Sexual reproduction sets in when resources become scarce. Under a wide range of conditions, the sexual species outcompete the asexual ones. Asexual species win only when survival conditions are harsh and death rates are high, or when resources are so little structured or consumer genotypes are so manifold that all resources are exploited to the same extent. ~ German evolutionary zoologists S. Scheu & B. Drossel
The first sex happened 1.2 billion years ago. Its evolution owes to a viral innovation.
Early on viruses developed a protein that lets 2 cell membranes fuse into 1, allowing combination of genetic material. This technique is essential for both viral and sexual reproduction.
Nature has a limited number of ways it can cause cells to fuse together into a single cell. The protein that first made sex possible – and is still used for sexual reproduction in many of Earth’s organisms – is identical to the protein used by certain viruses to enter human cells. This protein must have really put the spice in the primordial soup. ~ American cytologist William Snell
Having been handed the essential tool, early cells went their own way in becoming sexy. Sex chromosomes independently evolved in different lineages of eukaryotes.
In evolutionary terms, sex has been habit forming. Most protists are into sex, as are fungi, plants, and animals.
A few stopped having sex, including a class of freshwater rotifers (Bdelloidea). Despite parthenogenetic reproduction, these microscopic rotifers have evolved for tens of millions of years, differentiating into over 500 species, many with very distinct bodies. Bdelloidea demonstrate that sex is only one route to diversity.
Bdelloidea can survive desiccation: a necessity if you plan on living past your puddle drying up. Losing all the moisture in your body taxes the genome. Once wet again, Bdelloidea are able to repair the inevitable DNA damage from being bone dry.
This gene-patching skill is how Bdelloidea manage their multiplicity. Like viruses and prokaryotes, Bdelloidea are mavens of DNA manipulation. These rotifers put horizontal gene transfer to practical effect.
Parthenocarpic plants reproduce vegetatively and can produce fruit without fertilization. Sharks normally sexually reproduce but can dispense with the sex to parthenogenetically reproduce.
Sex is expensive. It necessitates additional organs which require developmental resources, and, more pressingly, taxes organisms with energies spent in mating and breeding.
Sex evolves when the need for adaptability is high, as sex more actively shuffles the genetic deck than asexual reproduction. Greater diversity in offspring raises the odds that some may survive to reproduce.
Variety’s the very spice of life, that gives it all its flavour. ~ English poet William Cowper