Darwin himself put forward two theories. First, he conjectured that sex may be advantageous because it allows for the purging of harmful mutations. Second, he proposed that sex may stimulate evolutionary change by constantly reshuffling the genetic 'cards'.
Modern evolutionary theory generally supports the second of Darwin's ideas, that of sex as an engine for evolutionary change. Importantly, theoretical models indicate that sex can only promote evolutionary adaptation (i.e., increasing fitness) if the environment and the associated selective forces are changing.
For most organisms, environments fluctuate continuously due to changing weather patterns, altered food supplies, the spread of disease, the invasion of competitors and so on. Under such conditions, recombination and sex help sustain adaptation by producing novel gene combinations. Importantly, because novel gene combinations are unlikely to be harmful and potentially beneficial, most offspring produced through sexual reproduction have fitness close to that of their parents (a fact known as 'Muller's ratchet'). This robustness allows populations to survive environmental change and persist in the long term.
The long-term advantage of sex is the maintenance and evolution of genetic diversity, ensuring the persistence of species across geological timescales.
