Natural selection takes place in the following conditions:
1. There should be a struggle for existence, which means that there are more individuals that are born than can survive.
2. There should be a presence of heritable variation.
3. There should be a different fitness rate among individuals to initiate the “survival of the fittest.”

According to Darwin, the unity in all the living things can be observed through a process called descent with modification. He proposed that the relationship between individuals can be traced using an evolutionary tree to show how modern organisms are all linked to one another by their common ancestors.

He also noted that the evolution in the species, which took place over the years, is responsible for the great diversity of life that we have on Earth.

If the organism has a high fitness, their survival and reproduction are easier because the organism is adapted well to their environment. Being able to reproduce offspring would ensure that the existence of their species and their role in the ecosystem would continue over time. In this way, they help keep the ecosystem to be balanced and highly functional.

One example is the polar bears in the arctic tundra. They have special adaptations that enable them to survive and reproduce in their biome. Their existence in the ecosystems found in the Arctic region is highly valuable because they keep the ecosystem functioning. They control the seal population by feeding on them.

The Anole species that are found on the same island have differences in body shape, behavior, and niche. These characteristics help them survive in their environment. For an instance, the species that live near the ground have long back legs that enable them to run and jump quickly. This trait is helpful when escaping their predators. On the other hand, the species that live in the upper part of the tree have short legs and stickier toes, which allow them to have a stronger grasp on the branches.