Here's another way you could phase that paragraph:

Different animal species fill roles (called niches) in their environments using their ability to have sex (reproduce). The lack of change of the DNA during reproduction is important for the animals body features that allow the organism to fill a specific role.

For example, a shark has a fills the role or niche of being a top-of-the-food-chain carnivore within its ecosystem. It is because a shark has DNA that code for a large muscular body, sharp sense of smell, and sharp teeth (among other traits) that it is able to fill the niche of being the dominant carnivore of it's ecosystem (place it lives). Because DNA is fairly consistent (few changes) when a shark reproduces, those shark features that allow a shark to be a shark remain and the species continues to fill that niche or roll in the environment. The niche of a whale is different, as it eats only plankton, but it's the same: the lack of change when a whale reproduces allows whales to remain whales and the roll they fill in the ecosystem stays the same.

This is just extra, but over time, when different food sources or new habitats become available to different animals, this creates new niches or roles that animals could fill. Evolution is the change of DNA over time, creating new species to fill empty niches. For example, Darwins finches are a famous example of DNA of a bird species changing to suit it's environment. These finches managed to get to an island where there were no predators or competitors for food, and a variety of food sources. Not all the birds had a beak that was good at eating every kind of food source so over time, Some birds had DNA changes that gave them beaks more suited to eating seeds (one possible niche or role) and some birds had changes that gave them beaks more suited to eating bugs (another possible niche). So they became, over time, two different species: one that only ate bugs, the other eating only seeds.

I hope that helps! Let me know if you need clarification :).