The idea that entire populations of species change over time to become better adapted to their environment is called .

The idea that entire populations of species change over time to become better adapted to their environment is called Evolution.

Here are the steps to understand this concept:

1. Variation: Within any given population of organisms (say, a type of bird), there is genetic variation. Some birds might have slightly longer beaks, some might be a bit faster, some might have better eyesight, etc. This variation is crucial for evolution.

2. Selection Pressure: This is an environmental factor that makes certain traits more advantageous than others. For example, if a population of birds lives in an area with deep, narrow flowers, having a long beak might be advantageous because it allows the bird to reach nectar that birds with shorter beaks can't.

3. Survival and Reproduction: Birds with longer beaks are able to get more food, so they're healthier and more likely to survive and reproduce. When they reproduce, they pass on their genes for long beaks to their offspring.

4. Over Time: Over many generations, the proportion of birds in the population with long beaks increases. This is because the birds with long beaks are more likely to survive and reproduce, so more and more of the population inherits the long-beak genes.

5. Adaptation: The population has now adapted to its environment. It's evolved to have longer beaks, which are better suited to getting food from deep, narrow flowers.

This process is known as natural selection, and it's one of the main mechanisms of evolution.