Why asexual reproduction is bad

In truth, however, sex does not always increase variation. Imagine, for instance, the simple case of a single gene that contributes to height in a diploid organism ; here, individuals with genotype aa are shortest, those with genotype Aa are of intermediate height, and those with genotype AA are tallest Figure 1. Now, for the sake of argument, imagine that the shortest individuals can hide safely, the tallest individuals are too big to be eaten by predators, and the intermediate-height individuals are heavily preyed upon.

Among those lucky few organisms who survive to reproduce, there will be a great deal of variation in height, with plenty of tall individuals and plenty of short individuals.

What would sex accomplish in this case? Here, mating would bring the population back to Hardy-Weinberg proportions, producing fewer offspring at the extremes of height and more offspring in the middle. That is, sex would reduce variation in height, relative to a population that reproduces asexually. Figure 1: Variability, built up by selection, is decreased by sex.

Because the fitness surface exhibits positive curvature, the result of selection is a population with a great degree of variability in height middle panel. Asexual reproduction in such a population preserves this variation bottom left , but sexual reproduction with random mating brings the population back into Hardy-Weinberg proportions and reduces variation bottom right.

This example illustrates the fact that sex does not always increase variation. Figure Detail. This example is overly simplified, but it serves to illustrate a general point: Selection can build more variation than one would expect in a population in which genes are well mixed.

In such cases, sex reduces variation by mixing together genes from different parents. This problem arises in the case of a single gene whenever heterozygotes are less fit, on average, than homozygotes. In this case, the heterozygote need not have the lowest fitness ; rather, its fitness must only be close to that of the least-fit homozygote.

In general, mathematical models have confirmed that selection builds more variation than expected from randomly combined genes whenever fitness surfaces are positively curved, with intermediate genotypes having lower-than-expected fitness. In such cases, sexual reproduction and recombination destroy the genetic associations that selection has built and therefore result in decreased rather than increased variation among offspring. The term " epistasis " is used to describe such gene interactions, and cases in which the intermediate genotypes are less fit than expected based on the fitness of the more extreme genotypes are said to exhibit "positive epistasis.

Interestingly, even when sex does restore genetic variation , producing more variable offspring does not necessarily promote the evolution of sex. Again, this reality refutes one of the arguments often raised in the attempt to explain the relationship between sex and evolution.

To understand how this operates, consider another simple case involving a single gene, but this time, assume that heterozygotes rather than homozygotes are fittest. The gene responsible for sickle-cell anemia provides a great real-life example.

Here, people who are heterozygous for the sickle-cell allele genotype Ss are less susceptible to malarial infection yet have a sufficient number of healthy red blood cells; on the other hand, SS homozygotes are more susceptible to malaria, while ss homozygotes are more susceptible to anemia. Thus, in areas infested with the protozoans that cause malaria, adults who have survived to reproduce are more likely to have the Ss genotype than would be expected based on Hardy-Weinberg proportions.

In such populations in which heterozygotes are in excess, sexual reproduction regenerates homozygotes from crosses among heterozygotes. Although this indeed results in greater genetic variation among offspring, the variation consists largely of homozygotes with low fitness. Yet again, this simple example illustrates a more general point: Parents that have survived to reproduce tend to have genomes that are fairly well adapted to their environments.

Mixing two genomes through sex and genetic recombination tends to produce offspring that are less fit, simply because a mixture of genes from both parents has no guarantee of functioning as well as the parents' original gene sets.

In fact, mathematical models have confirmed that when selection builds associations among genes, destroying these associations through sex and recombination tends to reduce offspring fitness. This reduction in fitness caused by sex and recombination is referred to as the "recombination load" or the " segregation load" when referring specifically to segregation at a single diploid gene. The reason that the recombination load is a problem for the evolution of sex is better appreciated by looking at evolution at the level of the gene.

Imagine a gene that promotes sexual reproduction, such as by making it more likely that a plant will reproduce via sexually produced seeds as opposed to some asexual process e. Carriers of this gene will tend to produce less fit offspring because sexual reproduction and recombination break apart the genetic associations that have been built by past selection.

The gene promoting sex will fail to spread if the offspring die at too high a high rate, even if the offspring are more variable. Indeed, theoretical models developed in the s and s demonstrate that genes promoting sex and recombination increase in frequency only when all of the following conditions hold true:.

Unfortunately, empirical data have not indicated that fitness surfaces curve in just the right way for these models to work in real-life situations. To make matters worse, sexual reproduction often entails costs beyond the recombination load described earlier.

To reproduce sexually, an individual must take the time and energy to switch from mitosis to meiosis this step is especially relevant in single-celled organisms ; it must find a willing mate; and it must risk contracting sexually transmitted diseases. This last cost is often called the "twofold cost of sex. These are substantial costs—so substantial that many species have evolved mechanisms to ensure that sex occurs only when it is least costly.

For instance, organisms including aphids and daphnia reproduce asexually when resources are abundant and switch to sex only at the end of the season, when the potential for asexual reproduction is limited and when potential mates are more available.

Similarly, many single-celled organisms have sex only when starved, which minimizes the time cost of switching to meiosis because mitotic growth has already ceased. Asexual organisms are subject to many outside and predatory forces. However, this type of reproduction process can endure better than sexual reproduction. While humans and mammals continuously reproduce sexually, not everyone is going to find a mate.

Also, many humans and mammals usually have a limited number of offspring. They are more concerned about survival. Asexual organisms have an advantage because they can perform various functions to improve their ability to survive and thrive.

Asexual beings do not require mobility to find a mate. Humans and mammals generally have to leave home to find a mate. This is a part of the process of acquiring a mate to reproduce with. Asexual organisms do not have this problem. One of the reasons why humans engage in so much sex is because they have a stable environment. This is also true for a lot of animals. It is a proven scientific fact that people reproduce less when fighting for survival or living in harsh conditions that make it hard to survive.

Also, pregnant women under a lot of stress, discomfort, and chaos run an extremely higher risk of losing their children. These organisms can make necessary adjustments to thrive in different conditions.

Here is another thing that asexual reproduction offers to people. Human beings can have babies, but they are also required to provide lots of energy, money, resources, and time in raising their children.

This is not a requirement for asexual life forms. They can carry out their reproduction functions and then leave their offspring to grow and develop independently.

Not all asexual organisms take this approach with their offspring, but most do. They realize that their offspring will survive and find a way to exist. This is an important part of the reproduction process. Humans are notoriously picky about whom they are going to have children. While this might be a wise thing to do in some circumstances, many people often waste their reproductive years because of their strict requirements for a mate.

Asexual beings are not aware of other members of their species in terms of reproduction. Since its self-contained, it keeps the species moving forward without wasting time trying to find the perfect mate.

Asexual reproduction is functional. It is an easy process that allows lifeforms to keep reproducing themselves without dying out. This type of sexual reproduction allows life forms to live long and flourish. Asexual reproduction is also a good thing for predator species. They provide a great source of nutrition for these creatures.

Sexual reproducers that are used for food take time to produce new offspring. Asexual beings can reproduce in living conditions that would be difficult for other species to survive. Certain asexual plants, organisms, and bacteria can survive within marine environments where it would be impossible for people to live.

This way of reproducing is only done among organisms that often stay in one place. They do not need to move to other places just to produce offspring. Most animals and plants that rarely move to other places are extremely able to create their offspring. Just little time and resources are used. Much time and energy is not needed to produce offspring asexually. As you can see, certain asexual animals and plants could reproduce without considering the amount of time or energy to be consumed.

It is more environmentally friendly. When it comes to asexual reproduction, there are no concerns with regards to the environment. But with sexual reproduction, organisms may not survive when they deal with harsh environments.

Because of the susceptible or delicate organs or stages of sexual reproduction make it impossible for organisms to live. Reproduction will be boosted. When certain asexual organisms have been established in suitable habitats, they can reproduces rapidly to make more new individuals.

It requires just a little investment. Those that reproduce asexually can produce more than one offspring at a time and do not have to carry their offspring for long periods of time. This quick and inexpensive process means a little investment for time and money. It can hinder diversity. Take note that asexual reproduction does not have genetic diversity. Finding a mate can be very difficult, especially for organisms that are living in desolate environments, such as deep ocean ecosystems.

Now, asexual reproduction takes the need to find a mate away, allowing organisms to multiply on their own when it is time. It is friendly to the environment. When it comes to this form of reproducing, there are no concerns with regards to its impact on the environment. On the other hand sexual reproduction would cause some organisms not to survive in harsh environments due to their susceptibility, fragile stages in the process and their delicate organs.

It is pretty handy in case of emergency. In difficult situations, asexual plants and animals are still able to keep themselves alive and continue to produce offspring without other reproductive sources. Basically, there is no big problem regarding environmental emergencies when it comes to asexual reproduction. It does not require any true investment.

Those that reproduce asexually do not have to carry their offspring for long period of time, unlike those that breed through sexual reproduction, which are also mostly limited to one offspring. As you can see, there is no need for much energy and time to produce offspring, plus certain asexual plants and animals can even produce as many clones or offspring as they can without considering any investment to make on your end. Moreover, this process of reproducing has not been complex, which only requires less energy compared with its counterpart.

It hinders diversity.