Evolution is defined as a change in species over time. There are many processes that can occur to drive evolution including Charles Darwin's proposed idea of natural selection and the human-created artificial selection and selective breeding. Some processes produce much quicker results than others, but all lead to speciation and contribute to the diversity of life on Earth.
One way species change over time is called convergent evolution. Convergent evolution is when two species, that are not related via a recent common ancestor, become more similar. Most of the time, the reason behind convergent evolution occurring is the build-up of adaptations over time to fill a certain niche. When the same or similar niches are available in different geographical locations, different species will most likely fill that niche. As time passes, the adaptations that make the species successful in that niche in that particular environment add up producing similar favorable traits in very different species.
Species that are linked through convergent evolution oftentimes look very similar. However, they are not closely related on the tree of life. It just so happens that their roles in their respective environments are very similar and require the same adaptations in order to be successful and reproduce. Over time, only those individuals with favorable adaptations for that niche and environment will survive while the others die off. This newly formed species is well suited to its role and can continue to reproduce and create future generations of offspring.
Most cases of convergent evolution occur in very different geographic areas on the Earth. However, the overall climate and environment in those areas are very similar, making it a necessity to have different species that can fill the same niche. That leads those different species to acquire adaptations that create a similar appearance and behavior as the other species. In other words, the two different species have converged, or become more similar, in order to fill those niches.
One example of convergent evolution is the Australian sugar glider and the North American flying squirrel. Both look very similar with their small rodent-like body structure and thin membrane that connects their forelimbs to their hind limbs that they use to glide through the air. Even though these species look very similar and are sometimes mistaken for each other, they are not closely related on the evolutionary tree of life. Their adaptations evolved because they were necessary for them to survive in their individual, yet very similar, environments.
Another example of convergent evolution is the overall body structure of the shark and the dolphin. A shark is a fish and a dolphin is a mammal. However, their body shape and how they move through the ocean is very similar. This is an example of convergent evolution because they are not related very closely via a recent common ancestor, but they live in similar environments and needed to adapt in similar ways in order to survive in those environments.
Plants can also undergo convergent evolution to become more similar. Many desert plants have evolved somewhat of a holding chamber for water inside their structures. Even though the deserts of Africa and those in North America have similar climates, the species of flora there are not closely related on the tree of life. Instead, they have evolved thorns for protection and the holding chambers for water to keep them alive through long periods of no rain in the hot climates. Some desert plants also have evolved the ability to store light during the daytime hours but undergo photosynthesis at night to avoid too much water evaporation. These plants on different continents adapted this way independently and are not closely related by a recent common ancestor.