All single-celled organisms, both eukaryotes and prokaryotes, reproduce by simple division. An individual that replicates by fission cannot be said to "die" if its offspring live on. Individuals can die, as a result of a hostile environment or predation, but reproduction by fission affords a kind of "immortality" for the few. When the first multicellular eukaryotes appeared, between 1,500 and 1,000 million years ago, they probably reproduced by shedding parts of themselves. Each portion grew into a new individual genetically identical with the parent. (Spores are a specialised way of doing this.) Again, the genome continued; "immortality" of a sort. This is not true of sexually reproducing organisms. They all die.
Why did sex evolve? For a prokaryote with only 1000 or so genes and a very fast replication rate, sex is unnecessary. Even if the mutation rate is high or environmental stresses mount, some progeny will survive. However, a eukaryotic cell with tens of thousands of genes reproduces slowly. In this situation, a high mutation rate or a stressful environment is likely to terminate the lineage. Sex insures against this eventuality in at least two ways.
First, males redistribute genes among otherwise "all-female" lineages, mixing the genomes and generating many variants, giving the species a better chance of surviving environmental change. Males redistribute genes in the gene pool rather as taxation and public spending redistribute money in society. Sex only works within species; that is, among organisms with essentially the same genomes. Second, because sexual reproduction fuses the genomes of two individuals, it ensures that every individual has two copies of nearly every gene. As we saw in chapter 11, this insures against potentially damaging mutations.
However, these insurances are long-term. They are advantageous only over many generations. Natural selection operates on immediate advantages, not long-term ones. It concerns the current generation, not the future. So how and why did sex evolve? Perhaps it first appeared at a time of high environmental stress, when conditions were changing rapidly. For an organism that contrived to reproduce both asexually and sexually, the sexual offspring - comprising more variants - might have had an advantage over the less varied asexual ones. They might have had a better chance of survival in the changing world.
But this still begs the question of how sex originated. One idea, favoured by Margulis and others, is that it resulted from incomplete cannibalism. A protist devoured another protist of the same species, but the nucleus of the prey survived and fused with that of the predator. Such events can be observed today, so the idea is not implausible. Nevertheless it is a big step from such a coincidental beginning to the formation of specialised reproductive cells, which are found in all sexually reproducing multicellular organisms today. These reproductive cells continue the lineage; the individual that houses them ages and dies.
Sex in humans and some other primates is chromosomal. It depends on whether the individual carries two X chromosomes (female) or one X and one Y (male). The X chromosome has several thousand genes, the Y chromosome only a few dozen. These few dozen include specifically male genes, such as those needed for manufacturing sperm, but the Y chromosome also has 19 genes in common with X. These 19 are found in four groups on the X chromosome. Comparative genetic mappings of several species has shown that chromosomal sex evolved in four stages, the first occurring around 300 million years ago and the most recent around 40 million years ago, when the ape-monkey line parted evolutionary company with the ancestors of the lemurs.
In most animals and plants, sex is not fixed by the presence or absence of a Y chromosome. In many species of fish, for instance, one individual in a school becomes male and the rest remain female; if the male dies, another individual becomes male instead. Change the temperature just a few degrees and young salmon develop as females rather than males.
Why did sex originate? Why, in species such as ours, did it become chromosomal? We can only speculate, but attempts to answer these questions have led to new insights. In particular, the discovery that evolution can occur in jumps, as in the evolution of the Y chromosome, shows that it is not always the steady gradual process that Darwin originally envisaged.
One of the most important points about sex is that it allows more complex organisms to flourish: it increases the redundancy of the genome and thereby stabilises it. The more complex the organism, the bigger the genome. The bigger the genome, the higher the risk of fatal damage to the DNA between one generation and the next. The faster the accumulation of DNA damage, the faster the extinction of the species. It is because sex insures against mutational damage that it allows complex organisms to survive. There is a theoretical limit, beyond which any further increase of complexity would over-stretch the DNA repair machinery, but perhaps only organisms as complex as ourselves approach that limit.
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