Sewell Wright's key insight (one of his many key insights — he's one of the hot shots of post-Darwinian evolutionary biology) was suggesting that genetic drift is the mechanism that lets a population move from one fitness peak (one really great genotype) to another because, unlike natural selection, genetic drift lets populations wander around low-fitness parts of the adaptive space. In his hypothesis, a small population could descend (genetically speaking) into an adaptive valley (an area of low fitness) and then climb a different, higher, adaptive peak as a result of natural selection. Essentially, a small population has a chance of hitting the genetic jackpot and ending up on a high adaptive peak.
Genetic drift is like a lottery; no guarantee exists that genetic drift is going to make it possible for a species to wander over to another peak. Wright's argument is just that it can do so some of the time. The low part of the fitness landscape means that those are genotypes that have a lower chance of leaving descendants. But they might get lucky once in a while.
When a small population does hit the genetic jackpot and gets to the slope of a higher peak, natural selection will kick in again and the population will climb up the new higher peak.
With a really fit combination of alleles, the population can spread through the actual physical landscape. Because the individuals have higher fitness (they're at the top of a really high fitness peak), they increase in frequency and can migrate in the physical landscape, spreading their selectively advantageous genetic combination far and wide. In other words, one small population hits the fitness jackpot; then these individuals sow their oats in other populations.
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