Do you know that Aristotle referred to dolphins as fish? It is not surprising at all, as they look alike with their streamlined body shape and a fin on the back. This similarity can be quite unsettling when you see a fin approaching you during a swim. And that's not the only example of a resemblance found in living nature. Have you ever wondered why the wings of a bird and the wings of a fly, which are so different in structure, are still… wings? How did echidnas, hedgehogs, and porcupines manage to grow spiny protection even though they belong to different families? The answer is convergent evolution. Let's take a closer look at it.
Analogous vs. homologous
Convergent evolution is a process whereby not closely related species independently evolve similar features. It creates analogous body parts that share similar forms and purposes though they have different origins. Bird's and fly's wings are analogous structures: they assume different structural features but are both used for flight. On the contrary, homologous body parts share a common origin but have dissimilar forms as they were adapted to different functions. The human arm and dolphin's flipper are homologous structures: derived from the same forelimb of the ancient vertebrate, but the former helps you grab a fruit, and the latter turns a dolphin into a top-swimmer.
Some structures are homologous at one level, but analogous at another. Wings of birds, bats, and pterosaurs are functionally analogous, as they all serve as wings, and the last common ancestor of those groups didn't fly. However, the bones of these wings are homologous because the tetrapod ancestor had them in its forelimbs. Later on, some descendants of this ancient tetrapod evolved their forelimbs into wings independently from each other.
Briefly: homologous = same origin but different functions, analogous = different origin but same function
Another convenient term to describe a shared trait is homoplasy. Homoplastic traits were absent in the common ancestor of analyzed organisms, for example, reptile-like ancestors of birds and mammals lacked warm blood. Homoplastic traits arose independently through convergent evolution. They could lead to the incorrect analysis of the evolutionary relationship as they make organisms look more closely related than they really are.
Convergent vs. divergent
But why do evolutionary distant species evolve to look alike? To answer this question, let's go from the opposite process when closely related organisms accumulate differences. The classical example of this divergent evolution is Darwin's finches. A long time ago, a flock of finches came to the Galapagos islands. At first, they had looked more or less the same, but after a couple of millions of years, they diverged into several species. To put it simply, it happened because the Galapagos islands offered a variety of food to those finches. Some finches preferred bugs, and others liked fruits, so their beak shape changed to better fit their preferences. Thus, different environmental pressures (in the case of finches, each type of food requires a special approach) caused the divergence of a morphological trait. On a larger scale, specific environmental conditions might result in the formation of homologous organs: legs for running, wings for flight, and flippers for a swim.
Organisms should match the environment they live in or they will die. But no matter how diverse the environment may seem to us, it offers a limited set of roles to the organisms. These distinctive ways of life are called ecological niches. Darwin's finches split up and occupied different ecological niches (bug-eaters, fruit-eaters, and so on), which is manifested in their divergence. So, it is not a big surprise that if evolutionary distant organisms inhabit similar environmental niches, they resemble each other in behavior and morphology. They will grow similar traits like analogous organs if they like similar food, live in similar climates, and fight similar obstacles. For example, both Indian vultures and Andean condors eat carrion and have bald heads to keep them clean when feeding, but they are not close relatives. They develop the same solution to the same problem — it is convergent evolution in a nutshell.
There are clear examples of convergent evolution not only at the level of organisms but at the molecular level too. Many hydrolase and transferase enzymes (the former break chemical bonds with a water molecule, and the latter transfer chemical groups) have a set of three coordinated amino acids with specific traits in an active site. The identical geometric arrangements of amino acid residues in catalytic triad evolved independently more than 20 times in separate enzyme superfamilies.
During divergent evolution, related species accumulate differences as they inhabit different ecological niches. During convergent evolution, not closely related species evolve similar features as they inhabit similar ecological niches.
Parallel evolution
Species that descend from the same ancestor share a lot in common. In case of similar environmental pressure, they easily evolve similar traits as they already have the same basis for their development. It is called parallel evolution. The prime example of parallel evolution is two main mammal branches: the marsupials (kangaroos, koalas, etc) and placentals (most mammals). Their evolutionary paths parted 100 million years ago, and further, they developed on separated continents. Evolution leads to surprisingly similar animals living in Australia and other parts of the world: burrowing marsupial and placental moles, hopping jerboas and kultarrs, carnivorous Tasmanian wolves, and gray wolves. Other examples are beetles and some bugs, which independently modified their forewings to be hard and protective, and flying squirrels and flying lemurs, which evolved a gliding membrane.
The border between convergent and parallel evolution is blurred since all the organisms, even evolutionary very distant, have a common ancestor somewhere in the past. However, the evolution is clearly convergent, if different structures were modified for a similar function like in the case of octopuses and human eyes. On the contrary, if similarities have evolved from the common trait an ancestor had, it is parallel evolution.
Conclusion
There are three main trends that the process of evolution can take. During convergent evolution, organisms develop similar traits as they inhabit similar ecological niches. If the last common ancestor lacked them, these newly-derived characteristics are called homoplastic. Convergent evolution may lead to the emergence of analogous structures, which have different origins but perform similar functions. If the common ancestor of the organisms has a trait, which its descendants independently developed to the same result, this is the case of parallel evolution. During divergent evolution, organisms adapt to different ecological niches and diverge their traits. It may cause the formation of homologous structures, which have the exact origin but were modified for different functions.