Meiotic drive

Meiotic drive is the transmission advantage a gene gains when it biases meiosis in its own favor. By turning the normally equal lottery between alleles into an unequal one, a segregation distorter can spread even when it harms the body carrying it. The process makes a central tension in evolution visible: what benefits a gene’s representation in later generations can be disastrous for the organism and for the other genes with which it shares a body.

Mechanism and definition

In ordinary meiosis, the cell division that produces sperm or eggs, just one of two alleles enters any given reproductive cell. Across many sperm or eggs, each allele has an equal chance: half receive one, half the other. The book compares this with a fair coin toss, while noting that meiosis, like a coin toss, is a physical process whose outcome can be influenced by circumstances.

A mutant gene may affect meiosis itself, making it more likely than its allelic partner to enter an egg or sperm. Such genes are called segregation distorters; the resulting bias in transmission is meiotic drive. The distorter’s advantage is not necessarily an improvement to the organism’s traits. It can arise from the way the gene alters the machinery of inheritance, giving it more copies in the next generation regardless of whether its effects help the body.

The t gene in mice

The mouse t gene provides the book’s clearest example. A mouse with two copies of t either dies young or is sterile, so the gene is lethal in the homozygous state. A male with one copy, however, can be healthy and otherwise normal. The unusual result appears in his sperm: as many as 95 per cent carry t, while only 5 per cent carry its normal allele, sharply departing from the expected 50–50 distribution.

That skew gives t a substantial advantage in the meiotic lottery. When a t allele arises in a wild population, it can spread rapidly. As its frequency increases, more mice inherit two copies—one from each parent—and die young or fail to reproduce. The gene’s transmission success can therefore undermine the survival of the population in which it spreads. The book notes evidence that wild mouse populations have, in the past, become extinct through epidemics of t genes.

Selection and its costs

Meiotic drive exposes a mismatch between selection’s effects at different levels. A segregation distorter can be favored because it increases its own representation in the gene pool, even as its effects reduce bodily welfare and damage the interests of other genes. In the t gene’s case, the same transmission bias that helps the allele spread also generates homozygous offspring that die or are sterile; the advantage to the gene and the cost to organisms are linked consequences of its inheritance pattern.

The example qualifies the familiar tendency to explain selection through traits that improve an organism’s survival or reproduction. When a gene’s effects benefit the gene while harming the body, the body-level balance may be wholly negative even though the gene is favored. The book stresses, however, that such distorters are not common. Their existence poses a further question: why is meiosis usually fair, and why do organisms’ systems of inheritance resist the proliferation of elements that exploit them?

A gene among cooperating genes

The conflict is striking because genes ordinarily work within highly interdependent systems. A survival machine contains thousands of genes; development is a cooperative process in which each gene’s effects interact with those of many others and with the environment. A gene may influence a particular outcome, but it does not build a body part alone. Meiotic drive is an exception in the sense that it pits one gene’s success against the welfare of the rest of the genetic system, not evidence that genes generally act as independent agents.

The book’s account also treats purpose-language as shorthand, not as a claim that genes consciously plan or cheat. A gene described as spreading is one whose effects result in more copies in future gene pools; a “selfish” gene is not a thinking agent. On this reading, meiotic drive is a concrete case of the gene-centered perspective: selection can favor a replicator’s transmission advantage even when the advantage does not serve the organism that carries it. The phenomenon leaves the reader with a biological puzzle rather than a simple moral: how did organisms and normally impartial meiosis arise and persist amid the potential conflict among replicators?