|MadSci Network: Evolution|
Speciation occurs when the adaptations of a subset of organisms within a species isolate that subset genetically from the rest of the species. In other words, there are many adaptations which occur within species that simply extend the gene pool - it isn't until there are enough different adaptations to prevent a small group from reproducing with the rest of the species that that group becomes a separate species of its own. The easiest way, that I can think of, to explain this is to give some examples.
The first example of adaptation on the brink of speciation comes from work done by Don McPhail and his colleagues at the University of British Columbia. They examined the fish life in several small isolated lakes on Vancouver Island, and found that the Sticklebacks in many of the lakes had adapted to two separate habitats. Those fish that swam near the surface, picking up free-swimming food were categorized as limnetic, and those fish that stayed near the bottom eating material in the muck were categorized as benthic. Since becoming isolated to the lakes, the limnetic fish had become thinner, smaller, and faster - adaptations that improve their ability to exploit their niche. In contrast, the benthic fish become larger, fatter, and slower, adapting to their niche. Now for the speciation... the two forms of sticklebacks can be mated to produce a viable and fertile hybrid (arguing against their being separate species), however the hybrid cannot compete for food in either niche so few reach sexual maturity in the wild; furthermore, the mating seasons in the lake depend on water temperature such that the limnetic form mate about a month before the benthic form, so crossbreeding rarely, if ever, occurs in the wild. So it becomes a point of semantics whether the limnetic and benthic forms in a given lake can be considered separate species.
The second example comes from Sean Carroll and his colleagues at the University of Wisconsin. They examined a species of African butterflies (Bicyclus anynana) which exhibit rapid adaptation to environmental conditions. During a normal year, the butterflies that appear during the wet season have several, large eyespots on their brown wings to frighten predators when they light on the green vegitation. Conversely, the butterflies of the dry season have very small eyespots, so that their wings look more like the surrounding dead leaves. As above, the differences in mating seasons prevent these forms from contacting each other sexually, and they had been classified as separate, though closely related, species. Interestingly, if the "dry" form was forced to emerge during the wet season, the predation on the butterflies with the smaller spots induced rapid genetic alterations in the homeotic genes responsible for eyespot formation, such that within a couple generations, the "dry" species was indistinguishable from the "wet" species. So again, one "species" can differentially adapt to exploit separate habitats such that the different forms become genetically isolated and considered different species.
There are several other examples of recent examples of speciation, including induced and natural speciation in fruitflies (Drosophila species) and several species of plants, that are frequently sited in the literature, so I won't go into them here. To sum up, adaptation becomes speciation when the genetic separation between the group of organisms and the rest of the original species becomes permanent.
Try the links in the MadSci Library for more information on Evolution.
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