Birds reproduce sexually

Why sex?

Sex is not absolutely necessary for reproduction, as a look back into evolutionary history shows: Many organisms manage to reproduce without exchanging body fluids. Why sexual reproduction prevailed in nature is still not fully understood. The existence of the male sex in particular remains a mystery.

In the beginning there was no sex. Of course we would miss a lot without sex: If nature hadn't invented it, many of the most beautiful things in life - apart from sex itself - would not exist: Plants would not bloom, birds would not sing, the peacock would not turn a wheel and deer do not wear antlers. Men wouldn't drive sports cars and women wouldn't wear mini skirts. The enormous effort that is made in nature and in humans for sex seems paradoxical - measured in terms of the apparently low benefit. In addition, without sex and without two genders, things would become a lot easier: There would be no more rivals and no heartache, no pointless boasting and no stupid lies.

A look back at evolutionary history proves that life can also exist without sexuality. The first living things such as bacteria and blue-green algae, which emerged four to five billion years ago, reproduced asexually. And they still do it today. The advantages are obvious: A single parental organism copies its genetic information, divides and completely passes on its construction plans to the offspring. Thus, all of the progeny are genetically identical. The tedious search and choice of a partner becomes superfluous.

Evolutionary further development is of course also possible in this way: Occasionally, there is an accidental change in the genetic material. If such a "mutation" is advantageous, this clone can colonize a new habitat or adapt to other changes, for example through increased cold tolerance. In any case, the bacteria were doing extremely well with their strategy, and so it came about that hardly any place on earth remained free of them. One reason for this is their short generation times: some of these simple organisms can divide every twenty minutes.

99 percent sexualization. In the meantime, however, over 99 percent of all existing animal and plant species reproduce sexually, which means that they combine their DNA with that of a sex partner of their kind and thus generate genetically new offspring. But why did it happen one to two billion years ago that bacteria began to exchange their body fluids or their DNA via cytoplasmic bridges? So why did the recombination of genetic information between two organisms gradually establish itself, i.e. sexual reproduction?

As undisputed as the success of sexual reproduction may be in terms of evolutionary history, the justifications of biology are inconsistent and controversial. Which possibly has to do with the fact that in biology the importance of sexual selection was underestimated for a long time. Mainly because male biologists of the 19th century could not believe that female animals actively choose their sexual partners, explanations of sexuality in evolutionary terms remained in short supply for a long time. And to this day the belief is widespread that the only driving force of evolution is natural selection in the narrower sense - that is, the "survival of the fittest".

Competition of theories. Evolutionary biology has long since advanced to become a central research area in the sciences of life. And accordingly there are now more than twenty different and sometimes contradicting theories about why sex has prevailed in nature - contradicting also because the experimental data are still rather sparse. But evolutionary biologists have long been working on checking their theories with the help of molecular biology and complicated computer simulations.

Observations on living organisms are the exception to the rule, which is in the nature of things: "Evolution in the making" is an extremely lengthy process; only a few model organisms multiply so quickly that a researcher's life is sufficient to glimpse long-term evolutionary processes. One such organism is baker's yeast, which the American researchers Clifford Zeyl and Graham Bell examined more closely for their sexual and non-sexual life. They concluded that, over time, the sexually reproducing populations will accumulate few harmful mutations, while the asexual ones will degenerate more and more. The harmful changes are like in purgatory. Bad mutations would be weeded out before they get the better of a population.

Are Men Superfluous? The German zoologist Manfred Milinski is one of those researchers who use complicated computer simulations to track down the evolution of life and sexuality. When asked why animals and humans reproduce sexually, he too has no answer, as he frankly admits: "In particular, we cannot explain why the male sex exists. In theory, there would be no problem with females who are asexually, thus reproduce through parthenogenesis, "explains the director of the Max Planck Institute for Limnology.

Milinski came to this finding, which - at least theoretically - endangered the existence of himself and his co-workers by running programs on his computer that were based on a certain number of "genetically" different individuals that were supposed to multiply under certain environmental and competitive conditions. After a few virtual generations, it was possible to observe which of the original individuals in the predetermined environment had the greater success - that is, more viable offspring.

The surprising result: it was the asexuals who prevailed in Milinski's computer-generated tribal stories. In a direct comparison, it took hardly more than ten generations until the living beings that were intent on togetherness were by far outnumbered by the loners who reproduced by means of virgin generation. And a little later those individuals that formed pairs for reproduction were finally extinct.

Sex as an Adaptation Strategy? Now it is obvious that the actual evolution gives the virtual lie the lie. Then why did the computer model fail? It was simply not realistic enough, in other words: the computer simulation had not taken into account the changing living conditions enough. So the virtual environmental conditions, such as the climate, have slowly changed over several generations. And lo and behold: the faster the changes, the longer the group of sexually reproducing survivors - only to sooner or later die out at some point and leave the field to the loners.

The only exception: if there were drastic changes from one generation to the next, the asexuals suddenly had no chance. But where are similarly drastic upheavals outside of well-defined computer simulations? Where is the tropical climate in one generation and polar weather in the next?

Of course, it is not the climate that changes overnight, so to speak. Viruses and bacteria - all the causative agents of infectious diseases - are incredibly versatile and can be changed very quickly. It was the British biologist William Hamilton who came up with this sparkling idea back in the 1960s: What is needed is a highly flexible immune system that quickly recognizes foreign - and thus new foreign - as foreign and destroys it before it can cause damage to the body. The answer to changing environmental conditions and flexible parasites is called recombination: the mixing and random recombination of the genetic material of two individuals. Just sex.

Genetic arms race. A very similar explanation is the so-called Red Queen Hypothesis, which was formulated in the 1970s by the evolutionary biologist Leigh Van Valen. Parents with a "good" mutation each have "super fit" offspring that combine both advantages. Here, too, the success of an individual depends on how well they can fight off pathogens and parasites. As the attackers' strategies become more sophisticated, the immune system must also be flexible. There is a dogged arms race between parasites and their hosts.

Living beings that are better equipped against pathogens through genetic innovations therefore have higher chances of survival and reproduction. As a result, an initially rare defense pattern becomes more and more common, and the pathogens find a way to outsmart it again. The hypothesis about the genetic arms race of a species and its attackers takes its name from the Red Queen from "Alice in Wonderland": In the legendary land of the Red Queen you have to run to stay in the same place. If you want to get ahead, you have to be twice as fast.

Organisms that can reproduce both sexually and asexually - such as aphids - are in favor of answering the question "Why sex?" naturally of particular interest. In any case, New Zealand freshwater snails support the assumptions of the parasite theory: In densely populated lakes they reproduce sexually and thus produce new variants of the immune system. If, on the other hand, they live in waters with few parasites, they rely on asexual reproduction.

Immune to "sniff". Another proof that the development of sexual reproduction has something to do with the development of a highly flexible immune system was provided by the discovery of the MHC molecules, named after the so-called "major histocompatibility complex". These molecules are programmed by around a dozen genes, and the more colorful these gene variants are, the broader the spectrum of pathogens that can be responded to.

So if females knew which gene variants they themselves carry, they could proceed in a targeted, evolutionarily promising way when choosing a partner. And apparently they always do it automatically. US researchers provided the first evidence of this in experiments on mice, in which it was shown that one can "smell" immune genes. Female mice "recognize" the males' MHC gene variants by scents, and somehow they apparently "know" which ones they themselves carry. This makes the choice of partner, which is the most favorable in evolutionary theory, a lot easier. How exactly this smell, recognition and knowledge work is still largely unknown.