Computer simulations of the evolution of a hypothetical lineage over 50 thousand generations have found that beneficial mutations arose earlier in the lineage's history than mutations with a mildly negative effect. This pattern of evolution allows species to adapt and move on without having to wait for the harmful mutations to be removed.

Computer simulation:

the hypothetical lineage starts in the middle and begins evolving to the right. Beneficial mutations are represented as green squares while slightly negative mutations are small red squares. The line that connects the squares shows the lineages' genotype; mutations that are later in the lineage are higher on the line while mutations that occurred earlier are lower.

As the lineage evolves and acquires new mutations (new squares are added to the line), the negative effects of the slightly negative mutations are cancelled out by the positive effects of the beneficial mutations. This allows the lineage to continue to adapt over time even though it carries mutations with slightly negative effects.

It was also found that the fitness improvement associated with beneficial mutations that occurred later in the lineage was smaller; this is because many of the mutations that had large beneficial effects had already arisen earlier.

The findings contrast with the traditional perspective of evolution which focuses on the removal of harmful mutations as the primary way in which lineages adapt. They also highlight that the genetic background may allow beneficial mutations to have disproportionally larger effects because many of the mutations causing smaller negative effects have already been removed.