Question

Lungs ventilated with water occur in some animals-most notably sea cucumbers- but are rare. Why would water lungs be unlikely to be favored by natural selection? Give as many reasons as possible.

Short answer

Water lungs are not commonly favored by natural selection due to reasons such as lesser oxygen content in water as compared to air, higher energy requirement to move water than air across respiratory surfaces, and slower diffusion of oxygen through water. These factors contribute to decreased survival and reproduction advantages, thus making them less prevalent in nature.

Step-by-step solution

Understand the role of lungs

The primary function of lungs is to facilitate the exchange of gases, primarily oxygen and carbon dioxide. Oxygen is needed by cells to carry out various metabolic processes. In most animals, air is a more efficient medium for this exchange compared to water due to its higher concentration of oxygen.

Understand the properties of water vs. air as a respiratory medium

When considering water as a respiratory medium, one must consider that water contains less oxygen per unit volume compared to air. Moreover, moving water across respiratory surfaces requires more energy than moving air. Oxygen diffusion through water is also slower compared to air.

Discuss natural selection and evolutionary advantage

Natural selection favors characteristics that provide an advantage for survival and reproduction. If lungs ventilated with water provided such advantages, they would be more common in nature. However, given the above factors related to efficiency and energy usage, water lungs do not appear to confer substantial benefits over lungs ventilated with air.

Conclude

While water ventilation might work for specific organisms in certain niches, like sea cucumbers, it generally does not confer an evolutionary advantage over air ventilation, primarily due to efficiency of oxygen absorption and energy cost. Hence, such lungs are unlikely to be favored by natural selection.

Key concepts

Natural Selection's Role in Organ Function

Natural selection is a key mechanism of evolution that favors traits enhancing survival and reproduction. The concept revolves around the idea that organisms better adapted to their environments are more likely to survive and pass their genes onto the next generation.

• For traits like water lungs, natural selection would only favor them if they provided a significant survival advantage over air lungs.
• Given the role lungs play in gas exchange and the inherent inefficiencies of water as a medium for respiration, such an adaptation is less advantageous.

Water lungs are not widespread because they do not meet the baseline efficiency needed to be naturally selected. Traits that don't improve survival or reproduction are less likely to be inherited by future generations.

Efficiency in Respiratory Systems

Respiratory efficiency is crucial for any organism's survival, as it influences how well oxygen is supplied to cells and tissues. Efficiency in respiratory systems is determined by the medium through which oxygen is absorbed—either air or water.

• Air is a more efficient medium than water, containing more oxygen per unit volume. Consequently, air-breathing lungs need to move less medium to acquire the same amount of oxygen compared to water-breathing lungs.
• Moving a dense liquid like water requires more energy, making the process of breathing more effort-intensive.

These factors indicate why terrestrial animals have evolved air-based respiratory systems to maximize efficiency and energy use.

Understanding Gas Exchange in Different Media

Gas exchange is a critical biological process where oxygen is taken in, and carbon dioxide is expelled. The medium facilitating this exchange significantly impacts its effectiveness.

• Oxygen in water is less concentrated compared to air. Hence, acquiring sufficient oxygen through water requires a more complex and energy-demanding system.
• Moreover, oxygen diffuses slower in water than in air, slowing down the exchange process further.

Because of these physical properties, lungs that ventilate air have an advantage in most environments, explaining why water lungs have not evolved widely. These differences highlight why adaptation to air-based gas exchange systems is predominant.

Insights from Evolutionary Biology

Evolutionary biology examines the changes in organisms over time and the mechanisms behind these changes. It provides insight into how certain traits are favored or phased out.

• Water lungs in some marine animals like sea cucumbers can be a specialized adaptation, but they are exceptions rather than the rule.
• In the broader context, air-breathing systems have evolved recurrently among diverse terrestrial species due to the respiratory efficiency and advantages they offer.

Evolutionary biology helps us understand why traits like air breathing have become standard, demonstrating a clear evolutionary preference for systems that offer greater efficiency and survival benefits. By analyzing the evolutionary path of respiratory systems, it becomes clear why certain traits thrive while others remain uncommon.