The evolution of sexual reproduction describes how sexually reproducing animalsplantsfungi and protists could have evolved from a common ancestor that was Sexual reproduction biology single celled eukaryotic species.
The evolution of sex contains two related, yet distinct, themes: The maintenance of sexual reproduction in a highly competitive world had long been one of the major mysteries of biology given
Sexual reproduction biology asexual reproduction can reproduce by budding, fission, or spore formation not involving union of gametes, which reproduce at a much faster rate to sexual reproduction.
Since hypotheses for the origins of sex are difficult to test experimentally outside of evolutionary computationmost current work has focused on the maintenance of sexual reproduction. Sexual reproduction must offer significant fitness advantages to a species because despite the two-fold cost of sex, it dominates among multicellular forms of life, implying that the fitness of offspring produced outweighs the costs. Sexual reproduction derives from recombinationwhere parent genotypes are reorganized and shared with the offspring.
This stands in contrast to single-parent asexual replicationwhere the offspring is identical to the parents. Recombination supplies two fault-tolerance mechanisms at the molecular level: The issue features in the writings of Aristotleand modern philosophical-scientific thinking on the problem dates from at least Erasmus Darwin in the 18th century.
August Weismann picked up the thread inarguing that sex served to generate genetic variationas detailed in the majority of the explanations below. On the other hand, Charles Darwin Sexual reproduction biology that the effects of hybrid vigor complementation "is amply sufficient to account for the Biologists - including W.
HamiltonAlexey KondrashovGeorge C. Hopf and Richard E. Michod - have suggested several explanations for how a vast array of different living species maintain sexual reproduction. This section will briefly focus on the ostensible disadvantages of sexual reproduction as compared to relative
Sexual reproduction biology in asexual reproduction. Given that Sexual reproduction biology reproduction abounds in multicellular organisms, this section is followed by a lengthy overview of theories aiming to elucidate the advantages of sex and sexual reproduction.
An asexual population can grow much more rapidly
Sexual reproduction biology each generation. Assume the entire population of some theoretical species has total organisms consisting of two sexes i.
If all capable members of this population procreated once, a total of 50 offspring would be produced the F1 generation. Contrast this with an asexual species, where each member of the organism population is capable of bearing young.
If all capable members of this asexual population procreated once, a total of offspring would be produced. This idea is sometimes referred to as the two-fold cost of sexual reproduction. It was first described mathematically by John Maynard Smith. Technically Sexual reproduction biology problem above is not that of sexual reproduction but a problem of having a subset of organisms incapable of bearing offspring. Indeed some multicellular organisms isogamous engage in sexual reproduction but all members of the species are capable of bearing offspring.
Sexual reproduction implies that chromosomes and alleles segregate and recombine in every generation, but not all genes transmitted together to the offspring. This is a consequence of the fact that gametes from sexually Sexual reproduction biology species are haploid. Again however, this is not applicable to all sexual organisms. There are numerous species which are sexual but do not have a genetic-loss problem because they do not produce males or females.
Yeast, for example, are isogamous sexual organisms which have two mating types which fuse and recombine their haploid genomes. In these species e. The concept of sex includes
Sexual reproduction biology fundamental phenomena: Depending on the presence or absence of these phenomena, the existing ways of reproduction can be divided into asexual, hermaphrodite and dioecious forms. The sexual process and sexual differentiation are different phenomena, and, in essence, are diametrically opposed.
The first creates increases diversity of genotypes, and the second decreases it by half. Reproductive advantages of the asexual forms are in quantity of the progeny and the advantages of the hermaphrodite forms — in maximum diversity. Transition from the hermaphrodite to dioecious state Sexual reproduction biology to a loss of at least half of the diversity. So, the main question is to explain the advantages given by sexual differentiation, i. It has already been understood that since sexual reproduction is not associated with any clear reproductive advantages, as compared with asexual, there should
Sexual reproduction biology some important advantages in evolution.
For the advantage due to genetic variation, there are three possible reasons this might happen. First, sexual reproduction can combine the effects of two beneficial mutations in the same individual i. Also, the necessary mutations do not have to have occurred one after another in a single line of descendants.
However, Sexual reproduction biology organisms containing only one set of chromosomes, deleterious mutations would be eliminated immediately, and therefore removal of harmful mutations is an unlikely benefit for sexual reproduction. Lastly, sex creates new gene combinations that may more fit than previously existing ones, or may simply lead to reduced competition among relatives.
The advantage of complementation to each sexual partner is avoidance of the bad effects of their deleterious recessive genes in
Sexual reproduction biology by the masking effect of normal dominant genes contributed by the other partner.
The classes of hypotheses based on the creation of variation are further broken down below. Any number of these hypotheses may be true in any given species they are not mutually exclusiveand different hypotheses may apply in different species.
However, a research framework based on creation of variation has yet to be found that allows one to determine whether the reason for sex is universal for all sexual species, and, if not, which mechanisms are acting in each species. On the other hand, the maintenance of sex based on DNA repair and complementation applies widely to all sexual species. In contrast to the view that sex promotes genetic variation, Sexual reproduction biology,  and Gorelick and Heng  reviewed evidence Sexual reproduction biology sex actually acts as a constraint on genetic variation.
They consider that sex acts as a Sexual reproduction biology filter, weeding out major genetic changes, such as chromosomal rearrangements, but permitting minor variation, such as changes at the nucleotide or gene level that are often neutral to pass through the sexual sieve.
Sex could be a method by which novel genotypes are created. Because sex combines genes from two Sexual reproduction biology, sexually reproducing populations can more easily advantageous genes than asexual populations.
If, in a sexual population, two different advantageous alleles arise at different loci on a chromosome in different members of the population, a chromosome containing the two advantageous alleles can be produced within a few generations by recombination. However, should the same two alleles arise in different members an asexual population, the only way that one chromosome can develop the other allele is to independently gain the same mutation, which would take much longer.
Several studies have addressed counterarguments, and the question of whether this model is sufficiently robust to explain the predominance of sexual versus asexual
Sexual reproduction biology. Ronald Fisher also suggested that sex might facilitate the spread of advantageous by allowing them to better escape their genetic surroundings, if they should arise on a chromosome with deleterious genes.
Supporters of these theories respond to the balance argument that the individuals produced by sexual and asexual reproduction may differ in other respects too — which may influence the persistence of sexuality. For example, in the heterogamous water fleas of the genus Cladocerasexual offspring form eggs which are better able to survive the winter versus those the fleas produce asexually.
One of the most widely discussed theories to explain the persistence of sex is that it is maintained to assist sexual individuals in resisting parasitesalso known as the Red Queen Hypothesis.
When an environment changes, previously neutral or deleterious alleles can become favourable. If the environment changed sufficiently rapidly i. Such rapid changes in environment are caused by the co-evolution
Sexual reproduction biology hosts and parasites. Imagine, for example that there is one gene in parasites with two alleles p and P conferring two types of parasitic ability, and one gene in hosts with two alleles h and Hconferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele hand P to H.
Such a situation will lead to cyclic changes in allele frequency - as p increases in frequency, Sexual reproduction biology will be disfavoured. In reality, there will be several genes involved in the relationship between hosts and parasites. In an population of hosts, offspring will only have the different parasitic resistance if a mutation arises.
In a sexual population of hosts, however, offspring will have a new combination of parasitic resistance alleles. In other words, like Lewis Carroll 's Red Queen, sexual hosts are continually "running" adapting to "stay in one place" resist parasites.
Evidence for this explanation for
Sexual reproduction biology evolution of sex is provided by comparison of the rate of molecular evolution of genes for kinases and immunoglobulins in the immune system with genes coding other proteins. The genes coding for immune system evolve considerably faster.
The number of sexuals, the number asexuals, and the rates of parasite infection for both were monitored. It was found that clones that were plentiful at the beginning of the study became more susceptible to parasites over time.
As parasite infections increased, the once plentiful clones dwindled dramatically in number. Some clonal types disappeared entirely. Meanwhile, sexual snail populations remained much more stable over time. However, Hanley et al. Contrary to expectation based on the Red Queen hypothesisthey found that the prevalence, abundance and Sexual reproduction biology intensity of mites in sexual geckos was significantly higher than in asexuals sharing the same habitat.
Inresearchers used the microscopic roundworm Caenorhabditis elegans as a host and the pathogenic bacteria Serratia marcescens to generate a host-parasite coevolutionary system in a controlled environment, allowing them to conduct more than 70 evolution experiments testing the Red Queen Hypothesis. They genetically manipulated mating system of C.
Then they exposed those populations to the S. It was found that the self-fertilizing populations of C. Critics of Sexual reproduction biology Red Queen hypothesis question whether the constantly changing environment of hosts and parasites is sufficiently common to explain the evolution of sex.
In particular, Otto and Nuismer  presented results showing that species interactions e. They concluded that, although the Red Queen hypothesis favors sex under certain circumstances, it alone does not account for the ubiquity of sex. As discussed in the earlier part of this article, sexual reproduction is conventionally explained as an adaptation for producing genetic variation through allelic recombination.
Sexual reproduction biology acknowledged above, however, serious problems with this explanation have led many biologists to conclude that the benefit of sex is a major unsolved problem in evolutionary biology.
An alternative " informational " approach to this problem has led to the view that the two fundamental aspects of sex, genetic recombination and outcrossingare adaptive responses to the two major sources of "noise" in transmitting genetic information. Genetic noise Sexual reproduction biology occur as either physical damage to the genome e.
The repair and complementation hypothesis assumes that genetic recombination is fundamentally a DNA repair process, and that when it occurs during meiosis it is an
Sexual reproduction biology for repairing the genomic DNA which is passed on to progeny. Recombinational repair is the only repair process Sexual reproduction biology which can accurately remove double-strand damages in DNA, and such damages are both common in nature and ordinarily lethal if not repaired.
For instance, double-strand breaks in DNA occur about 50 times per cell cycle in human cells [see DNA damage naturally occurring ]. Recombinational repair is prevalent from the simplest viruses to the most complex multicellular eukaryotes.
It is effective against many different types of genomic damage, and in particular is highly efficient at overcoming double-strand damages. Studies of the mechanism of meiotic recombination indicate that meiosis is an adaptation for repairing DNA. In some lines of descent from the earliest organisms, the diploid stage of the sexual cycle, which was at first transient, became the predominant stage, because it allowed complementation — the masking of deleterious recessive mutations i.
Outcrossingthe second fundamental aspect of sex, is maintained by the advantage of masking mutations and Sexual reproduction biology disadvantage of inbreeding mating with a close relative which allows expression of recessive mutations commonly observed as inbreeding depression. This is in accord with Charles Darwin who concluded that the adaptive advantage of sex is hybrid vigor; or as he put it, "the offspring of two individuals, especially if their progenitors have been subjected to very different conditions, have a great advantage in Sexual reproduction biology, weight, constitutional vigor and fertility over the self fertilised offspring from either one of the same parents.
Sexual reproduction: algae: Reproduction and life histories: Sexual reproduction is characterized by the process of meiosis, in which progeny cells receive half. Revise reproduction, the genome and gene expression for Edexcel for BBC Bitesize. Organisms produced by sexual reproduction have two parents and are. Sexual reproduction is
Sexual reproduction biology mode of reproduction that involves fusion of female gamete (ovum) and male gamete (spermatozoon).
The fusion of.