Evolution of Sexual Reproduction

process. Male apomixis is rare but possible, in which the pollen or male gamete makes the genetic material of the entire embryo.

As mentioned, some species have the ability to alternate between asexual and sexual reproduction. This is called heterogamy. Aphids will do this, depending on the environmental conditions. So can some bee species, reptiles, birds, and some amphibians. Daphnia is a freshwater crustacean that will undergo parthenogenesis under conditions of a sparse population but will move on to sexual reproduction when competition is great enough.

Some animals engage in polyembryony. This is when the egg is fertilized but the early embryo breaks up into multiple identical clones. This is obligatory in some animals but a sporadic thing in other animals, such as mammals, including humans.

Throughout evolution, there are some that have been successful in reproducing asexually throughout the course of evolutionary time. Bdelloid rotifers create only females and do so asexually. They have done this for millions of years. Stick insects engage in parthenogenesis only—also for the past many millions of years.

EVOLUTION OF SEXUAL REPRODUCTION

When it comes to evolution, sexual reproduction came along after asexual reproduction. Many types of organisms participate in this process. A few organisms already mentioned have lost the ability to reproduce sexually or have developed things like parthenogenesis and apomixis in order to circumvent dependence on sexual reproduction. Prokaryotes developed sexual reproduction about 2 billion years ago, and eukaryotic organisms developed it as well from a common eukaryotic ancestor.

In sexual reproduction, recombination occurs in the making of gametes, and the genotype of the offspring is a mixture of the genotypes of the parents. It does not create identical offspring as the parent, which is what happens in asexual reproduction. As you will see, there are evolutionary advantages to sexual reproduction, which is why it has persisted for so long in the evolutionary process.

Even so, there are disadvantages to the process of sexual reproduction. There is a population expansion disadvantage. Because sexual reproduction requires a male and female member, only half of the population can carry the offspring, leading to a decreased number of offspring than can be gotten through asexual reproduction. This is referred to as the two-fold cost of this type of reproduction.

If there was a species that developed a mutant that could reproduce asexually along with the population that needed to reproduce sexually, the numbers of mutants would double with each generation, outstripping the sexual reproducers.

Even if there was no such thing as only females bearing offspring and all organisms could bear young, there would be an energy cost of copulation that involves adequate energy to come together at the right time and in the same space.

In addition, a sexually reproducing species passes on just half of its genetic material to the next generation. This is true only of those organisms that make haploid gametes. Other species don t make males and females but make one type of gamete. These organisms pass all of their genes onto their genetic offspring.

There are advantages to sexual reproduction that make it preferable to the organism. In general, the two parts to sex are the fusion of genetic material and the differentiation into two different genders. There are organisms that do not do both of these things. If you think about it, sex both increases genetic diversity, while the differentiation part decreases genetic diversity. Hermaphroditism does not decrease genetic material by half because it creates offspring that are of both genders.

Sexual reproduction can be advantageous by combining beneficial mutations in the same offspring. It increases the spread of good traits. Sex also brings together bad mutations in the same offspring to create such an unfit organism that the genes are removed from the population. Finally, sexual reproduction can create fitter organisms than the parents.

The process of DNA repair during the process of meiosis can decrease the chances of damaged DNA getting into the progeny and increases offspring fitness. Because there are homologous genes from the opposite parents, recessive genes that are deleterious are masked by normal genes that are dominant over the recessive gene.

Some theorize that sexual reproduction decreases genetic variation because it can weed out chromosomal rearrangements and other major genetic changes, while allowing minor genetic variations to get through to the offspring.

New genotypes can be created better through sex. As mentioned, advantageous genes can be combined from different parent populations to be allowed to spread faster through sexual reproduction. Recombination creates the presence of the two genes together in the same offspring after just a few generations. This just doesn t happen easily in asexual reproduction because the offspring of each organism is identical to each parent.

In some cases, there are differences in the offspring created sexually compared to those created through asexual reproduction that benefit the heterogamous offspring. These differences are not always explainable but can be seen in organisms that do both sexual and asexual reproduction. Certain water fleas can do either type of reproduction but the heterogamous offspring have better fitness capabilities for reasons unexplained.

There is a theory on the persistence of sexual reproduction called the Red Queen Hypothesis. This involves a greater resistance to parasites in organisms that reproduce sexually. In an environmental change, there is the possibility that bad alleles or neutral ones can become more favorable. Offspring might have a genetic makeup that confers resistance to parasitic organisms, leading to better survival of these offspring. There have been studies in support of this hypothesis and that do not support this hypothesis.

In sexual reproduction, there can be inbreeding or outbreeding. Inbreeding involves mating with close relatives that allows deleterious recessive genes to be expressed to a greater degree. In outbreeding or outcrossing, mutations are hidden by dominant normal alleles. This is also referred to as hybrid vigor, which is a phenomenon of sexual reproduction. Outbreeding is sometimes abandoned with parthenogenesis taking place if there is too high a cost of sexual reproduction, such as when the population is sparse.

How does sexual reproduction help to remove deleterious genes from the gene pool? There are two theories on this. Recombination, for example, only happens in meiosis. In asexual reproduction, it can be difficult to get rid of a bad gene, which can be better removed during the DNA repair that happens in meiosis. With asexual reproduction, you can t usually take back a mutation; you can only add more, which can reduce the species success rate.

In addition, sex takes multiple slightly deleterious genes and, through recombination, makes gametes that have fewer mutations than the parent along with those that have more mutations than the parent. Those that are more deleterious are at a much greater disadvantage and get cleared out easily in the natural selection process. The process of sex will compartmentalize bad genes in a few organisms.

When and why did sexual reproduction develop? It started about 1.2 billion years ago and probably started with a single eukaryotic ancestor. It probably started with an organism with the ability to repair its DNA, which does not happen in asexually reproducing organisms. The proteins that help meiosis are similar to those that allow for genetic transfer and transformation in archaea and bacteria. Meiosis itself is triggered by adverse circumstances in the environment. It means that, throughout evolution, meiosis could provide a benefit for the organism. Sex may