Structural Biochemistry/Evolution of Populations
Although it is true that natural selection takes an effect on individual organisms, the evolutionary influence of natural selection is only evident in a population of organisms over time in their changes as a whole. 
Genetic variations are the distinctions among individuals in the makeup of their DNA segments or genes; individual variations commonly reflect genetic variation. Genetic variation is the reason why evolution exists in the first place; it gives the raw material for evolutionary change. Often, the genetically determined components of phenotypic variation can have evolutionary effects. 
Variation allows for survival of the population by adaptation or flexibility to the environment. Genetic variation is advantageous for this reason. It allows for a population to survive despite a changing environment. A great amount of variation exists due to being able to draw from a huge pool of alleles and genes. Some new alleles allow for greater survivability and therefore get selected fore, while others are detrimental and die out. Also, there are neutral alleles that do not contribute to overall survivability so they exist due to their neutrality. Genes and alleles are mixed to contribute to variation through breeding. There are two types of breeding methods that are non-random. They are non-random because individuals choose potential mates based on specific qualities. Inbreeding describes how mates choose partners with similar traits to themselves while outbreeding is how mates choose for qualities polar to their own. 
- Biology 9th Ed., Campbell.
- Genetic Variation, November 20, 2012.
Variation within a Population
There are two different types of characters that will vary within a population. The first of the two is a discrete character, which can be categorized on an either-or-basis. The discrete characters are commonly decided by a single gene locus with differing alleles that create discrete phenotypes. The other type of characters are the quantitative characters, which differ along the range within a population. As opposed to discrete characters, quantitative characters are commonly decided by two or more genes on a single phenotypic character. 
Variation Between Populations
Species display geographic variation as well as variation within a population. Geographic variation, or the distinctions in the genetic makeup of different populations, often occurs when populations are geographically separated by environmental barriers or simply geographically located elsewhere with different environments. One example of geographic variation are clines, or graded changes in a character down a geographic axis. 
Sources of Genetic Variation
Genetic variation stems from when gene duplication, mutation, or other processes produce new genes and alleles. New genetic variants can be created in little phases of time in organisms that rapidly reproduce. However existing genes can be arranged in new ways as a result from genetic variation by sexual reproduction. So overall, the main sources of genetic variation are the formation of new alleles, the altering of gene number or position, rapid reproduction, and sexual reproduction. 
One of the most important tools population genetics gave to the study of evolution was the principle of Hardy-Weinberg equilibrium. This principle states that there is nothing in gene replication, meiosis, fertilization, or reproduction that changes the frequency of gene alleles over time. As long as no other forces act upon the population, a gene in that today has 25% allele "A" and 75% allele "a" will still have 25% A and 75% in a million years.
The Hardy-Weinberg equilibrium is based in five assumptions, which, if they held true in nature, would create a situation where no other forces were acting upon a population and no change in gene frequencies (evolution) would occur. These assumptions are:
1. The population is infinite, isolated, and panmictic, where all individuals have an equal chance of mating with any other individual in the population. 2. There is no mutation in the genetic material of the population. 3. There is no gene flow in the population (no individuals leave or join the population). 4. There is no genetic drift. 5. There is no difference in reproductive success between members in the population (natural selection is not acting on the population).
Since these are the assumptions that must hold true for the Hardy-Weinberg equilibrium to be maintained, their opposites are the causes of evolution, or the change of allele frequencies in populations. For example, mutation can cause changes in allele frequency by creating new, altered genetic material while gene flow can change allele frequencies by introducing new alleles to the population through immigration or removing alleles through emigration. Of these five causes of evolution, only the last one involves natural selection, or directional change imposed by survival of the fittest in a harsh environment. The first four, all of which can play critical roles in evolution, involve chance events.
By studying the allele frequency in a population, and doing some math, ecologists can determine whether allele frequencies occur as predicted by the Hardy-Weinberg equilibrium principle. Hardy-Wienberg allele frequencies are likely with large populations that have lots of gene flow and random mating, and are unlikely if populations are small, isolated, or have non-random mating patterns. If a study reveals allele frequencies that do not match Hardy-Weinberg predictions, the population is probably structured in some way (e.g., is made up of more than one isolated subpopulation or has non-random mating). This is what two teams of researchers have done with monarch butterflies.
- Biology 9th Ed., Campbell.