Convergent evolution is the tendency of species, often separated by vast distances, to evolve in similar ways around similar stresses. It can show in physical makeup, place within trophic structure, behavior, and/or feeding patterns; more complex species display subtler convergent patterns.
Convergence does much to validate Darwin's work. It also demonstrates that we're given shape, visibly and invisibly, by the things acting upon us.
Angiosperms, what we know as flowering plants, radiated 150 million years ago over a Pangea that was a third of the way through drifting apart. It was a successful design. The only plants we have today that aren't angiosperms are conifers like pine and spruce, and ferns. Plant evolution at the family level has stood nearly still for 80 million years. We know this partially because the trees of the world's tropical forests, some of the world's oldest biomes, fall into six families.
Forests are similar most obviously in vertical organization. Starting underground, the layers of a forest on Earth are rhizomes, herbaceous plants, woody undergrowth, understorey, canopy, and emergent layer. The same plant families occupy the same forest layers all over the world. Have, for eighty million years. The species within those six families have overturned many times; the
species we know today are recent ones, having evolved the same way,
More spookiness ensues when one compares plants at different elevations. Laurels grow at middle elevations everywhere in the world; heaths and daisies at higher elevations, everywhere in the world. Similar truths bear out across all plant life. Forest at 500 meters' elevation in Madagascar does not resemble forest at 10 meters in Madagascar; it resembles forest at 500 meters in Brazil, and Indonesia.
Hundreds of tree species in the wettest forests, all from different and comparatively recent lineages, evolved buttresses and stilts to prevent toppling. Conversely, the plants making up chaparral scrub the world over are uniformly forced into squat shapes by cascading heat and dead soil.
And on it goes.
There are myriad animals built the same ways for the same reasons.
The most obvious examples are in mimicry, whereby one (or more) species co-opts the successful characterstics of another. Toxic monarch butterflies are piggybacked by viceroy butterflies, for example. Experts die in jungles, because the snake species are so similar. Mimicry bears its own writeup.
Such convergences are interesting because they illuminate natural selection. More interesting still are convergences in response to physics. For example, because air experiences friction, wings have appeared independently on Earth four times: in insects, in pterosaurs, in birds, and in mammals.
The rhea, the ostrich, and the emu evolved independently (nb: three continents).
More interesting still are echoing relationships. Because animals that live in darkness have good ears, the feathers of owls' wings are crenellated.
And on it goes.
In an early effort to "plot" convergent animal species morphologically, orinthologists James Karr and Frances James examined bird communities in Liberia, Panama, and finally Illinois, as a control. Using measurements of standard features skimmed from museum specimens (beak dimensions, wing length, etc), they used myriad statistical fluorishes to plot the birds on a two-dimensional space, whereby similarly-built birds occupied similar coordinates. Results were mixed. The chart did group birds satisfyingly enough according to ecological roles, but the Illiniois birds did not converge with the Liberian nor Panamanian birds; Panamanian hummingbirds also complicated things, having no morphological counterpart in Liberia. Their role in the ecosystem, however, was echoed by non-hovering sunbirds.
Convergent evolution is easy to observe in plants because plants have no choice but to express their immediate surroundings. Similar ecological niches produce plants that look and behave similarly, and physics tend to require the same workarounds everywhere you go.
Behavioral convergence is an area of ongoing study, and tends to be function/guild based (see hummingbirds/sunbirds, above). The best statistical evidence for behavioral convergence comes from Pearson, who for two years observed tropical forest birds in Ecuador, Peru, Bolivia, Borneo, New Guinea, and Gabon--specifically, their nine shared foraging techniques. These he recorded, along with species of bird and number of times per hour, then ranked by order of decreasing frequency.
The rankings were consistent across the six forests.
Terborgh, John (1992). "Diversity and the Tropical Rainforest." Scientific American Library.
Science Daily. "Convergent Evolution."
PBS Evolution Library. "Convergent Evolution."
Pianka, Erik R. "Convergent Evolution."