Wright's background was in animal breeding.  He had a substantial biological background by the time he began his evolutionary theorizing and was under no misapprehension about the importance of selection.

He was impressed by some work on selection in rats,  it showed that, although strong mass selection in a random-mating population could provoke a rapid and marked evolutionary response, this was often accompanied by deleterious side-effects.  This result was well-known to the breeders of livestock, and the lesson which he drew was that such mass selection had seemingly built-in limitations in producing sustained evolutionary advance - it was 'inefficient' in the long run.  He came to see gene interactions & processes in small populations as important.

Much of his own early work concerned inbreeding, which he investigated experimentally in guinea-pigs.  This work persuaded him that genes often worked together in complex ways in producing phenotypic characters.  His analysis of the development of the Shorthorn cattle breed, also persuaded him of the potential importance of inbreeding in raising degrees of homozygosity, and thus exposing new variability to selection.  He was also involved with the problem of correlations among relatives, but particularly as applied to systems of inbreeding;  in the process, he invented the powerful general analytical technique of path coefficients, which permitted him to do many of the things by partitioning variance.  He noted that, in his various inbred lines,  different characteristics could become fixed by chance.  This simple observation was to influence his entire vision of processes in natural populations.  His animal breeding work also convinced him of the importance of interactions within and among gene systems.