The evolution of sex and the role of sex in evolution

Resampling the pool of genotypic possibilities: an adaptive function of sexual reproduction

BACKGROUND

Natural populations harbor significant levels of genetic variability. Because of this standing genetic variation, the number of possible genotypic combinations is many orders of magnitude greater than the population size. This means that any given population contains only a tiny fraction of all possible genotypic combinations.

RESULTS

We show that recombination allows a finite population to resample the genotype pool, i.e., the universe of all possible genotypic combinations. Recombination, in combination with natural selection, enables an evolving sexual population to replace existing genotypes with new, higher-fitness genotypic combinations that did not previously exist in the population. This process allows the sexual population to gradually increase its fitness far beyond the range of fitnesses in the initial population. In contrast to this, an asexual population is limited to selection among existing lower fitness genotypes.

CONCLUSIONS

The results provide an explanation for the ubiquity of sexual reproduction in evolving natural populations, especially when natural selection is acting on the standing genetic variation.

Cohesion of Clonal Life History, Senescence and Rejuvenation Induced by Autogamy of the Histophagous Ciliate Tetrahymena rostrata

The histophagous ciliate Tetrahymena rostrata was found as a parasite in the renal organs of the land snails Zonitoides nitidus and Cochlicopa lubrica. A starvation medium induced encystment, meiosis, autogamy, and development of new macronuclei. The cell division rate declined linearly with number of divisions from the last autogamy until senescence. The senescing strains were rejuvenated by further encystment-induced autogamy. It is expected, that these processes contribute to genetic variability among the local, small, and isolated T. rostrata populations. Consistent with this expectation, small divergences in the cox-1 sequences appeared even among these strains, which had been isolated from different specimens of the same host species at the same site. The divergences in this gene between our T. rostrata strains from Z. nitidus and other strains from C. lubrica, Helix aspersa, and Deroceras reticulatum in Spain (Segade et al. 2009, Parasitology 136:771-782), were beyond the limits of intra-species variability within the genus Tetrahymena. However there is the lack of inter-strain differences in the life history and cytology among our, the Spanish, and those T.rostrata strains, that are not available for "barcoding" anymore. Therefore, variability in the life history and morphology within T .rostrata is constrained by natural selection.

Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour

In a population whose members' genomes are subject to degradation by random mutations, the heritable vigour of the most common phenotypes is unquestionable (though not necessarily optimal), and that of fringe individuals is always suspect. Natural selection will therefore support the evolution of an affinity for modal mates (i.e. koinophilia). The population's genetic make-up can then not readily be invaded by non-cryptic mutations. This imposes considerable phenotypic conservatism on sexually reproducing creatures, and inexorably canalizes them into sexually isolated, phenotypically distinct species. The model predicts, and the empiric data confirms, that the phenotypic gaps between largely monomorphic sexual species do not characterize the taxonomy of longstanding apomicts, where variation below the genus level is often continuous. The bias against the propagation of all forms of phenotypic novelty and non-conformity stabilizes social animals against selfish mutants, thus removing the barriers to the evolution of "group adaptations".