It hinders diversity. Since the characteristics and traits of only one parent is passed on to its offspring, asexual reproduction would hinder genetic diversity all of its generations. This causes for the population of the organisms to be exactly identical. With sexual reproduction, this has been a huge advantage, as we are able to mix gene pools to ensure a diverse ecosystem.
It poses some inheritance issues. Most of the time, it would only require a single asexual parent from which we can copy chromosomes and genes, which means the genetic defects or mutations that are bred out in asexual reproduction would still exist in the offspring without any exception.
This disadvantage can even lead to more unfavorable mutations, which make asexually produced organisms susceptible to diseases, which also means a huge number of offspring would be destroyed.
It can lead organisms to being prone to extinction. All of the same traits and characteristics also entail all of the same weaknesses, so we can assume that a certain parasite or predator that has evolved to kill a particular asexual organism will be able to take out its entire population.
In simple terms, asexual reproduction can lead to struggle for existence. It carries problems with population control.
Basically, this form of reproduction has no control over the rapid increases of population of the subject organisms. As competition in the breeding process does not exist, each organism is highly proficient in reproducing by itself, which means that its population will even be doubled in each reproductive cycle. During the vegetative period of their life cycle which may be as long as years in some bamboo species , these plants may reproduce asexually and accumulate a great deal of food material that will be required during their once-in-a-lifetime flowering and setting of seed after fertilization.
Soon after flowering, these plants die. Polycarpic plants form flowers many times during their lifetime. Fruit trees, such as apple and orange trees, are polycarpic; they flower every year. Other polycarpic species, such as perennials, flower several times during their life span, but not each year. By this means, the plant does not require all its nutrients to be channelled towards flowering each year.
As is the case with all living organisms, genetics and environmental conditions have a role to play in determining how long a plant will live. Susceptibility to disease, changing environmental conditions, drought, cold, and competition for nutrients are some of the factors that determine the survival of a plant. Plants continue to grow, despite the presence of dead tissue such as cork.
Individual parts of plants, such as flowers and leaves, have different rates of survival. In many trees, the older leaves turn yellow and eventually fall from the tree.
Leaf fall is triggered by factors such as a decrease in photosynthetic efficiency, due to shading by upper leaves, or oxidative damage incurred as a result of photosynthetic reactions. The components of the part to be shed are recycled by the plant for use in other processes, such as development of seed and storage. This process is known as nutrient recycling. The aging of a plant and all the associated processes is known as senescence , which is marked by several complex biochemical changes.
One of the characteristics of senescence is the breakdown of chloroplasts, which is characterized by the yellowing of leaves. The chloroplasts contain components of photosynthetic machinery such as membranes and proteins. Chloroplasts also contain DNA. The proteins, lipids, and nucleic acids are broken down by specific enzymes into smaller molecules and salvaged by the plant to support the growth of other plant tissues. The complex pathways of nutrient recycling within a plant are not well understood.
Hormones are known to play a role in senescence. Applications of cytokinins and ethylene delay or prevent senescence; in contrast, abscissic acid causes premature onset of senescence.
Many plants reproduce asexually as well as sexually. In asexual reproduction, part of the parent plant is used to generate a new plant. Grafting, layering, and micropropagation are some methods used for artificial asexual reproduction.
The new plant is genetically identical to the parent plant from which the stock has been taken. Asexually reproducing plants thrive well in stable environments. Plants have different life spans, dependent on species, genotype, and environmental conditions. Parts of the plant, such as regions containing meristematic tissue, continue to grow, while other parts experience programmed cell death.
Leaves that are no longer photosynthetically active are shed from the plant as part of senescence, and the nutrients from these leaves are recycled by the plant. Other factors, including the presence of hormones, are known to play a role in delaying senescence.
Asexual reproduction results in plants that are genetically identical to the parent plant, since there is no mixing of male and female gametes, resulting in better survival. The cuttings or buds taken from an adult plant produce progeny that mature faster and are sturdier than a seedling grown from a seed. Asexual reproduction in plants can take place by natural methods or artificial methods. Because the reproductive process is easier to complete, for many asexual organisms, it happens more often than with sexual reproduction.
This means population numbers for a species can increase at a dramatic rate, especially when there are favorable environmental conditions which support the reproductive cycle.
Add in the fact that there is no competition for breeding and the possibility of the population of an organism doubling with every reproductive cycle becomes a possibility. There can be an inability to adapt.
Asexual organisms are not always able to adapt to a changing environment or habitat. This is especially true if there is some sort of predator or disease which can develop the ability to seek and destroy the asexual organism.
With its limited evolutionary access, any evolution that targets the organism could destroy the entire species in a short amount of time. Overcrowding can be a real issue. One parent can produce a high number of offspring in a limited period. As each generation progresses to the next, more organisms than what the environment can support may become a possibility.
Overcrowding creates a lack of resources that could stop the organism from future growth. Population levels will stabilize to support a maximum number of organisms, but that comes at the expense of starvation. Reproduction can create competition. Some forms of asexual reproduction create offspring that are in close relation to one another. Because they are so close together, a competition for resources begins.
Although food is an important resource, there are also space considerations in play for some species as well. Once change can eliminate an entire species. If the conditions of the environment around the colony were to change, the entire species could be eliminated. There are limited movement capabilities within most asexual species, which means the survival of many species are not fully in their own control.
Pest resistance is minimal with asexual reproduction. Plants that are grown through an asexual reproductive cycle tend to be less likely to resist pests that may be within the environment. Although injury or loss can be quickly replaced because of the speed and low energy requirements of this type of reproduction, the ongoing threat to species health can reduce crop yields, create poor quality crops, or produce additional health issues that can affect other species — or even people.
Asexual organisms typically have lower lifespans. The crops which are created through an asexual reproductive cycle have a lifespan that is usually shorter than plants that propagate through a regular sexual process.
For other crops, like an orchard, this is not the case. It is an expensive process. Indeed, researchers estimate that over But why is sexual reproduction so commonplace? People typically employ several arguments in their efforts to explain the prevalence of sexual reproduction. One such argument is that organisms engage in sex because it is pleasurable.
However, from an evolutionary perspective, this explanation arrived only moments ago. The first eukaryotes to engage in sex were single-celled protists that appeared approximately 2 billion years ago, over 1.
These bacteria as well as their modern counterparts engaged in genetic exchange via processes such as conjugation , transformation , and transduction , all of which fall under the umbrella of parasexuality. Surely, pleasure was not in a bacterium's realm of experience. A second, more serious argument is that sex generates variable offspring upon which natural selection can act.
This is one of the oldest explanations for sexual reproduction, tracing back to the work of German biologist August Weismann in the late s. Although this explanation may very well account for why sexual reproduction is so commonplace, the explanation is far more subtle than many people realize for two reasons.
First, sex does not always increase the variability among offspring. Second, producing more variable offspring is not necessarily favorable. In the next two sections, we describe these flaws in Weismann's explanation for sex, so that we can better understand the processes that help and those that hinder the evolution of sex.
To develop a better understanding of why sexual reproduction is so commonplace, it is helpful to start with an examination of some of the most common erroneous beliefs regarding the relationship between sex and natural selection , including those described in the following sections.
Many people assume that sexual reproduction is critical to evolution because it always results in the production of genetically varied offspring. 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.
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