What Is Sex?

In biological terms, sex is the combination of two distinct gametes or genomes to produce a new organism. Typically, these gametes are designated as male and female. One type of gamete is the egg, or ovum. The other type is the sperm. Each gamete carries a reshuffled half of the parental genetic material. These gametes fuse to form a zygote. It contains a new combination of genetic information from both parents and develops into a new organism. This organism is distinct from either parent, yet derived from both.

For decades, the prevailing explanation for the evolutionary value of sex has been genetic variation. By recombining genes in each generation, sex generates diverse offspring. Diversity is thought to give populations a better chance of adapting to changing environments, resisting disease and surviving. Yet, a recent study on serial cloning in mice suggests that variation is only part of the story.

The Cloning Experiments

Cloning, as demonstrated by Dolly the sheep, starts with a highly specialized cell. The nucleus, containing all the cell's genetic material, is carefully removed from this cell.

Next, an egg cell is taken from a donor animal. Its own nucleus is removed, leaving an 'empty' egg. The nucleus from a mature cell, commonly an adult skin cell, is then transferred into this enucleated egg. The modified egg is stimulated to start dividing and growing as a fertilized egg would. Finally, this developing embryo is implanted into the womb, much like in in vitro fertilization.

These new experiments used serial cloning: an original mouse, called Generation 0, was cloned to produce Generation 1. Then a Generation 1 mouse was cloned for Generation 2, and so on. Initially, cloned mice were healthy and fertile for several generations. As cloning continued, the birth rate, overall health, longevity and survival declined. By generation 58, the transferred nucleus no longer produced any viable progeny.

These mice were not subject to obvious external selective pressures. They lived under controlled conditions with similar environments and care. Thus, the progressive decline in fertility and survival cannot reasonably be attributed to Darwinian selection. The explanation points inward—to what happens within the transplanted nucleus’s genome as it replicates again and again.

This puts paid to the idea that we can develop an entire army of clones, as in the Star Wars movies. The study, however, challenges this popular fantasy of turning one perfect soldier into an endless line of identical copies. Biology simply doesn’t allow for perfectly identical copies; variation inevitably creeps in at the level of individual cells.

Mutation Accumulation in Cells

Every time a cell divides, it replicates billions of nucleotides. DNA replication is remarkably accurate. However, it is not perfect. The estimate is that at least six errors occur each time the human genome is copied, leading to potential mutations with each cell division. Over an organism's life, cells divide to create and renew the adult organism. Over a lifetime, the cells in one person are thought to undergo around ten quadrillion divisions. Even if only about one new mutation appears in the DNA each time, that adds up to roughly ten quadrillion new mutations scattered.

Also, not all cells replicate at the same rate. This matters for how sex limits the buildup of mutations. Eggs are made early and rarely divide, so they carry few mutations. In contrast, sperm are made continuously throughout a man’s life. The cells that produce sperm continue to divide as they age. Each time these cells divide, there’s a chance for new mutations to appear in the sperm. This is why children of older fathers are more likely to have certain genetic conditions. Older age can also lead to lower sperm quality and fertility. Somatic, or body, cells also divide constantly to renew tissues, leading to many more mutations over time.

A Biological Work of Art

As a result, not all body cells have the same DNA, despite claims in many modern texts. Each group of cells picks up its own set of mutations. An adult organism is a mosaic, made of cells that appear similar but are subtly genetically distinct. Adult organisms contain many, many variants that are not present in the original fertilized egg. Some may even become cancerous. Much like a mosaic artwork that looks seamless from a distance but reveals a patchwork of distinct tiles up close, our genetic landscape is functionally coherent yet built from countless genomic variations at the smallest scales.

When a nucleus is transferred into an egg to create a new animal, all the clone’s cells inherit these mutations. With each generation, repeating the process allows even more harmful mutations to build up. Over time, this accumulation can cause fertility and health issues, eventually making further cloning impossible.

Could Life Have Evolved Without Sex?

One might ask: could an organism evolve a strategy that preserves a pristine set of germ cells across generations, minimizing their replication so they do not accumulate harmful mutations, and use these cells for asexual reproduction?

In principle, such a system is possible. This would involve a lineage of germ cells that undergoes very limited replication. Each germ cell would be activated in turn to produce a new organism. Somatic cells would still be generated for each individual’s body, but they would not serve as the source of the next generation’s genome. With this hypothetical strategy, mutation accumulation in the germline could be reduced dramatically, thereby weakening one of the major arguments for sex.

However, with extremely limited replication and no recombination, genetic variation would be minimal. Consequently, populations might become evolutionarily stagnant, unable to adapt to new environments, emerging diseases or ecological changes. Over long timescales, the inability to generate sufficient variation could be as lethal as excessive accumulation of mutations.

A Little Balance Goes a Long Way

Evolutionary success requires a balance. Too little variation means species cannot adapt. Too much unchecked mutation causes lineages to succumb to genetic decay. Sexual reproduction, as seen in nature, strikes this balance. It creates variation but also provides ways to manage and limit the accumulation of harmful mutations.

The classic answer to "Why does sex exist?" is that it creates genetic variety, helping species adapt. The real answer is more complex. Sex does more than mix genes. It helps clear out damaging mutations and keeps the species’ DNA healthy. This dual role explains why sex is nearly universal among complex life. Sex is an essential compromise. It lets life adapt, thrive and endure.