Fitness and Selection

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    Absolute fitness is the total number of offspring one produces over its lifetime. Relative fitness, on the other hand, compares the absolute fitness of one to the average fitness of the complete population. While the absolute fitness of the organism stays the same, as the average changes, so does its relative fitness. We can observe this by looking at a population of butterflies. 

    Let's say we have two butterflies, one white and one brown. The brown butterfly will produce 40 offspring in its lifetime, so it has a relative fitness of 40. The white butterfly will produce only 30, giving it a relative fitness of 30. The average of this population is 35, so using the equation for relative fitness, we can see that the white butterfly has 1.14, while the brown butterfly has 0.86.

    Using this same population of white and brown butterflies we can explore positive selection on the trait of wing coloration. We will assume that this population of butterflies lives in an environment where brown individuals have an easier time camouflaging from predators. The first generation in this population consists of 60% brown and 40% white butterflies. The predation rate on white butterflies is higher since they are easier to spot, while the darker-colored butterflies blend in better, leading to higher survival rates. Due to the increased predation, when the second generation of this species comes about, there is a change in population percentages. We now have 10% white and 90% brown. The average absolute fitness of white butterflies is 40 and 100 for brown butterflies or an average population fitness of 94. Looking at relative fitness again we can see the brown butterflies have a score of 1.06 and the white have 0.43.

    As time continues, and new generations are born, we observe the population become even more predominantly brown due to their higher fitness. The main driving forces of this change are the predation pressures and environmental factors. 


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