A bat darting out of its cave at twilight. A butterfly flitting from flower to flower. A bird of prey circling above the tree tops. What do all these creatures have in common?
Not much, at least as far as their relationship on the phylogenetic tree of life is concerned, and that's what makes their shared capacity for winged flight such an interesting example of convergent evolution.
If it's been awhile since your last biology class, here's a brief refresher: Convergent evolution occurs when completely unrelated species evolve functionally similar features known as "analogous structures." The easiest way to understand whether a similar structure found within two different species is analogous is to ask yourself whether their most recent common ancestor also possessed the structure. In the case of bats, birds and butterflies — none of which share a common ancestor that flew — they all "converged" on the ability of flight as a useful trait in response to environmental stimuli and biological goals.
Of course, to fully understand the science behind analogous structures, it's important to talk about homologous structures, which are structures found in different species that originated from a common ancestor. While the wings of a bird and a bat (seen at right) are not homologous, the wings of a hawk and an owl are homologous because they both descend from a common flying ancestor that passed down its wings to future avian generations.
Another common example of a homologous structure can be observed in the bones of today's tetrapods, which are four-limbed terrestrial vertebrates that include amphibians, reptiles, mammals and birds. Despite their many physiological differences, every single one of these animals descend from a single common ancestor that is responsible for originating their basic skeletal structure almost 400 million years ago.
In the diagram below, you can compare the striking similarities between the homologous skeletal structures of several modern tetrapods: a human, a dog, a bird and a whale.
Although the basic arrangement of a tetrapod skeleton is uncanny, the notable differences you see between these four animals is the result of divergent evolution, which occurs when one species diverges into new species by developing variations in traits in response to environment and lifestyle.
One of the most dramatic, well-known examples of divergent evolution is found within the evolutionary history of cetaceans. Millions of years ago, the terrestrial ancestors of today's whales and dolphins abandoned their landlubbing lifestyle to live under the sea. Over time, these born-again sea creatures gradually streamlined their bodies to take on more fish-like characteristics — including transforming their limbs into paddle-like flippers and flukes for more efficient swimming. However, despite these drastic physiological changes, they still retain the homologous skeletal structure of a tetrapod, albeit in different proportions.
What's interesting about the evolution of cetaceans is that their adoption of more fishy characteristics represents not just an example of divergent evolution, but convergent evolution as well. This is why dolphins and sharks, despite coming from completely different branches of the animal kingdom, look so strikingly similar:
It should come as no surprise to learn that sharks and dolphins are very different. Dolphins are mammals, and sharks are fish. A dolphin's skeleton is made of bone, and a shark's skeleton is composed of just cartilage. While dolphins must come to the surface to breathe air, sharks use gills to extract oxygen from the water.
However, both sharks and dolphins have evolved specific analogous traits — streamlined bodies, dorsal fins, pectoral fins and flippers — to achieve the same goal, which is to swim quickly through the ocean and catch prey. In a nutshell, the concept of goal achievement is pretty much the essence of convergent evolution. That is, multiple species from different corners of the world all drawing similar evolutionary conclusions to the challenges and opportunities that they face.
Whether it's soaring through the sky, speeding through the water or trapping prey in sticky pits of doom, instances of convergent evolution are found all throughout nature on many different scales ... and not just in animals, but plants, as well! Continue below for just a few of the most interesting examples of convergent evolution that have manifested in nature.