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Do species converge or diverge as they evolve?

        Science | Evolution

Unrelated species can take on similar forms in they live in the same type of environment. See more saltwater fish pictures.
Digital Vision/Thinkstock

Waiting at the bus stop, you notice a living microcosm in the grass below. An ant drags potato chip crumbs to a nearby mound; a handful of mushrooms tower above the dirt; a hungry bird pecks strategically at the ground. Interestingly, what sets these organisms apart also draws them (and us) together: the process of evolution.

Evolution is defined as descent with modification, which means that existing organisms descended from ancestral ones. Over time, from a few generations to millions of years, evolution's genetic tinkering of traits and adaptations often result in the creation of new species and the extinction of others. Charles Darwin and Alfred Russel Wallace are best known for their descriptions of a major mechanism of evolution called natural selection, in which certain heritable traits become more common because they give organisms a leg up to survive and reproduce.

Natural selection and other aspects of evolutionary change create noticeable patterns in the fossil record (and even among living species today). But understanding evolution's intricacies isn't necessarily easy. How can unrelated species develop similar body parts, while genetically similar species appear to be complete opposites? In other words: Do species converge or diverge as they evolve?

In the scheme of evolution, species can both converge and diverge, depending on genetic and environmental factors. Evolution leaves its mark on living things and shows up in reoccurring patterns, including convergent evolution, parallel evolution, divergent evolution and coevolution, to name a few.

Convergent evolution occurs when one or more unrelated species (meaning they don't share a recent ancestor) evolve similar morphologies or behavior. Often, the species "converging" fill similar ecological niches in a given habitat or in different regions of the world. For instance, a torpedolike body among marine organisms is common in several unrelated animals. Marine mammals like dolphins, certain species of sharks, and even fossils of extinct marine reptiles, for example, share this shape -- but they don't share a recent ancestor. In most cases, similar morphologies evolve in organisms to overcome the same natural hurdles in different environments. Much like convergent evolution, parallel evolution occurs when species evolve similarly but share a recent ancestor.

Divergent evolution, on the other hand, happens when related members of a group possess enough variation to be considered a separate species. The term also applies to two or more related species that become increasingly dissimilar as they evolve [source: BioWeb]. A widely known process of divergent evolution is adaptive radiation. You may have heard the term while learning about Darwin's famous finches on the Galapagos Islands. Darwin hypothesized that an original species of finches arrived at the island and split up to exploit new environmental niches. This gradually resulted in the creation of several species. In addition to exploiting new environments, divergent evolution can be influenced by the physical demands of a given environment, competition for resources and geographic isolation [source: Schluter]. Extinction can be a consequence of divergent evolution as well.

Coevolution is best described as two or more species affecting each other's evolution in a mutual way [source: University of California Museum of Paleontology]. Species that coevolve usually have close relationships with one another -- they might be predator and prey duos or have a symbiotic relationship. In this case, species aren't necessarily converging or diverging, but rather evolving to match the adaptations of other species.

Generally speaking, these types of evolution are widespread but not exclusive. In some cases, evolution stabilizes populations, making extreme traits less common. It's important to realize that evolution isn't a solid march toward perfection -- it's a complex process that can manifest itself in many different forms. Ultimately, understanding evolution's ways lends us a greater appreciation for the natural world and our own existence in it.

Read more about evolution on the next page.