Garfinas flower petal colour can be red or purple. The petal colour depends on the nitrogen content in the soil.

Garfinas flower petal colour can be red or purple. The petal colour depends on the nitrogen content in the soil. If planted in soils with nitrogen above a certain threshold value the plant will produce red flowers; if planted in soils with nitrogen levels below the threshold value the plant will produce purple flowers.

You set up a growing area in your greenhouse. You control the amount of nitrogen in the soil such that 20% of the growing area is above the threshold value and 80% is below. Everything but nitrogen content of the soil is the same throughout the growing area.

You plant 100 Garfinas seeds, evenly distributed throughout the growing area. When the plants grow from the seeds you have 80 purple flowered plants and 20 red flowered plants. You think red flowers are prettier than purple flowers so you decide to 'select' just the red flowered plants for breeding. 

You breed the 20 red flowered plants with each other and destroy all the purple flowered plants. You collect the seeds from the red flowered plants (seeds come from breeding) for planting. You then plant these new seeds in a growing area with the same characteristics as before (80% below threshold; 20% above). 

There will be no significant change in the frequency of red flowered plants.