master regulators of clusters of others, while the effects of any particular gene depend on its interactions with the rest of the genetic system.

For that reason, expressions such as a “gene for long legs” are shorthand for a comparative influence, not a claim that one gene constructs a leg by itself. Leg development is a cooperative process involving multiple genes and the external environment; legs, like other body parts, are made from material supplied through food. A gene may nevertheless tend to make legs longer than they would be under the influence of its allele, with other relevant factors held constant or allowed to vary within limits. The analogy is nitrate in wheat: nitrate can increase plant growth without being sufficient to make a plant.

From Behavioral Effects to Extended Phenotype

Genes can influence behavior without issuing moment-to-moment instructions. By shaping survival machines and their nervous systems, they exert an ultimate influence; the nervous system makes immediate decisions. In this analogy, genes are policy-makers and brains are executives, with learning and simulation expanding the brain’s role. For a behavior to evolve, a gene influencing nervous-system development in a way that makes the behavior more likely must persist more successfully than a rival gene or allele. The claim concerns inherited influence on behavior, not a gene consciously directing an animal.

The conventional account limits phenotypic effects to consequences within the body that carries the gene. The extended account treats them as all the effects a gene has on the world. A gene’s influence may in fact remain confined to the bodies through which it passes, but that is a matter of circumstance, not a boundary built into the definition. The effects are tools by which genes influence their passage into later generations, and those tools can reach outside the body wall—for example, through structures such as beaver dams, bird nests, and caddis houses.

Consequences Beyond the Body

The extended account also encompasses effects on other organisms. When natural selection favors genes that manipulate another organism, it is legitimate to describe those genes as having phenotypic effects on the manipulated organism, even though the genes themselves are physically located in a different body. The target may be the carrier of the gene or another organism; what matters is the gene’s effect on the world and the resulting prospects for its propagation.

This extension reframes the relation between organism and gene. An animal’s behavior may tend to maximize the survival of the genes associated with that behavior whether or not those genes reside in the animal performing it. The principle is not restricted to behavior: the same reasoning may apply to color, size, shape, and other effects. Phenotype thus links the gene-centered account of selection to visible bodies, behavior, artifacts, and interactions beyond the body, without treating genes as conscious or purposeful agents.