1. OVERPRODUCTION. In a species [organisms that can interbreed], more individuals are born than survive to reproduce.
2. VARIATION. There is variation among individuals in a population, much of which is genetic.
4. DIFFERENTIAL REPRODUCTION. This differential reproduction will lead to gradual change in a population with an increase in characteristics adapted to the environment and alleles that code for those characteristics.
The gray squirrel (Sciurus carolinensis) is a species common in eastern North America. Most individual squirrels of the species are indeed gray, but in northern populations there are a number of squirrels that are all black. When the pioneers settled northern Wisconsin, a large proportion of the species population were black. By the early 1900s, due to hunting, the gray squirrel population was very low and black gray squirrels were extremely rare, probably under 1% of the population. Hunting regulations were passed that limited the shooting of gray squirrels. The gray squirrel population recovered and the percentage of black gray squirrels has again increased in northern populations. (About 30% black in my northern Wisconsin squirrel research.)
Briefly try to explain this increase in percentage of black gray squirrels in terms of natural selection. Include in your answer these components of natural selection - overproduction, variation, competition & survival, differential reproduction.
In natural selection, there are always more individuals produced than can survive in the environment. This means that some (or most) are going to die depending upon the conditions that exist. In the scenario presented, both black and gray squirrels were produced. The colors of the fur is a variation of the trait, adding to the selection pressures. Squirrels eat the same things, so both the black ones and the gray ones are in competition with each other for the same resources. As a result, those individuals that are best suited for the environment are going to beat out the others ...
This answer describes the process of natural selection as it applies to gray squirrels. It provides an overview of how natural selection occurs, and gives some suggestions about how the population of squirrels may be effected when environmental conditions change.