Edited by Luke When species evolve, they take on a multitude of shapes and sizes as they grow and adapt to survive in their changing homelands. It takes a long period of time for a species to evolve. This will mostly happen when a change happens in the everyday way of doing things. For example, something could change in the food chain meaning that the species that are part of that food chain would either have to adapt or suffer a great loss to their population.
But it turns out that not all evolution is different. Often, creatures evolve to fit into their surroundings and optimise their chance of life. Because of this, most animals tend to evolve in the same ways as others. This is called convergent evolution. Convergent Evolution can result in both singular and overall similarities in different species even if those species never cross paths or even being from the same gene pool. For example, the wings of pterosaurs, bats, birds, (look below) and even insects are similar in terms of their wing structure despite never being closely related.
This picture was taken from Wikipedia https://en.wikipedia.org/wiki/Convergent_evolution
Other examples of convergent evolution are the eyes of most vertebrates and most cephalopods. Vertebrates are species with cartilage or bone surrounding their spinal cords. Cephalopods are members of a particular class of species: the Molluscan class. Members of the Molluscan class are species like squid or octopi.
Even though the two types of species evolved in such different environments, their eyes still have lots in common. The camera eyes of a vertebrate and a cephalopod's developed independently yet still somewhat resemble each-other.