A drought hits the habitat of a semi-aquatic bird population. All ponds dry up, and fish populations decline. There are two groups of birds in the population that differ in leg length and diet. Long-legged birds eat fish, while short-legged birds eat insects. The drought has little effect on insect populations.

Question

What is the main selective pressure in this scenario?
• insect population
• fish population
• drought
• leg length

Answer 1

The main selective pressure in this scenario is the decline in fish population due to the drying up of ponds during the drought. This would have a greater impact on the long-legged birds that depend on fish as their main food source, and may result in natural selection favoring shorter legs in the population over time.