DNA

DNA is the hereditary replicator whose persistence across generations gives the book’s gene-centered account of evolution its material basis. Its molecules are copied and housed inside organisms, while their effects emerge through development, interaction with other genes, and, in some cases, changes to the world beyond the body. The image of the “immortal coil” captures this contrast: individual molecules are transient, but replicating sequences can endure through an immense succession of bodies.

Molecular structure and inheritance

A DNA molecule is a long chain of nucleotide building blocks, arranged as two chains twisted together into a double helix. There are four kinds of nucleotide, abbreviated A, T, C, and G. The building blocks themselves are shared across animals and plants; what distinguishes sequences is their order. An A, T, C, or G in a human is chemically the same kind of building block as its counterpart in a snail, but the sequences differ between species and, usually, between individual humans. Identical twins are identified as a special exception to that individual variation.

DNA is distributed through the cells rather than concentrated in one bodily location. In an average human body, about a thousand million million cells each contain a complete copy of the body’s DNA, apart from exceptions the book sets aside. The DNA functions as instructions for making a body, encoded in the four-letter nucleotide alphabet. The book compares each cell’s nucleus to a bookcase containing the plans for an entire building: in humans, the plans are organized into 46 chromosome volumes, with genes arranged along the chromosomes. The metaphor helps make an invisible molecular system imaginable, but it is not literal: there is no architect behind the plans; natural selection assembled them.

Copying, expression, and development

DNA performs two central tasks. First, it replicates. At conception, a single cell carries a master copy of the plans; as that cell divides, each daughter cell receives a copy, and successive divisions produce the many cells of the developed body. The copying process is described as highly faithful, with scarcely any mistakes at each division. This continuity links the DNA in a body to the replicators passed on through generations.

Second, DNA indirectly supervises the manufacture of proteins. Its nucleotide code is translated into an amino-acid sequence, and proteins contribute to bodily structure while also controlling chemical processes within cells, turning them on and off at particular times and places. The route from protein production to the development of a whole organism is complex; the book notes that embryologists may need decades or centuries to work out the details. Genes therefore influence the construction of bodies through developmental processes rather than building them directly, and acquired knowledge or other characteristics gained during an individual life are not inherited genetically. Each generation begins anew from inherited instructions.

That influence is powerful but not simple or independent. A survival machine contains many thousands of genes, and bodily development is a cooperative enterprise in which the effects of one gene depend on other genes and the environment. A single gene may affect several parts of the body, while any one bodily part may be influenced by many genes. Describing a “gene for” a trait is therefore a convenient way to refer to a genetic difference that tends to alter development when other influences are held sufficiently constant; it does not mean that one gene alone produces the trait. The book’s example of nitrate and wheat makes the distinction clear: nitrate can contribute to greater growth, but it cannot make a plant without seed, soil, sun, water, and other conditions. In the same way, DNA’s instructions operate within an interdependent developmental system.

Replicators and evolutionary time

The physical lifetime of an individual DNA molecule is short: perhaps months, and certainly no longer than a single lifetime. Its hereditary continuity is instead achieved through copies. A particular gene may, in principle, persist in copied form for a hundred million years, with copies distributed across the world inside the bodies of survival machines. This is the sense in which genes can be called “forever”: not that one molecule is unaltered or everlasting, but that a replicating pattern can outlast each body and each individual DNA molecule that carries it.

The book accordingly defines the evolutionary significance of a gene through its potential longevity as a copied replicator, not simply as a fixed stretch corresponding to one protein. Sexual reproduction shuffles genes into temporary combinations: individual bodies and their chromosome arrangements pass away, while gene variants can recur in new combinations down the generations. DNA is thus both material and vehicle for a continuity that is not the continuity of an individual organism. The language of genes seeking survival is figurative, not a claim that molecules are conscious: differential copying allows variants that leave more copies to become more common.

Effects beyond the molecule and the body

Natural selection does not inspect DNA molecules directly. DNA is enclosed in protein and membranes and is not itself exposed as an obvious object of selection; the consequential differences among genes appear through their effects. The book calls a gene’s bodily manifestation its phenotype. A gene may affect such features as eye color or hair, but most genes have multiple effects, and selection favors variants because of the consequences of those effects for survival and reproduction, not because of the molecule’s appearance in isolation.

The boundary of those consequences need not coincide with the boundary of the body containing the DNA. A gene’s effects may reach into the external world, altering objects or the behavior of other organisms; the book’s extended-phenotype account treats such consequences as part of the gene’s effects when they influence its propagation. DNA fragments called plasmids illustrate how genetic material can also cross the usual chromosomal boundaries. Found especially in bacterial cells, plasmids are small, free-floating DNA fragments, often consisting of only a few genes. Some can splice into a chromosome so seamlessly that the join is indistinguishable from the surrounding sequence, and later cut themselves out again. The book presents these fragments as a way to see how replicators may take routes outside the ordinary arrangements of chromosomes and bodies.

This broad view does not turn DNA into a purposeful agent. The book uses active language—genes that “try” to increase their numbers—as shorthand for a non-conscious process: genes whose effects tend to increase their representation are the ones more likely to be found in future populations. DNA’s significance lies in the reach of those effects, from cellular development to consequences outside an organism, and in the copying process that allows some sequences to persist while their carriers are replaced. Its enduring pattern, rather than any single molecule or body, is the foundation of the evolutionary argument.