In , researchers 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. In sexual reproduction, only half of each parent's genes are passed to the next generation. There has been much criticism of Kondrashov's theory, since it relies on two key restrictive conditions. Increased resistance to parasites[ edit ] One of the most widely discussed theories to explain the persistence of sex is that it is maintained to assist sexual individuals in resisting parasites , also known as the Red Queen Hypothesis. 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 H, conferring two types of parasite resistance, such that parasites with allele p can attach themselves to hosts with the allele h, and P to H. Genetic noise can occur as either physical damage to the genome e. 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. Novel genotypes[ edit ] This diagram illustrates how sex might create novel genotypes more rapidly. Kondrashov's model requires synergistic epistasis, which is represented by the red line   - each subsequent mutation has a disproportionately large effect on the organism's fitness. It has already been understood that since sexual reproduction is not associated with any clear reproductive advantages, as compared with asexual, there should be some important advantages in evolution. On the other hand, Charles Darwin concluded that the effects of hybrid vigor complementation "is amply sufficient to account for the The number of sexuals, the number asexuals, and the rates of parasite infection for both were monitored. Indeed some multicellular organisms isogamous engage in sexual reproduction but all members of the species are capable of bearing offspring. Disadvantages of sex and sexual reproduction[ edit ] This section will briefly focus on the ostensible disadvantages of sexual reproduction as compared to relative advantages in asexual reproduction. 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. Protection from major genetic mutation[ edit ] In contrast to the view that sex promotes genetic variation, Heng,  and Gorelick and Heng  reviewed evidence that sex actually acts as a constraint on genetic variation.