Part A: The MOST LIKELY cause of the sudden appearance of a new gene in the butterfly's species is a mutative process. Genes can emerge due to mutations, which are changes in the DNA sequence that can occur spontaneously or due to environmental factors.
Part B: The immediate effect of this change could possibly be this change could allow the butterfly to develop into a new species. Genetic mutations can lead to new traits that may eventually contribute to speciation over time, although the direct immediate impact may vary depending on the nature of the mutation.