Consider the following scenario: 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.
What is the main selective pressure in this scenario?
The main selective pressure in this scenario is the availability of food sources. With the drought causing the ponds to dry up and fish populations to decline, the long-legged birds that rely on fish as their main food source will face a shortage of food. This could lead to a decrease in their survival and reproductive success, as they may struggle to find enough food to sustain themselves. On the other hand, the short-legged birds that feed on insects, which are not affected by the drought, may have a higher chance of survival and reproduction in this scenario. This difference in food availability is driving the selective pressure on the bird population, favoring those individuals with traits that allow them to successfully obtain food in the changing environment.
