Sex is Evolution's Greatest Success

When a human child is born, it is only one of the billions of new organisms being produced every second on our planet. A prerequisite for us humans to exist and create another specimen of our kind is that all other organisms do the same. If crops, gut bacteria, mangrove trees, or earthworms did not constantly reproduce, there would be no life on Earth.

Among all methods of reproduction – and there are surprisingly many – sexual reproduction is the most sophisticated. In sexual reproduction, the female gametes (eggs) must be fertilised by the male gametes (sperm) for reproduction to occur. This act of mixing genetic material from two parents is called evolution's greatest success.

However successful the method is, it is also costly. Reproducing sexually is neither easy, safe, nor cheap.

Dies after 2 weeks of constant copulation

Just think of the female praying mantis that bites the head off her partner after mating. The male bee that dies immediately after ejaculating into the queen bee. The males of mouse-like marsupials called Antechinus that engage in continuous copulation for up to 14 hours a day for about two weeks until they literally disintegrate and die. The female California fiddler crabs that inspect up to 100 of the burrows males dig to impress them before choosing where to mate and lay their eggs.

Why does sexual reproduction have to be so costly, and why are some willing to pay with their lives to create offspring? According to population biologist Owen Jones from the Biological Institute, it makes sense:

- Yes, it is complicated to find a mate, attract the mate and defend your chances to mate against competitors. You may need displays, strong muscles, horns for weapon and thicker skin for defense. But it is worth the effort because it ultimately increases the chance of getting a mate and not being outcompeted by others. The competition is tough, even among the same species, he says:

Sex is also a defense against disease

- Sexual reproduction has the great advantage of ensuring genetic variation in a population. It produces offspring born with a larger range of genetic possibilities that provide a diverse toolkit for adaptation as conditions – such as climate – change. Offspring from sexual reproduction will be born with different palettes of genetic possibilities, increasing the likelihood that some of them can cope with the challenges that life presents.

Owen Jones also points out that sexual reproduction accelerates evolution; the more diversity in the genetic material, the faster a new trait can evolve. This is especially important when a population is affected by disease or parasites.

- In that light, you could say that sex evolved as a defense against disease, says Owen Jones.

Females don’t need males to reproduce

There are easier and less costly ways of reproducing, but they are not as widespread in more complex organisms as sexual reproduction. Asexual reproduction  is characterized by not requiring two parents.

For example, bacteria can reproduce by dividing. Among algae, it is common for the parent algae to let its cell nucleus divide, creating new cell nuclei. Many plants reproduce without pollination; for example, strawberry plants form runners that take root and become new plants, and tulips form new bulbs.

In the animal kingdom, it is not as uncommon as one might think for females to reproduce without help from a male. This has been observed in numerous invertebrates (animals without bones) and in about 80 vertebrates (animals with bones). It is most often seen in fish, amphibians, and reptiles – it is actually quite common, and the offspring becomes viable and capable of reproducing themselves.

For many, this is completely normal

This kind of reproduction is called parthenogenesis and is also known as virgin birth. Here, the female's egg develops into an embryo without the contribution of sperm. The offspring is not an exact clone of the mother but is genetically very similar, and the offspring will always be females.

- Most of us would probably find parthenogenesis completely crazy, but for many species, it is completely normal, says Owen Jones.

While some animals always use parthenogenesis, others alternate between sexual reproduction and parthenogenesis, and finally, there are animals that can use parthenogenesis as an absolute last resort if the female cannot find a male to mate with.

No males before autumn

- The aphid is an example of an animal that alternates. During the summer, they reproduce exclusively by parthenogenesis, and for 6-7 generations over the summer, there are only females. But when autumn comes, they switch to mating with males, says Owen Jones.

Their summer parthenogenesis can occur faster than sexual reproduction, and since there is plenty of food available in summer, the aphids use the summer to reproduce explosively, accepting that the genetic variation is low.

When autumn comes, they again pay attention to genetic variation and the females begin to produce both female and male offspring. During autumn, these mate and produce eggs that overwinter until the following spring, when they all hatch as females, and the cycle starts over.

Virgin born crayfish all over the place

In 1995, a crayfish gained attention because it unexpectedly reproduced in an aquarium in a living room in Germany. It turned out that the crayfish, a female marbled crayfish, had produced hundreds of eggs and fertilized them herself.

The living room in Germany was not the only place, that suddenly swarmed with small crayfish; marbled crayfish females now also live in many lakes across Europe, where they thrive so well that researchers are concerned they will displace other species.

So far, it seems that nothing can stop the march of marbled crayfish females, but it may only be a matter of time: They are, all things considered, more vulnerable to diseases because they have lower genetic diversity in their genetic material.

Mammals are in a league of their own – but then scientists created this mouse

While parthenogenesis is common among fish, reptiles, etc., it does not work for mammals. Mammals need DNA from both a father and a mother for genomic imprinting, which ensures that specific genes are active only if inherited from a particular parent, and its disruption can prevent successful reproduction.

However, the world knows of a litter of mice that was created by parthenogenesis. This happened in 2022 at Shanghai Jiao Tong University, where researchers manipulated the genomic imprinting in the genes of a female mouse's egg so that the eggs eventually resembled eggs fertilized by a male mouse.

The eggs were then placed in a female mouse's uterus, where they continued to grow. All in the litter survived birth, and one offspring even reached adulthood and had offspring of her own.

Genetic variant for virgin birth has been found

Researchers from the University of Cambridge have also created artificial parthenogenesis in the laboratory. This happened after they found the gene variant that controls parthenogenesis in a specific species of fruit flies and inserted it into another species of fruit flies that normally mate through sexual reproduction.

The result: As long as there were enough males, the genetically modified females preferred to mate. But when researchers restricted the females' access to mates, they switched to parthenogenesis if a male did not appear in the first 40 days of their approximately 80-day lifespan. The researchers also found that the ability to undergo parthenogenesis was inherited by the next generation.

- Parthenogenesis can have its advantages – in the short term. It is a "cheaper" and less resource-intensive way of reproducing, and it can lead to population growth. But it also provides the offspring with less flexibility and fewer options to cope with environmental and climate changes, posing a long-term threat to the population, says Owen Jones.

Fruit fly, human, mouse, bacteria or algae: Whether we reproduce sexually or asexually, "completely crazy" or "normal", all organisms contribute to a continuous renewal of the diversity of life that exists on Earth. Just think of all the little new ones that have come into the world while you've been reading this article.