Which Mechanism of microevolution caused the the beak of the medium gound finch population on dafne major to increase in size in size from 1976 to 1978?

Natural selection

Explanation:

Microevolution refers to changes produced at a lower level than species. In genetics, microevolution is the change in the allelic frequency perceptible in a few generations. Most of these naturally produced changes by mutation, natural selection, genetic flux, genetic drift.

After the drought on Daphne Major, many of the plants producing small-sized seeds decreased their reproductive rate drastically. Consequently, there were almost no seeds available for the medium ground finch to feed. The population of this species also decreased to only a hundred birds over two years. Weather conditions and food availability influenced the survival of the animals.

With time, the finched population increased again, but now, the average size of the beaks was larger. The trait modification was related to the availability of only larger seeds with thick husks.

Eating large seeds with medium or small-sized beaks was impossible, so Finches needed to adapt, developing larger beaks to crack open the husks and eat the contents of the seeds.

Natural selection was responsible for the rapid change in the finches´ population beaks size after the drought. The evolutive force modifies the allelic frequencies, increasing the frequency of genetic variants that expressed the larger beak size and declining the frequency of the alleles that expressed smaller beak size.  

a covalent bond, specfically triple bond