Evolution describes change over time. This could be the radical change seen between a single and multi-cellular organism to something as subtle as a change in the shape of a beak or the color of some plumage. Natural selection is the mechanism by which organic evolution occurs. An organism-be it bird, mammal, reptile or bacterium-can only survive and reproduce if outfitted with the proper equipment to survive in its environment. Those that are deficient in some way-too slow to escape predators, unable to acquire food or water-will ultimately perish before passing on their genes to a new generation. This tends to be a positive thing for the species as a whole, as it will be better able to survive its current environment and adapt to changes.
Convergent evolution: An overview
One of the processes observed while studying evolutionary theory is convergent evolution. This manner of evolution can be seen between species that have similar physical characteristics without sharing a common evolutionary ancestor. These species may be separated by vast amounts of time and location yet still have similar physical structures.
Compare the echidna (a monotreme) and the hedgehog (a member of the order Eulipotyphla). Taxonomically speaking, the closest relation that they share is that they're both mammals. Echidnas are more closely related to platypuses, and hedgehogs share an order with shrews. Echidnas are found only in Australia and New Zealand, while hedgehogs are native to Europe, Eurasia and Africa, separating the species by oceans and continents. Both animals, however, evolved to have rows of pointy spikes on their back and roll up into a defensive ball when threatened.
A broader example would be the use of a poisonous sting via hypodermic needle as a defense mechanism. It appears in species of jellyfish, snakes, spiders, scorpions, sea snails, stingrays and the duck-billed platypus. As it is also found in the stinging nettle plant, it is a mechanism that isn't even contained to a specific kingdom.
Why convergent evolution occurs: A finite number of evolutionary solutions
This phenomenon occurs because of the nature of natural selection. If an organism must adapt, genetics is going to select for the attributes that are the most successful. On the planet Earth during any given geologic age, the conditions are relatively specific. There are certain biomes, levels of UV radiation, atmospheric conditions, temperature ranges and chemical pathways that, on a grand scale, are fairly unchanging. Because all organisms must conform to inhabit Earth-specific conditions, there is a relatively limited number of ways in which organisms can and will adapt with success.
Consider carnivorous plants. Due to various environmental circumstances, a number of unrelated plant species found themselves faced with a nitrogen deficiency. Independently, they developed the ability to capture insects and "eat" them for their nitrogen content. They've developed different methodologies for doing this (pitcher plants versus flypaper versus the spring action of the Venus Flytrap), but the underlying problem and solution is the same.
Same environment, same adaptations
In general, similar environments will produce examples of convergent evolution. The pronghorn is native to the midwestern United States. In form and function it greatly resembles the antelope species of Africa. Antelope, however, are more related to cattle, while pronghorns share a closer familial relationship with giraffes. When compared side by side, the African savanna and the dry grass lands of the United States where the pronghorn live are fairly similar: low average rainfall, grassy plains with fewer trees and comparable temperature ranges. The pronghorn and the antelope also inhabit the same ecological niche: herbivorous prey for larger terrestrial mammalian predators. For any given set of environmental conditions, there are going to be certain evolutionary ideals. If two regions have similar environmental conditions, it then follows that the organisms inhabiting these areas are going to develop similar characteristics.
Same obstacles, same adaptations
Threats of predation can drive organisms to adapt similar traits, as well. Thorns, prickles and spines have developed among thousands of unrelated species of plant because it's simply the most efficient way to deter and prevent consumption by herbivores. Tea and coffee bean plants, even though they are only distantly related, both developed deposits of caffeine for a similar reason.
Sometimes organisms with incredibly different environments and different places in their respective ecological niches can develop similar attributes. Certain species of bats and whales have both adapted to use echolocation as navigation and a hunting mechanism. Though they inhabit very dissimilar circumstances, both whales and bats face a common problem. They must hunt and live in environments where sight cannot be relied on to hunt effectively. Both these types of organism had to adapt a different methodology for getting around, and echolocation was the adaptation that was most successful.
Convergent evolution, as a whole, is what results when natural selection is doing the best it can with what it has. An adaptation that works for one organism can and often does work equally well for another organism, and evolution will, in general, take the most sustainable course.