In this situation, the swan can see the brown fish compared to the blue fish who blends in. The swans eat more and more of these brown fish until there aren't any left. This then takes away from the gene pool. Now, the blue fish will start making blue offspring, and soon the species becomes primarily blue due to a dominant blue gene.
Brown fish, YUMMY!
Here, the lion and cheetah compete for the same resource: food. However, because the cheetah was faster, it got to the gazelle first. In ecological competition, species that do not have an adaptation or natural trait to help them attain a resource usually fall into extinction due to competition. Evolution occurs because that species gradually gets another adaptation or the species able to get the food dominates.
Over time, natural selection and other processes have occurred. Once a cat, now a lion or a tiger. All in all, they cam from one species, or common descent.
*Note: This isn't actually where dolphins, orcas, sharks, and penguins live; I just needed a body of water. In convergent evolution, none of these animals have a common evolutionary history. However, they have analogous structures, meaning they are similar but not bound by history. Notice how all of them have similar streamline bodies and fins to help them swim.
Coevolution is due to changes in the relationship between two organisms. In this scenario, we have a bee and a plant. If the plant does not have the specific/right shape, color, odor, and other characteristics, then the bee will not come to pollinate. This one change in the bee leads to another change in the plant.
Throughout evolution, some changes occur rapidly over a short period of time juxtaposed to long, stable periods of time taken. These long periods are interrupted by the short periods of time. Although up for debate, it is mostly commonly accepted that evolution has happened at different rates of time.