Final answer:
Allele frequencies in populations can change due to genetic drift, which causes random fluctuations, or natural selection, where advantageous alleles become more prevalent. Without the graph, a specific explanation for the change between the 30th and 40th generations cannot be provided.
Step-by-step explanation:
The question refers to the change in allele frequency within three populations between the 30th and 40th generations. Without a provided graph, I can draw on general scientific principles to explain that allele frequencies can change due to various evolutionary mechanisms. One such mechanism is genetic drift, which can cause random fluctuations in allele frequencies, particularly in small populations, due to chance events. Another mechanism is natural selection, where alleles that confer a survival or reproductive advantage increase in frequency over time. If the context indicates that no migration or mutation is affecting the populations, and the population size is large, the likely cause of change in allele frequency is either genetic drift or natural selection. However, without more context or the actual graph, a precise explanation is not possible.