Final answer:
The findings about monarch butterflies' toxicity can best be explained by natural selection promoting non-toxic butterflies, as these butterflies may have been less frequently targeted by predators, allowing their increase in the population. Laboratory experiments and environmental factors also play a role in the evolutionary dynamics of monarch butterflies.
Step-by-step explanation:
The best explanation for the findings about the monarch butterflies' toxicity to the bird species is option D, Natural selection has increased populations of non-toxic butterflies. Monarch butterflies feed on milkweed plants and ingest chemicals that make them toxic to predators. This is an evolutionary advantage as it deters predation. However, some variations within species can lead to certain monarchs being less toxic if they have not consumed enough milkweed or due to genetic differences. If birds feed on these less toxic individuals without adverse effects, they might do so more frequently, thereby increasing the proportion of non-toxic butterflies through natural selection.
Considering the ecological interactions, such as the monarch butterflies' role in pollinating flowers and their co-evolutionary relationship with milkweed, the selective pressures from predation by bird species directly influence butterfly populations. This process is also evidenced by laboratory experiments mentioned, where the yellow buntings responded differently to butterflies based on visible traits like eyespots, further highlighting the role of predator-prey dynamics in natural selection. Moreover, environmental changes caused by human activities, such as the planting of certain GE crops that reduce milkweed availability, could also affect monarch populations and their evolutionary traits.