Question

define genetic drift

Answer 1

Drift is changing slightly . genetic drift is used in context of viruses which changes their antigen structure continuously, such as HIV

Answer 2

The wiki definition is "Genetic drift or allelic drift is the change in the frequency of a gene variant (allele) in a population due to random sampling."

So basically the genetics of a population change (not just through natural selection - this isn't through natural selection) through random events. Some animals will die and not be able to reproduce, not as a result of fitness but just through random events (eg. rockfalls, being stepped on). That organism will then not be able to reproduce and the alleles it holds will be lost.

If you have a population with a small number of offspring the effects of random evens will be more dramatic. Say you have a population where the probability of an allele of occurring is 50% (so you have two alleles each of equal change of occuring). Then one unlucky animal becomes victim to a meteor falling out of the sky (okay i'm using silly examples here). Say there were only four animals in this generation (so 2 with allele A and 2 with allele a). The organism that died had the a allele so now in the next generation the probability of receiving an a allele is reduced. Slowly as the probability is reduced initially the allele cannot be returned or is "killed out".

Anyway, take a look at these sites as they give good descriptions:

http://highered.mcgraw-hill.com/sites/dl/free/0072835125/126997/animation45.html

http://www.biology.arizona.edu/evolution/act/drift/about.html