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Causes of Evolution: Natural Selection, Mutation, and Gene Flow

Darwin’s notion of natural selection is not necessarily the only cause of evolution, but it is the key that unifies the theory. Two other important factors are mutation and gene flow. Before we begin our discussion of these three processes, it is important to note that evolution occurs in populations, not individuals. A population is defined as a group of individuals of the same species occupying a given area. Therefore, the remainder of our discussion will focus on populations of individuals, and how changes arise in these populations.
  • Natural selection: Natural selection is the result of the interaction of environment and individual variations. When environmental conditions change, selective pressure is placed on a population. Only organisms possessing beneficial variations that allow them to exist in the different environment will survive and reproduce. It is important to understand that natural selection can only work with the variations already present in the population. New variations cannot be produced to help organisms survive.
    For example, during the evolution of terrestrial vertebrates, the ancestral species was a tetrapod (possessing four legs). Therefore, all terrestrial vertebrates have “four legs.” Human evolution involved a transition from walking on “all fours” to walking erect, and an alteration of the forelimbs into arms and hands. Natural selection did not create this situation, but made due with the materials it had to work with. Made due, you say? Aren’t humans pretty well off? Well, yes, but ask anyone who’s ever had back or knee problems, and they’ll tell you these structures are far from perfect. For all intents and purposes, our intellect and logic allowed us to survive, not our physical strength and prowess. Natural selection is responsible for both.
  • Mutation: As we discussed above, Darwin was not aware of how variations arose or how traits were inherited, as the field of genetics would not take shape for decades after his death. Today, genetics is central to our understanding of evolution. In Chapter 6, we discussed how mutations alter protein production. This alteration produces new alleles, or variations of genes that may result in a change in the phenotype of an organism. New alleles may change the genetic make up of the population, and this may lead to evolution.
  • Gene flow: Gene flow is described as the movement of alleles from one population to another. This movement could cause changes in the population, either by eliminating variation (as some organisms may leave the population) or by enhancing it (as some organisms enter the population). 
Although mutation and gene flow play critical roles, natural selection is cited as the main cause of evolution. As you may have already deduced, the three are not mutually exclusive, and, indeed, natural selection often involves mutation and gene flow. Perhaps the most important thing to remember is that evolution is due to both environment and genetic variation within species. Natural selection is the result of the interaction of these factors and produces changes in the population.

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