When plants grow up in an environment without much stress, they tend to be relatively similar. For traits, we can easily measure, such as height, it is easy to see. The plants are all roughly the same height.
But when you add stress to the environment, something changes. The plants respond differently. Some plants do just fine, while others do not fare well at all. The variation is what is important. You might expect that, in a drought, all the plants are short, or all the plants do not produce as much corn. But that is not the case. Some plants continue to thrive despite the stressful surroundings. And the key to their success lies in their genotype.
In a way, people are like this, too. In an ideal setting, many people grow up happy and healthy. But in a tough situation, some people succeed, while others suffer a great deal.

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2017.00048