Why do some people call Evolution EVILution

Under sexual reproduction we generally imagine the fusion of a small sperm with a much larger egg cell to form what is known as a zygote. It is used to mix the genetic information of two individuals who do not need this size difference in the germ cells in any way. In fact, this difference did not exist in the early history of evolution: The first sexual processes took place between individuals who had germ cells of roughly the same size - which can still be observed today in many algae and fungi.

Small sperm, large eggs
In nature, however, there are hereditary differences between individuals in almost all characteristics. So there were individuals who distributed their precious building materials for the production of germ cells to a few more cells, which, however, were smaller as a result. These individuals, which we might call protomales, had a small numerical advantage when it came to colliding and fusing with germ cells from others; so they spread. One would expect this type of selection, but it would have to result in all individuals in a population producing smaller germ cells. We have to consider another factor here. The zygote resulting from the merger must contain a certain amount of building materials so that it can develop optimally. Fusions between two small germ cells resulted in smaller individuals with reduced chances of survival. It was therefore worthwhile for protomales to avoid such fusions and instead to specialize in fusions with larger germ cells. Soon the small cells were so numerous that two large cells rarely fused, and the increasing competition between these protosperms also favored the faster ones among them. The other individuals, on the other hand - let's call them proto-females - were increasingly forced to provide the building materials for the zygote on their own. This division of labor can definitely be interpreted in such a way that the proto-males with their smaller germ cells as parasites profited from the investment of the proto-females. Sperm and eggs, and thus also the male and female sex, were created! In fact, there is no more fundamental characteristic for differentiating between the sexes than the size of the germ cells: the male sex makes the small cells, the female the large cells. The differences in size of the cells in animals can literally be astronomical: while an ostrich egg weighs around 1.5 kilos, the sperm of an ostrich is only about as long as a hair is wide - a difference that is more than ten billion times that is significantly greater than that between the sun and the moon. But there are also surprisingly large sperm cells. The record is held by the males of the fruit fly Drosophila bifurca, which, although only three millimeters long, produce six centimeters long sperm. Nevertheless, researchers estimate that in this fly, too, an egg represents an investment that is around six times greater than that of a sperm.

The small size of the sperm has led many biologists to suggest that reproduction is cheap for males. A single male can potentially fertilize many females, and we may wonder what the optimal ratio of the sexes is. Shouldn't there be many more females than males in many animal species? It is true that every sexually reproducing individual arises from the merging of two mostly very different germ cells, which probably also make a rather unequal contribution to building materials. But both germ cells contribute an identical proportion of both paternal and maternal genes to the zygote: each individual has exactly one father and one mother. This means that the entire reproductive success is always shared fairly between the sexes - even though the material contribution of the male, at least potentially, can be very small. As a male you can, at least in theory, gain a lot with little investment. Darwin was well aware of equality in reproductive success between the sexes, but he wasn't sure how it would affect the relationship between the sexes. It was the German biologist Carl Düsing who, with the help of what was probably the first mathematically derived evolutionary rule, was able to show in 1884 that equality in reproductive success automatically leads to a balanced relationship between the sexes. If, for example, there are more females than males in a population, mothers who produce an excess of sons due to a genetic predisposition have a higher reproductive success. This predisposition would thus spread out evolutionarily until the success of sons and daughters is the same again with a balanced relationship between the sexes. The males have to divide the theoretically high profit among themselves somehow. For example, the sex ratio in the deer is close to 1: 1, even though a single successful top deer can potentially fertilize a large number of females. For every successful stag there are many unsuccessful ones who either lost in the fight against a top dog or were spurned by the females. How unequal the distribution of reproductive success between males and females is and what factors influence this distribution is an important aspect of research on sexual selection.

How does a hermaphrodite feel?
The animals we are familiar with, such as insects, birds and mammals, are so-called gonochorists, in which all individuals spend their entire lives as the same sex: whoever is born as a male, dies as a male. This way of distributing the sexes among the individuals of a population also corresponds to the idea that most people have of sexual reproduction - but it is by no means the only possible one. We find other types of gender distribution in hermaphrodites, in which individuals either wear both sexes at the same time (simultaneous hermaphrodites) or can decide whether and when to change sex (sequential hermaphrodites). The latter usually begin their life either as males or females and change their sex only once in a lifetime, while a few sequential hermaphrodites, such as certain reef fish, can change sex several times. Since hermaphrodites have both sexes, the question arises whether and how they will make a decision about how much energy to invest in their male and female gender function. Does this depend on the availability or the attractiveness of the partners? And how does a hermaphrodite actually feel? Is he driven by his male side to search for a partner, while his female side thinks carefully about who she is getting involved with? Or are there such gender roles in these animals at all? Our alienation towards hermaphrodite animals probably results from our own, innate or acquired gender roles, which we often project onto them when we look at animals. We don't do this well with hermaphrodites. In the case of a flower, on the other hand, we are not particularly surprised to see both Darwin and the Evolution UNI NOVA 111/2009 19 the male stamens and a female pistil. Since we usually do not associate any behavior with plants, we do not resist their hermaphroditism. In addition, it has become clear in recent years that hermaphrodites have their impressive, so far only sparsely investigated peculiarities when they reproduce. If two hermaphrodites give each other sperm, but both prefer not to receive sperm, then an interesting cooperative conflict arises, the solution of which one can often only wonder. Hermaphrodites repeatedly force us to question our preconceived notions about the sexes. An understanding of their peculiarities will allow us to gain a new perspective and thereby a deeper insight into the evolution of the sexes.