The role of genetic drift in evolution

Genetic drift is one of the major mechanisms that drive evolution. Genetic drift is the gradual change in the genome of populations over time, even in the absence of natural selection. Natural selection works on mutations which impact survival, favoring those which enhance the chances of survival. It is therefore the mechanism behind adaptation, the process of gradually “fitting” an organism to a particular niche in a given environment. Genetic drift, on the other hand, works on all mutations, including those which offer no survival advantage. Over time, it can produce changes in the genome which can lead to speciation even without environmental pressure. It can even resist and counter the effects of adaptation to weaker natural selection forces, which is why many harmful mutations continue to exist.
Genetic drift is a stochastic process, or it operates on random chance, and is affected by factors such as breeding population size, the geographical spread and consequent isolation of breeding groups, ecological disasters such as famine, epidemics, volcanoes and earthquakes, and yes, even asteroid strikes. Here's how it works:
The DNA of organisms is made up of genes. Each gene encodes for one protein. Many organisms are diploid, having paired homologous chromosomes. Each chromosome in a pair has a separate copy of the gene, so in a diploid organism, there are two copies of each gene, called alleles. If they are identical, then the organism is homozygous for the trait expressed by that gene. If the two alleles of the gene are not identical, then the organism is heterozygous for that trait. While in a given diploid organism, only two copies or alleles of a gene can exist, in the population there might be many more. For example, for a given gene, there might be 9 variants or 9 alleles in the population living in one country. Any individual in that population has 2 out of those 9 possible alleles. The distribution of the 9 alleles in that population is static at any given time, but over the course of time it may change. It may be that right now alleles 2 and 7 are more prevalent, accounting for 50% of the entire population. But perhaps a few thousand years ago, alleles 5 and 9 were the most prevalent. This change in the frequency of different alleles over time is in fact evolution, and genetic drift is one important cause for it.
 There are many ways in which genetic drift works. The most simple is a random disaster. Suppose there is a breeding population of 10,000 living in a given area. A volcano erupts nearby, and all the organisms living in proximity die. This has nothing to do with natural selection, because no member of the population is evolutionarily better suited to surviving hot molten lava, or to breathing searing hot poisonous gases.  In other words, this kind of disaster does not selectively kill the weaker or the ones with bad genes, it simply kills whatever happened to be close by. As a result of this disaster, half the population dies. The remaining half will probably not have exactly the same allele distribution as the whole population did. For example, for a certain gene A, which has 9 alleles, it may be that prior to the disaster , the population as a whole had an allele frequency distribution such that allele 1 occurred in 32% of the population. But because of random chance, more organisms with allele 1 lived near the volcano, therefore they died in disproportionately high numbers, and after the disaster the frequency of allele 1 falls to 18%. The same principle applies to all alleles of all genes.

Visit these websites for more information about genetic drift:

 http://www.talkorigins.org/faqs/genetic-drift.html

http://anthro.palomar.edu/synthetic/synth_5.htm