Speciation occurs when individual members of a particular species develop different genetic characteristics to the point where they cannot mate with the original species and produce a fertile offspring. These creatures have become separate species. Horses and donkeys are an example. They descended from a common ancestor, but their genetic makeup diverged. Their offspring, a mule, is infertile.
Charles Darwin thought about speciation while on his 1835 voyage to the Galapagos Islands, an isolated land that is home to a diverse ecosystem. His observations led to two big breakthroughs. Darwin decided that isolation allows a species slowly to improve itself and could lead to the production of a new species. He also deduced that a species that covers a broad territory would be more likely to form new species in order to adapt to the different conditions it finds in different regions.
Naturalists recognize four types of speciation, but there are overlaps; some species fall into more than one category:
- Allopatric speciation occurs when a large number of individuals are separated from others of their species by a physical geographic boundary. This is the type of speciation Darwin observed on the islands when he studied the types of finches that lived there.
- Peripatric speciation is similar to allopatric speciation, but it takes place when a small group is separated from a larger group of its species. The genetic change in peripatric speciation is much more rapid because the gene pool is much smaller; there simply aren't as many genetic options, so a new species appears in a shorter period of time.
- Parapatric speciation occurs when a species migrates from its original home and enters a new environment. Darwin said natural selection produces individuals more suited for the new conditions. One example is grasses that thrive in the contaminated soils near mines.
- Sympatric speciation occurs when the individuals adapt to changing conditions. Scientists are studying the Rhagoeitis pomonella fruit fly, which originally fed on the fruit of hawthorn trees. Then some individuals shifted to feed on apples, which were more common, about 150 years ago. Now, the apple-eating flies seem to be becoming a new species [source: Science Daily].
Whenever a species diverges, forming two separate species, the event that splits the lineage is known as a speciation event. When one of these events occurs, it results in a loss of genetic flow between the two species produced in the split; they become genetically isolated. Genetically isolated species, despite initially sharing a common ancestor, can no longer procreate.
Speciation might be caused by geographic isolation, such as when continents drift across oceans or even when rivers change courses and separate populations, or they may result from reduced gene flow over generations. This gets a little more complicated and doesn't always cause speciation, but sometimes some members of a species' population simply stop mating with others in it. Once these barriers arise, a species may have reproductive isolation. For example, a type of bird called the European blackcap in southern Germany and Austria seems to be splitting its migrating pattern. Some of the warblers are heading north in the breeding grounds instead of the traditional southerly direction. The birds mate at different times in the two locations and scientists studying them believe that at some time in the future, the birds that migrate in different directions might not be able to mate with one another [source: Genome].
Here's another example of speciation you may be more familiar with than the European blackcap: Many scientists believe that at one point in history, the polar bear was a close relative of the brown bear and fossil samples show the connection [source: PBI]. Over time, the species began to spread out into different areas, forming genetic niches and eventually becoming two entirely separate species of bear. It's very clear that polar bears are white because of genetic adaptation to their surroundings; their fur could no longer be brown like that of their relatives or else they wouldn't blend in with the harsh, colorless arctic terrain they call home.
Most speciation occurred in the past, as with the polar bears. Scientists often can isolate when the events occurred and continue to study how they happened.
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