Scientists once believed in the morphological species concept -- that similar traits were basically all it took to group organisms together into one species. By the 1940s, they had begun to instead accept the biological species concept, which adds reproduction to the mix [source: University of Michigan]. To reproduce, however, species must live in the same location.
When one population of a species becomes isolated from the rest of the species, that population forms its own distinct gene pool. Over time, by means of natural selection, the gene pool of this isolated population will produce mutations and acquire new traits that other groups of the same species will not develop. Eventually, over a very long period of time, so many mutations build up that the two different populations of this species become so different and incompatible with each other that they are unable to interbreed. At this point, the two groups have become two distinct species.
Scientists have proven the existence of species that evolved through isolated evolution. Along with the still-living examples of genetic diversity -- Galapagos tortoises, komodo dragons and many others -- scientists have one major ally in support of their theories on isolated evolution: the fossil record. The fossil record is an organized total of all currently discovered fossils on Earth. When aligned properly, they form an evolutionary timeline for all life on Earth, past and present. This record also offers evidence for speciation, as fossil evolution can be tracked over time to show species developing divergent traits before splitting off entirely into new ones.
Scientists say that it takes more than simple geography, such as island isolation, for reproductive isolation to form new species. It's difficult to study much of how species form because the process takes time and so most of the evolutionary changes have occurred far in the past. Eventually, gene flow between two species that once were related is blocked and the species become distinct, forming at least one new species.
China has provided a large number of fossils to the fossil record -- the natural history of Earth captured by all of the fossilized remains discovered to this point. This skeleton was displayed in Shanghai in July 2007. (China Photos/Getty Images)
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