Why can't humans just reproduce asexually?

The reproductive strategies of animals

Asexual reproduction

In principle, the scientists differentiate between asexual and sexual reproduction. Asexual reproduction is probably the most original form of reproduction and occurs mainly in unicellular organisms. With their help, the tiny organisms can quickly adapt to changing environmental conditions.

The principle of this type of reproduction is always the same: the single-celled cells simply duplicate their genetic material and then distribute it to two cells. One becomes two, four, eight, ... organisms in no time at all. Special germ cells are not necessary for this.

The advantages

This type of reproduction has advantages. On the one hand, the offspring of asexually reproducing organisms are identical copies of the parent cell - apart from small genetic changes (mutations) that can later change their genetic material individually. Tried and tested gene combinations are retained in the next generation.

On the other hand, this type of reproduction is time-saving: no special sex cells have to be produced and there is no need to search for a partner. In this way, single-celled organisms can react quickly to current environmental changes, such as a heavy downpour, and increase their population density - for example in the puddle that has just formed - from now on.

It works the other way around, of course, in that the organisms simply stop dividing when the drought continues, and so their population shrinks. The good thing: Even with extremely few individuals of a species, this type of reproduction still works.

The disadvantages

As simple and useful as asexual reproduction may be, it has a major catch: the genetic material always remains the same over generations. Grandmother, mother and daughter are genetically identical as there is no mixing of genes.

Instead, the organisms clone themselves over and over again. So there are hardly any individual differences within a population, i.e. organisms that are particularly good at dealing with heat or with a lot of rain.

This restricts asexually reproducing organisms in their adaptability and also makes them easy prey for attackers - because what kills one member of the population also works for the others.

Single-sex reproduction

When animals reproduce sexually, they usually carry out what is known as parthenogenesis or virgin generation. The offspring are created from unfertilized egg cells of the females.

With this type of reproduction, special germ cells are required, but a sexual partner is superfluous - just as with asexual reproduction. Instead, hormones trick the unfertilized egg cells into a fertilization situation, whereupon they begin to divide. This creates new organisms in the female body.

Aphids, as well as some fish and lizard species, water fleas, bees and wasps reproduce unisexually. However, the females only temporarily do without the males and sooner or later also engage in bisexual reproduction.

Bisexual reproduction

Most animal species use bisexual reproduction. It is particularly widespread among the more highly developed organisms, as it offers the possibility of creating new and possibly more advantageous gene combinations for the current environmental conditions.

For bisexual reproduction, two different germ cells have to come together, for example egg cell and sperm cell. The offspring therefore receive genes from both parents. Only when these merge can offspring arise.

As a rule, two different types of germ cells (male and female) are involved in bisexual reproduction, but externally these can be the same. Sometimes the two sex cells come from the same individual. In such a case one speaks of autogamy or self-fertilization.

The advantages

The almost limitless possibilities of recombining genes are considered an evolutionary advantage of bisexual reproduction. This means that the offspring do not exactly match one of the parents or one of the siblings in their blueprint. Instead, they are endowed with individual features.

As a result, they can, for example, (by chance) be better adapted to climatic changes and their consequences than their family members. This may make them more resilient and thus more successful in their own reproduction - and only those who reproduce can survive.

The (genetic) diversity within a species also makes it less sensitive to enemies and pathogens, as viruses or bacteria, for example, have to come up with a tailor-made attack for practically every individual.

In addition, every individual who results from bisexual reproduction has two genes of the same type (one from each germ cell), so that a possibly disadvantageous change in one gene does not always have to come into play. Its function can be replaced by that of the second gene.

The disadvantages

In order to reproduce as two sexes, two individuals of different sexes must meet - with the exception of hermaphrodites. This search for a partner is often energy and time consuming and is often unsuccessful, especially with small populations, since the sexual partners simply do not meet. In extreme cases, the restriction to bisexual reproduction can lead to the extinction of a species.

Author: Lena Ganschow

Status: 20.06.2017, 12:00