Question

Ladybird beetles are distasteful to predators because of toxic chemicals they secrete, yet they also have dark melanic forms (Majerus \(1998,\) p. 221 ). Melanic ladybirds have declined in frequency in central England along with the peppered moth during the past 50 years as air quality has improved. If ladybirds are not eaten by predators, how might you explain these changes in melanic frequency?

Short answer

Improved air quality removed the camouflage advantage for melanic ladybirds, leading to a decline in their frequency.

Step-by-step solution

Understanding Melanism

Melanism refers to the dark pigmentation in organisms, such as ladybirds, often resulting from environmental pressures. In the past, these organisms had a higher frequency of melanic forms due to pollution, which made darker individuals more adapted to the sooty environments as they were better camouflaged and therefore survived and reproduced more successfully.

Link to Environmental Changes

With the decline in air pollution over the past 50 years due to improved air quality measures, the environmental pressure that selected for melanic forms has decreased. This means that lighter-colored ladybirds now have a better chance of survival as the landscape has become less sooty, offering better camouflage for non-melanic individuals.

Exploring Mechanisms for Decline

Even though ladybirds secrete toxic chemicals making them distasteful to predators, other selective forces likely influence their frequency. Improved air quality could indirectly select against melanic forms by making them more visible to non-predatory selective pressures such as temperature regulation, breeding, or finding mates, given the lighter environmental backdrop.

Conclusion on Frequency Shift

The shift in melanic frequency in ladybirds, similar to that observed in peppered moths, can be attributed to the reduced selective pressure for melanism due to cleaner air. The improvement in air quality led to an environment where melanic pigmentation provided no camouflage advantage, influencing the evolutionary pressure acting on the species.

Key concepts

Environmental Changes

Environmental changes have a profound impact on the natural world, affecting species and their adaptations. The shift in environmental conditions is often driven by human activities, such as industrial pollution. For instance, in the past, pollution emitted by factories led to sooty environments, particularly in urban and industrial regions. In these settings, darker forms of organisms, like melanic ladybirds, thrived because they could blend into the darkened landscape, reducing the risk of being spotted by predators or other threats.
However, as efforts were made to clean up the environment, such as implementing stricter air quality regulations, the landscape began to change. The air became clearer, and surfaces that were once covered in soot became lighter. These changes altered the habitats where these organisms lived.
This shift in environmental conditions shifted the selective advantage from darker (melanic) forms to lighter-colored forms, which now possessed better camouflage against the cleaner, lighter backgrounds. Hence, environmental changes can significantly influence which traits are advantageous, altering the frequency of specific forms or species in an area.

Selective Pressures

Selective pressures are forces in the environment that influence the survival and reproduction of organisms with certain traits. These pressures can arise from interactions with predators, competition for resources, or environmental conditions. The concept of selective pressure is foundational in understanding natural selection and evolution.
When it comes to ladybirds, selective pressures historically favored melanic forms when industrial pollution created darkened environments. Melanic ladybirds, with their dark pigmentation, had an advantage in these sooty surroundings. They could avoid predators more effectively, and hence, were more likely to survive and reproduce.

• However, as air quality improved, these pressures changed. The darker ladybirds could no longer blend into their environment as effectively.
• Lighter-colored ladybirds began to thrive because they fit the changing background better, reversing the selective advantage.

Hence, selective pressures are not static; they vary with changes in the environment, leading to shifts in the traits that are deemed advantageous or disadvantageous for survival.

Air Quality Improvements

Air quality improvements have been a significant factor in changing the frequencies of melanic forms in various species, such as ladybirds and the infamous peppered moth. As governments and communities enacted environmental protection laws and reduced industrial emissions, the quality of air improved significantly over time. Cleaner air contributed to the reduction of pollution in soils and surfaces which had previously favored darker pigmentation. With a decrease in pollutants, species that had adapted to previous conditions faced new challenges.
Cleaner air meant a return to lighter-colored landscapes, which diminished the camouflage advantages previously held by melanic individuals. This change played a role in the decline of melanic forms, as lighter-colored forms became less visible and therefore less likely to be targeted for exposures that would select against them.
As a result, air quality improvements demonstrate the complex interplay between environmental policy and biological adaptation, showing how reducing pollution can lead to evolutionary shifts in wildlife populations, highlighting the importance of good environmental stewardship.