El Nino involves very wet weather condions that cause extensive flooding. The floods of an El Nino can damage habitats, kill organisms and destroy most of the resources required to sustain an initial popualtion size. El Nino might also wipe out a large proportion of a population. This, therefore, wipes out a large proportion of alleles from the population. If the population was very small, there will be a dramatic shift in allele frequency since the small population left (with very low allelic variability) will have the responsibility of repopulating. This is called the bottleneck effect.
