Greenflies

Greenflies are presented as an example of reproduction without sex: a female can give birth to live daughters without a father, and those daughters contain copies of all her genes. The example makes a central evolutionary puzzle concrete. If an individual could reproduce this way, why do so many organisms combine their genes with those of a partner? In The Selfish Gene, Greenflies help distinguish the reproductive interests of an individual from the longer-term interests of the genes that make up its body.

Asexual reproduction

A Greenfly female can produce a daughter without sexual reproduction. The offspring inherits copies of the mother’s genes, rather than receiving one set from each of two parents. This makes Greenflies a contrast to organisms whose reproduction involves mixing genes before an offspring is made.

The example extends beyond a single generation: an embryo developing inside its mother may itself contain a smaller embryo. A Greenfly female can therefore give birth to a daughter and granddaughter simultaneously. In genetic terms, both are equivalent to the mother’s identical twins, illustrating how closely successive generations can resemble one another under this mode of reproduction.

The question of sex

Greenflies make the prevalence of sexual reproduction seem puzzling. If each child produced asexually carried all of its mother’s genes, an individual might appear to pass on twice as much of its genetic material per child as in sexual reproduction, where each child receives half from that individual and half from its partner. The book presents this as an apparent advantage, not as a complete explanation of reproductive success.

The author declines to give a full account of why sex evolved, noting that serious explanations often require sophisticated mathematical reasoning. Instead, the Greenfly example serves to expose the limits of judging reproduction only by how many of an individual’s genes enter each child. Under the gene-centered argument, sexuality can be treated as an attribute influenced by genes: a gene associated with sexual reproduction may spread when it benefits its own persistence, even if sex appears inefficient from the perspective of the individual organism.

Significance in the argument

Greenflies are not described as having a defective or incomplete form of reproduction. Their fatherless offspring provide a clear comparison that sharpens the book’s distinction between an organism as a temporary vehicle and genes as replicators whose interests need not match an individual’s apparent reproductive efficiency. The example opens the question of sex without resolving it; its role is to show why that question looks different when evolution is considered from the perspective of genes rather than whole individuals.