Why did evolution keep a costly form of reproduction? Harvard scientists find clues in yeast

Sexual reproduction seems, at first glance, like an evolutionary puzzle. It requires organisms to find mates, compete for reproduction and, in many species, maintain populations in which only some individuals can give birth. Asexual reproduction, by comparison, can appear far more efficient: an organism can reproduce without a mate, and every individual can potentially produce offspring.So why has sexual reproduction persisted across much of the natural world?New research from Harvard biologists offers an important clue. Using yeast that can reproduce both sexually and asexually, researchers tracked evolution across 960 generations and found that sexual reproduction provided a measurable advantage — particularly when the environment changed.A question evolutionary biologists have debated for decadesAs Harvard Gazette reported, sexual reproduction comes with what evolutionary biologists describe as significant costs. In asexual reproduction, offspring can be produced directly, without the need to find a mate.That raises a fundamental question: if cloning oneself is simpler, why take the more complicated route?To investigate, researchers in the laboratory of Michael Desai, Fisher Professor of the Natural Sciences at Harvard, used yeast as a model organism. The study was led by PhD student Shreyas Pai and Parris Humphrey, then a postdoctoral fellow.Yeast offered the researchers an unusual advantage. It can reproduce both sexually and asexually.It also produces a new generation roughly every 90 minutes, allowing scientists to observe evolutionary changes over hundreds of generations within months rather than waiting centuries.What happened over 960 generations?The researchers began with 12 genetically identical yeast populations growing under favourable conditions.All the populations initially reproduced asexually. However, selected populations were induced to reproduce sexually for one generation after every 100 generations.Over nearly 1,000 generations, the sexually reproducing populations showed a greater increase in fitness.According to the research, populations that reproduced sexually increased their fitness by 8% compared with the ancestral population, while the asexual populations showed a 5.7% increase.The researchers then introduced the yeast to changing conditions, including environments that were saltier, hotter, less acidic or had lower phosphate levels.Here, sexual reproduction again showed an advantage, with sexually reproducing populations outperforming asexual ones by between 2% and 5.6%.Why can even a small advantage matter?A few percentage points may not sound dramatic. But in evolution, even a small advantage can accumulate over many generations.“Selective differences of fractions of a percent in large populations can drive total shifts in population composition,” Desai told Harvard Gazette.He added that a several-percentage-point advantage in microbes could be significant enough for one population to take over within 50 to 100 generations.The researchers wanted to understand not only whether sexual reproduction offered an advantage, but why.The problem of genetic ‘baggage’The answer may lie in genes that influence more than one trait.Some mutations can be beneficial in one way but carry neutral or mildly harmful effects in another. When organisms reproduce asexually, offspring are genetically very similar to their parents, making it difficult to separate beneficial genetic changes from the less useful or potentially harmful traits travelling alongside them.Harvard Gazette describes this as a kind of “hitchhiking load”.That baggage may not cause major problems in a stable environment. But when conditions change, mutations that were previously harmless or only mildly disadvantageous can suddenly become a liability.Sexual reproduction changes that equation.By reshuffling genetic material between parents, it can help separate useful traits from harmful hitchhikers over generations.Why changing environments may favour sex“We showed that, without sex, it’s not just that the populations are accumulating deleterious baggage, they’re accumulating baggage that maybe is not so bad in the environment they’re evolving in, but would hurt you elsewhere,” Desai told Harvard Gazette.In sexually reproducing populations, genetic reshuffling can reduce this burden and prevent a population from becoming too specialised for one particular environment.That could become particularly valuable when conditions shift.The research was conducted using microbes, but the scientists believe the underlying mechanism could apply much more broadly. Asexual reproduction exists in other forms across nature, including plants that can grow from cuttings and some animal species capable of parthenogenesis.“We believe that this general mechanism by which sex speeds adaptation and purges hitchhiking deleterious load should be a general advantage of sex across all sorts of organisms,” Desai said.The study, published in Science, does not end the evolutionary debate over why sexual reproduction exists. But by watching nearly 1,000 generations unfold in just four months, the Harvard team has provided new experimental evidence for one powerful explanation: sex may be costly, but when the world changes, genetic diversity can be worth the price.Disclaimer: This article is based on research reported by Harvard Gazette and published in Science. The findings and interpretations have not been independently verified by TOI Education.Get the latest Education News and Live updates. Download the TOI app.