What are directional selection across populations?

Directional selection across populations is when one type of trait becomes more common in different groups because it helps them survive better.

Imagine you have two groups of kids playing a game on the beach, one group has to run through sand, and the other runs through grass. The kids who are faster win candies at the end of the game. In the sand group, the fastest kids win more often because running through sand is harder. Over time, the sand group starts having more fast runners. In the grass group, maybe being a little slower is okay, they still get some candy too. So, over time, that group has more average-speed runners.

This is like directional selection across populations, different groups are picking out different traits because those traits help them in their specific environment.

• In each group, the trait that helps the most gets passed down.
• Over time, the groups start looking and acting differently from each other.
• It's like playing a game where each team has its own rules, and the best players for each rule win more often.