Consider two species of organisms, species A and species B. Species A has a population in a community. Another population species B arrives into the same community, rather suddenly, being brought into the community by man (there could be other ways by which the new species can enter into a community). Let us assume that the food organisms of the two species are identical (it can be insects or seeds etc.). The range of food particle size fed upon by each individual species falls into a normal curve. By normal curve we mean that there is a variation in the given trait, for example size of the seeds or insects in this case, or beak sizes which are all distributed in such a way that the largest number of individuals in the population has optimum or mean size or measurement and a very few individuals have extreme sizes or measurments. The normal distribution as shown in Figure takes the fo!m of a bell shaped curve. You could observe from Figure that there is a significant overlap in the normal curves of food capturing devices (mouth, beak or tooth sizes) of species A and B. The region of overlap implies that the members of the two species have to compete for their food over a larger area in the community. Essentially, there is an ecological interspecific cornpetiton for food. You may also note in the figure that some food particles (food particles of both large and small sizes) are out of the reach of each species. Although the population as a whole can feed on a range of food particle size, any individual in the community is restricted in the size of the food particles it can feed on.
Assuming that the number of food particles of each size is the same, then individuals outside the region of overlap have better accessibility to the food particles and thcy may have to expend only less energy in acquiring them. Compare these individuals with those which lie within the region of overlap. They have not only to compete with conspecific individuals for their food, but with those of the other species as well. The net result is that the individuals outside the region of overlap have more energy for reproduction, and naturally will tend to leave more offsprings to thc next generation. These offsprings will essentially carry the alleles of their parents including those involved in the determination of food particle size. The effect of this natural selection.