An Australian forest fire wipes out over 92% of a population of terrestrial gastropodan slugs. The slug's gene pool originally consisted of 4 alleles for dermis color CM , CS, CB and CA these occurred at frequencies of 0.3, 0.4. 0.1 and 0.2 respectively. After the fire the slug's gene pool now consists of only 2 alleles CS , and CB at frequencies of 0.2, 0.8 respectively. This type of forest fire mediated reduction in allelic variation is called