Populations evolve for many reasons. suppose there is a population of plants that have either purple flowers or white flowers, and the allele for purple flowers is dominant. this means that plants with two purple alleles have purple flowers. plants with one purple allele and one white allele also have purple flowers. only plants with two white alleles have white flowers.

for each event or condition described below, answer the following questions.

which mechanism of evolution is at work?
how does this event affect the population�s gene pool? do the frequencies of the two alleles change, and if so, how?

natural selection

Explanation:Natural selection, gene flow, gene drift and mutation are the mechanisms for EVOLUTION.

The dominant purple allele must have been acquired from previous parent gene, by natural selection

Definitely the available   alleles from previous flower population were white and purple flowers. But, because the parents of purple flowers exhibit VARIATION, in their gene pool,  they showed dominance over  the recessive white colour flowers in the same population.  

◼ from Darwin’s observation and deduction, Organisms who acquired certain characteristics from variation  tend to survive than others, and therefore pass these inheritable characteristics to their offspring.

◼Thus the purple flowers dominated the population for the available resources, among the other flowers colours, (competitions) therefore it leads to a stable population in size after a period of time., while the white colour flowers are recessive

◼Darwin concluded that, variants of best adaptation will be selected for by natural conditions operating in the environment of the population at that particular period (natural selection).

Based on this, natural selection takes place, and the purple flowers with best variation will have selective advantage (dominance)above white colour flowers(recessive).

Consequently, they have higher survival rates compare to white flowers in the population this is   survival of the fittest by natural selection of purple flowers

The frequency of the white colour flower will reduced because of less manifestation of traits from generation to generation from reduced gene pool due to recessive trait, however the frequency of Purple flower will increase because the number of copies of allele of purple flowers is greater than the total number of all the flowers population( white flowers+purple)