Harvard's yeast run puts a number on what sex is actually worth

A group of Harvard biologists grew 12 identical yeast populations for 960 generations and measured what an occasional generation of sexual reproduction was worth in fitness terms ^[3][7][8]. The result, published in Science, converts a long-running theoretical argument about why sex persists despite its costs into a measured effect with a mechanism attached ^[4].

The cost side is not in dispute. Sexual reproduction requires competition for mates and produces populations in which only half the individuals can give birth, while an asexual lineage skips both and makes every individual a potential mother ^[1][2].

The experiment was run in the lab of Michael Desai and led by Ph.D. student Shreyas Pai and Parris Humphrey, then a postdoctoral fellow ^[3]. Yeast was the test organism because the same strain can reproduce either way, and because a new generation arrives every 90 minutes, which let the team cover 960 generations in four months ^[5][6][7]. Note that 960 generations at 1.5 hours each accounts for about 60 days of growth, so the four-month figure covers more than doubling time alone ^[24]. All 12 populations started asexual in ideal conditions; designated lines were then induced to reproduce sexually for a single generation every 100 generations ^[8]. That is roughly nine bouts of sex across the run, on the order of 1 percent of generations ^[22].

In the constant, favorable environment, sexual populations gained 8 percent fitness over the ancestor against 5.7 percent for asexual ones, a gap of 2.3 percentage points ^[9][21]. The team then assayed the evolved populations in four altered conditions: saltier, hotter, less acidic, and lower phosphate ^[10]. There the sexual populations held advantages of 2 to 5.6 percent over asexual populations ^[11]. At the top of that range the benefit of sex is roughly two and a half times the gap seen in the environment the yeast had adapted to ^[25].

Small percentages in microbes are not small. Desai said that selective differences of fractions of a percent in large populations can drive total shifts in population composition, and that a several percent effect in a microbe is large, taking perhaps 50 to 100 generations to take over ^[12][13][20]. At 90 minutes per generation, that takeover window is about three to six days of growth ^[23].

The mechanism is the part that answers the theory question. Many genes are pleiotropic, affecting several traits at once, some adaptive and some harmful ^[14]. Strongly harmful traits get removed by selection, but neutral or mildly harmful ones can ride along as hitchhiking load when the same gene carries an advantage ^[15]. Change the environment for the worse and some of that load starts to bite ^[16]. Asexual offspring are clones and cannot shed it; sexual offspring shuffle parental genes, and some combinations arrive without the hitchhikers ^[17][18]. According to Desai, populations without sex accumulate baggage that is tolerable where they evolved but would hurt them elsewhere ^[19].

Worth watching: whether the effect scales with the frequency of sex, since one generation in a hundred is a design choice rather than an organism's habit ^[8]; whether the 2 to 5.6 percent band holds under harsher shifts than salt, heat, pH and phosphate ^[10][11]; and whether the same purging shows up in organisms that cannot switch modes on command ^[5].