Variation between and within populations may be brought about by the four main forces of evolution. The main forces of evolution include gene drift, genetic flow, mutation and natural selection.
Natural selection is a process that ensures survival for the fittest. Individuals, who have the traits that help them to survive in a given environment, continue to reproduce offsprings. As usual, they have the same traits as their parents. Therefore, the population can evolve due to natural selection. As the result, the population with a certain trait develops after the characteristics that have been passed on to the next generations. Populations differ in traits that help them to survive in a given environment (Mayr, 1963).
Genetic drift is a process, according to which an individual with certain traits may survive owing to a chance. For example, closely related individuals mate their offsprings, whose chances to survive are low, due to a condition called homozygous condition.
Gene flow is a process where genes that did not exist in the population are brought by an individual from another population. For example, when a pollen grain is carried by wind and falls on a plant that have not had that genetic material before, the genetic differences between the two plants will be minimized in the next generation (Maynard, 1993).
Mutation is a way an offspring differs from its parents due to change of its DNA. An individual may differ from other individuals within the same population because of the change of its DNA, leading to new genes that bring about different traits in the individual.
Isolating mechanism is a separation of different species from each other to prevent them from mating. Isolating mechanism prevents the mixing of the genes in different species.Speciation is a process of finding out differences between species. A group of species that have unique traits can be distinguished from other species after speciation (Sober, 1993).