Sex is risky. In the short term, it even seems detrimental to survival. So then why do most organisms reproduce sexually rather than asexually?
Was it the lack of sex that led to Paranthropus robustus‘ demise?
Reproduction – the process of producing offspring from one or more “parent” organisms – is essential for all life on Earth and without it no species would be able to survive over time. The majority of organisms reproduce sexually, meaning that they produce gametes – cells that contain half of an individual’s genetic information – which need to be fertilised by another gamete in order to grow into a new individual. Sexual reproduction most often involves two individuals of different sexes, each contributing one gamete. However, some organisms such as Angiosperms reproduce by self-fertilisation, which is the fusion of two gametes which have been produced by the same individual. Asexual reproduction is the process whereby all prokaryotes, as well as protists and some other eukaryotes, produce offspring by creating genetically identical copies of themselves. There are various different ways of reproducing asexually, the most common being fission, which is when a cell replicates its DNA and then divides so that two or more daughter cells are formed from the original cell. Budding, another form of asexual reproduction, is carried out by organisms including the yeast fungi and the animal genus Hydra. Budding involves a small growth developing on a parent cell. The parent cell’s nucleus containing its genetic information then divides, and one section migrates into the newly formed daughter cell. The daughter cell continues to grow while attached to the parent cell until it has matured, when it then detaches from the parent cell and can reproduce by budding itself.
For most multicellular species, sexual reproduction has one main advantage over asexual reproduction, and this is the fact that it creates variation in the population. This is explained by Mendel’s fundamental laws of heredity; both chromosomes during meiosis and gametes during fertilisation are independently assorted, meaning that when cells reproduce sexually, any combination of chromosomes from the two parents could be inherited. When gametes are produced, each one will receive either the paternal or maternal copy of each chromosome. The chromosomes that are passed on to each gamete are randomly selected, so every chromosome in a gamete is of equal likelihood to have been inherited from either parent. This ensures that meiosis produces two daughter cells that are genetically different to each other, and their parents.
Meiosis also involves recombination, which is another contributing factor to increasing variation. It involves two homologous chromosomes crossing over and exchanging genetic information. Recombination ensures that the chromosomes a daughter cell inherits will not usually be an exact copy of one of the parent’s chromosomes, as the alleles of some genes linked to each chromosome will have been exchanged on to its homologue. This means that it is highly unlikely that siblings will inherit exactly the same genetic information; although their genotype can be anywhere from 0% to 100% similar, these extremes are very rare and siblings most often share approximately 50% of their genetic information.
The creation of variation is highly important as it is essential to the survival of most species and also drives evolution. Variation ensures that individuals of a species are all different and this makes the species much more likely to establish and survive over time. If there is a widespread disease affecting a population with variation in their genotypes, it is likely that some individuals will carry an allele that makes them resistant to the disease. For example, when the bubonic plague spread in the 14th century, the majority of survivors in Europe had a mutation in an allele that gave them resistance to the pandemic. As a result, most of the current population of Northern Europe now carry this mutation in their genomes. Conversely, if individuals of a species all shared the same genotype, a disadvantageous mutation such as one that impacted functionality or survival would spread through the population extremely quickly and soon affect the majority of individuals of that species.
Sexual reproduction does have its disadvantages for organisms, and these are mainly linked to the increased chance of contracting a disease through sexual rather than asexual reproduction. Sexually transmitted diseases are widespread throughout the animal kingdom, and range from bacterial infections similar to those found in humans, to parasites such as Coccipolipus hippodamia, small mites that live on their ladybird hosts and whose larvae infect the host’s mate during intercourse.
Chance of disease in many animals is also increased due to sexual reproduction because of their sex determination systems – in most mammals and some insects this is the XY chromosome system, and in birds and various other animals it is the ZW chromosome system. In the XY sex determination system, a zygote will either inherit an X chromosome from their mother and a Y chromosome from their father meaning it will develop into a male (XY), or it will inherit an X chromosome from each parent and develop into a female (XX). The ZW chromosome system works in much the same way, except the female is the heterogametic sex. A small part of each of the X and Y chromosomes, called the pseudoautosomal region, are homologous, which allows the chromosomes to pair and segregate properly in meiosis in males. However, the rest and vast majority of these chromosomes cannot pair and therefore cannot recombine to exchange genetic information. This increases the likelihood of X-linked disorders becoming more abundant in the population. In humans, recessive X-linked disorders are the most common; if a woman carrying the allele for the disorder becomes pregnant her sons will have a 50% chance of inheriting the gene and therefore being affected by the disorder, and her daughters will have a 50% chance of inheriting the gene and being carriers for the disorder. Y-linked disorders are extremely rare in humans, but they theoretically have the ability to spread very rapidly through a population, as although no females would be affected, a male with a Y-linked disorder would pass the deleterious allele on to all of his sons. The Y chromosome has evolved to counteract this by eroding almost all of the genes that were once linked to it, except those with a function that is specifically related to the male sex. As a result, the Y chromosome is very small compared to other chromosomes, so there is less chance of a deleterious mutation to occur on it. This also means that if a mutation does occur, it is likely to affect a gene that has an essential function in the male reproductive system. So any individual with a Y-linked disorder will in all probability be infertile and therefore unable to pass the deleterious mutation on to any offspring. Some insects, such as crickets and grasshoppers have an XO sex determination system, whereby offspring that inherit two X chromosomes will become female and those which inherit just one will develop into males. These invertebrates share a common ancestor which used the XY system, so it is thought that the Y chromosome has degraded so much over time it has been lost. It is possible that this is due to selection pressures from Y-linked diseases in these organisms, and perhaps this is what is slowly happening with the erosion of the Y chromosome in other organisms.
Asexual reproduction has many advantages and one of these is that offspring can be produced very quickly, without the need for individuals to invest time and energy into finding a mate, copulating, or rearing their young. When conditions are favourable, some strains of bacteria can reproduce as quickly as every 20 minutes, meaning a single individual could produce a population of millions in less than 24 hours. Large colonies of bacteria can be produced in a very short period of time, and the larger the colony, the more chance it has of outcompeting others and ultimately surviving. The fact that asexual reproduction produces clones of the original organism also has its advantages. As offspring will be a genetically identical copy of their parent organism, they will share the same fitness and will be able to reproduce just as the original organism did. There is no risk of fitness decreasing, unlike there is in sexual reproduction due to half the offspring’s genetic information being provided by another individual.
Organisms that reproduce asexually tend to be smaller and less complex than those that undergo sexual reproduction to produce offspring. This is largely due to the fact that there is less scope for natural selection to act upon organisms that reproduce asexually; the genetic information of a population will be exactly the same, so the only cause of variation in an individual will be through mutations in its DNA. Because of the lack of recombination in the genetic material, although an advantageous allele would spread through the population quickly, there is less chance of one individual being significantly fitter than the rest. If a deleterious mutation occurred, this would also spread quickly through generations and the whole population would be affected by it. Similarly, if a disease affected one individual in a population, it would affect them all as the lack of variation in the genotype means that none of the organisms in the population would be resistant. This could quickly wipe out an entire population.
For the majority of higher organisms, sexual reproduction seems to be the best strategy as it creates variation in the population and makes the species as a whole more likely to survive. However, this seems paradoxical as in evolutionary terms, asexual reproduction is more advantageous – an individual’s fitness increases with their reproductive success, which will be normally be higher for asexual organisms as their offspring are identical genetic copies of themselves. Organisms that reproduce asexually will produce offspring who too will be able to reproduce as they share the same fitness. Although sexual reproduction is more advantageous for the long term survival of the species, evolution is only driven by enhancing a single organism’s fitness in the present. The question to why a large portion of organisms have evolved to reproduce sexually when evolution should select for increased fitness in the present time and not look ahead to the survival of a whole species remains unanswered. In conclusion, it seems that asexual reproduction is more suited to smaller organisms who need to be able to produce offspring quickly and efficiently or perhaps cannot move around enough to find a mate, whereas sexual reproduction is highly advantageous for higher organisms whose offspring need a significant amount more time to grow and develop due to their size and complexity.