Evolution of sexual reproduction in Chlamydomonas (MSc. 2002, McGill University).
The evolution of sex is a major puzzle in modern evolutionary biology. It seems that a 
cumbersome sexual cycle is maintained because it improves the quality of progeny (fitness), despite reducing the overall number of offspring (twofold cost of sex). Sexual reproduction is expected to facilitate the elimination of mildly deleterious mutations and thereby increase the mean fitness of a sexual population relative to an asexual population (Kondrashov 1988, see the wikipedia article Evolution of Sex for an exhaustive explanation about the different theories on the maintenance and origin of sexual reproduction).
In order to test this mutation clearance hypothesis, populations ofChlamydomonas were maintained in a benign laboratory environment as obligatory sexual or asexual populations for five years. Fitness in competition and in pure culture was measured. In neither of the fitness assays, both in solid and liquid cultures of Chlamydomonas, was a fitness advantage of sexual reproduction seen, even though the results varied depending on the definition of fitness.
Moreover, we detected significant genotype by environment interactions. This would indicate that individuals in nature may face constantly changing conditions. Subsequent experiments also demonstrated that sexual populations, despite having a similar fitness than their asexual counterparts, are also more variable. This may provide fuel for continued selection as individuals face constantly changing environmental conditions. (Renaut_04_Msc thesis, Renaut et al. 2006 Evolution).