Can a species evolve by pure luck, with no selection at all?

Yes. Chance alone can make a gene version more common, or wipe it out, with no help from selection. Biologists call this genetic drift. Picture a jar of 20 marbles, half red and half blue. To fill the next jar, draw a marble at random, note its colour, put it back, and repeat 20 times. The new jar is rarely exactly half and half. Keep going, jar after jar, and one colour can vanish for good, though neither colour was ever better.

Drift is strongest in small groups. In 1775, a typhoon left only 20 people alive on the Pacific island of Pingelap. By chance, the survivors carried a rare recessive gene for complete colour blindness. Today about 10% of islanders are completely colour-blind, and another 30% carry the gene.

The same thing happens when a few people found a new community. This is called the founder effect. The Amish in the United States grew from very few founders, took in few newcomers, and mostly marry within the community. Extra fingers or toes, though still rare, are more common among them than in the wider American population.

A crash in numbers, called a bottleneck, can do this to a whole species. Hunted for the oil in their blubber, northern elephant seals fell to perhaps 20 to 40 animals by the end of the 19th century. Protected since, they now number more than 200,000. But the crash left them with very little genetic diversity.

For decades, most biologists thought drift barely mattered. In 1968, Motoo Kimura argued the opposite for DNA itself. Most changes that spread through a population at the molecular level, he said, are neutral mutations carried along by drift.