By describing adultery as a force that shaped our mating system, I am not “justifying” it. Nothing is more “natural” than people evolving the tendenc… - Matt Ridley

Progress and success are always relative. When the land was unoccupied by animals, the first amphibian to emerge from the sea could get away with being slow, lumbering, and fish-like, for it had no enemies and no competitors. But if a fish were to take to the land today, it would be gobbled up by a passing fox as surely as a Mongol horde would be wiped out by machine guns. In history and in evolution, progress is always a futile, Sisyphean struggle to stay in the same relative place by getting ever better at things.

The gene contains a single 'word', repeated over and over again: CAG, CAG, CAG, CAG ... The repetition continues sometimes just six times, sometimes thirty, sometimes more than a hundred times. Your destiny, your sanity and your life hang by the thread of this repetition. If the 'word' is repeated thirty-five times or fewer, you will be fine.
Most of us have about ten to fifteen repeats. If the 'word' is repeated thirty-nine times or more, you will in mid-life slowly start to lose your balance, grow steadily more incapable of looking after yourself and die prematurely.

It is no harder to explain than a game of cards. There are aces and kings and twos and threes in any deck of cards. A lucky player is dealt a high-scoring hand, but none of his cards is unique. Elsewhere in the room are others with the same kinds of cards in their hands. But even with just thirteen kinds of cards, every hand is different and some are spectacularly better than others. Sex is merely the dealer, generating unique hands from the same monotonous deck of genetic cards shared by the whole species.