Remember that both varieties were present at the start, with the mix of genes producing lights favored over the mix of genes producing darks. As the environment changed, the dark variety had greater opportunity to pass on their genetic mix, and percentages changed. All the while, the two types were interfertile. No new genes were produced, and certainly no new species resulted. This is natural selection in action, but not evolution.
Adaptation happens, but the changes are limited.
The textbooks seldom point out that in recent decades, as England has cleaned its atmosphere, the shift has reversed and now the lights are the more common form once again. Remember, this shift and shift back again have nothing whatsoever to do with the origin of moths, or how moths and people could share a common ancestor. Furthermore, we now know that neither dark nor light moths ever spend their days on exposed tree trunks or rocks as depicted in the famous textbook pictures. His original associates have even admitted that the photographs were faked, that the moths were glued onto the tree. Thus the star witness for evolution has perjured itself, and knowledgable evolutionists are recommending it not be used.