http://news.bbc.co.uk/2/hi/science/nature/4708459.stm
Butterfly unlocks evolution secret
By Julianna Kettlewell
BBC News science reporter
Why one species branches into two is a question that has haunted evolutionary biologists since Darwin.
Given our planet's rich biodiversity, "speciation" clearly happens regularly, but scientists cannot quite pinpoint the driving forces behind it.
Now, researchers studying a family of butterflies think they have witnessed a subtle process, which could be forcing a wedge between newly formed species.
A Russian-US team discovered that closely related species living in the same geographical space displayed unusually distinct wing markings.
These wing colours apparently evolved as a sort of "team strip", allowing butterflies to easily identify the species of a potential mate.
This process, called "reinforcement", prevents closely related species from interbreeding thus driving them further apart genetically and promoting speciation.
Although scientists have speculated about this mechanism for years, it has rarely been witnessed in nature.
"The phenomenon of reinforcement is one of the very few mechanisms that has natural selection playing a role in speciation," said Nikolai Kandul, who co-authored the Nature paper with Vladimir Lukhtanov and colleagues.
"It might be very widespread but it is hard to find good evidence of it," the Harvard University researcher added.
Geographical isolation
For speciation to occur, two branches of the same species must stop breeding with one another for long enough to grow apart genetically.
The most obvious way this can happen is through geographical isolation.
If a mountain range or river divides a population of animals for hundreds of generations, they might find that if they meet again they are no longer able to breed.
But geographical isolation is not enough to explain all speciation. Clearly, organisms do sometimes speciate even if there is no clear river or mountain separating them.
The other mechanism that can theoretically divide a species is "reproductive isolation". This occurs when organisms are not separated physically, but "choose" not to breed with each other thereby causing genetic isolation, which amounts to the same thing.
Reproductive isolation is much hazier and more difficult to pin down than geographic isolation, which is why biologists are so excited about this family of butterflies.
Butterfly clue
The Harvard team made the discovery while studying the butterfly genus Agrodiaetus , which has a wide ranging habitat in Asia.
The females are brown while the males exhibit a variety of wing colours ranging from silver and blue to brown.
Dr Kandul and his colleagues found that if closely related species of Agrodiaetus are geographically separate, they tend to look quite similar. That is to say, they do not display a distinctive "team strip".
But if similarly closely related species are living side-by-side, the researchers noticed, they frequently look strikingly different - their "teams" are clearly advertised.
This has the effect of discouraging inter-species mating, thus encouraging genetic isolation and species divergence.
"This butterfly study presents evidence that the differences in the male's wing colouration is stronger [when the species share a habitat] than [when they do not]," said the speciation expert Axel Meyer, from Konstanz University in Germany.
"This pattern would therefore support the interpretation that it was brought about by reinforcement, hence natural selection."
The reason evolution favours the emergence of a "team strip" in related species, or sub species, living side-by-side is that hybridisation is not usually a desirable thing.
Although many of the Agrodiaetus species are close enough genetically to breed, their hybrid offspring tend to be rather weedy and less likely to thrive.
Therefore natural selection will favour ways of distinguishing the species, which is why the clear markings exist.
"For me, this is a big discovery just because the system is very beautiful," said Dr Kandul. "As much as we can we are showing that [reinforcement] is the most likely mechanism."
This research was published in the latest edition of Nature magazine.
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Rapid Evolution of Reproductive Isolation in the Wild: Evidence from Introduced Salmon
Abstract
Colonization of new environments should promote rapid speciation as a by-product of adaptation to divergent selective regimes. Although this process of ecological speciation is known to have occurred over millennia or centuries, nothing is known about how quickly reproductive isolation actually evolves when new environments are first colonized. Using DNA microsatellites, population-specific natural tags, and phenotypic variation, we tested for reproductive isolation between two adjacent salmon populations of a common ancestry that colonized divergent reproductive environments (a river and a lake beach). We found evidence for the evolution of reproductive isolation after fewer than 13 generations.
http://adsabs.harvard.edu/abs/2000Sci...290..516H
"Three species of wildflowers called goatsbeards were introduced to the United States from Europe shortly after the turn of the century. Within a few decades their populations expanded and began to encounter one another in the American West. Whenever mixed populations occurred, the specied interbred (hybridizing) producing sterile hybrid offspring. Suddenly, in the late forties two new species of goatsbeard appeared near Pullman, Washington. Although the new species were similar in appearance to the hybrids, they produced fertile offspring. The evolutionary process had created a separate species that could reproduce but not mate with the goatsbeard plants from which it had evolved."
http://www.talkorigins.org/faqs/speciation.html
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Lessons About Speciation
http://www.evoled.org/lessons/speciation.htm
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Ring species
Ring species provide unusual and valuable situations in which we can observe two species and the intermediate forms connecting them. In a ring species:
· A ring of populations encircles an area of unsuitable habitat.
· At one location in the ring of populations, two distinct forms coexist without interbreeding, and hence are different species.
· Around the rest of the ring, the traits of one of these species change gradually, through intermediate populations, into the traits of the second species.
Ring species:
a ring of populations in which there is only one place where 2 distinct species meet. A ring species, therefore, is a ring of populations in which there is only one place where two distinct species meet. Ernst Mayr4 called ring species "the perfect demonstration of speciation" because they show a range of intermediate forms between two species. They allow us to use variation in space to infer how changes occurred over time. This approach is especially powerful when we can reconstruct the biogeographical history of a ring species, as has been done in two cases.
California salamanders exhibit ring species traits.
Ensatina salamanders
One well-studied ring species consists of salamanders in the Ensatina eschscholtzii group, distributed in mountains along the west coast of North America. In 1949, Robert Stebbins5 described a fascinating pattern of geographical variation in these salamanders:
· Two distinct forms of Ensatina salamanders, differing dramatically in color, coexist in southern California and interbreed there only rarely.
· These two forms are connected by a chain of populations to the north that encircles the Central Valley of California, and through this ring of populations the color patterns of the salamanders change gradually.
DNA analysis supports a common ancestor for these salamanders. Stebbins thought that this situation arose when an ancestral population of salamanders, in northern California, expanded southward along two fronts, one down the Sierra Nevada mountains, and the other down the coastal mountains. The two groups gradually became different as they moved south. When they met again in southern California, the two expanding fronts were so different that they rarely interbred, and were therefore different species. More recently, a team of researchers led by David Wake6-8 has examined genetic relationships among salamander populations using DNA sequences and other molecular traits, and the genetic evidence has supported Stebbins' hypothesis. The geographical variation, when combined with the inferred history revealed by the molecular traits, allows us to envision the small steps by which a single ancestral species in the north gave rise through evolutionary divergence to two species in southern California. Greenish warblers, a ring species, are found in parts of Asia and eastern Europe.
Greenish warblers
Another ring species that has provided valuable insights into speciation consists of the greenish warblers (Phylloscopus trochiloides). These small, insect-eating songbirds breed in the forests of central and northern Asia and eastern Europe. In the center of Asia is a large region of desert, including the Tibetan Plateau and the Taklamakan and Gobi Deserts, where the warblers cannot live. Instead, they inhabit a ring of mountains surrounding this region, as well as the forests of Siberia to the north. The warblers have remarkable geographic variation:9-11
· In Siberia, two distinct forms of greenish warblers coexist, one in the west and one in the east, their distributions narrowly overlapping in central Siberia, where they do not interbreed. These forms differ in color patterns, the songs that males sing to attract mates, and genetic characteristics. Also, males of each form usually do not recognize the song of the other form, but respond strongly to their own.
· The traits that differ between the two Siberian forms change gradually through the chain of populations encircling the Tibetan Plateau to the south.
· Thus two distinct species are connected by gradual variation in morphological, behavioral, and genetic traits..............
http://www.actionbioscience.org/evolution/irwin.html
