Ring species in the making.
Posted on 06/08/2004 3:30:58 AM PDT by PatrickHenry
The first eyewitness to the birth of a new species may be a University of Arizona graduate student. Her new findings could help biologists identify and understand the precise genetic changes that lead one species to evolve into two separate species.
Laura K. Reed and her advisor Therese Markow, a UA Regents' Professor, made the discovery by observing breeding patterns of fruit flies that live on rotting cacti in western deserts. Whether the closely related fruit fly populations, designated Drosophila mojavensis and Drosophila arizonae, represent one species or two is still debated by biologists, testament to the UA researchers’ assertion that the insects are in the early stages of diverging into separate species.
The team's findings will be published the week of June 7 in the online early edition of the Proceedings of the National Academy of Sciences.
The seeds of speciation are sown when distinct factions of a species stop reproducing with one another. When the two groups can no longer interbreed, or just prefer not to, they stop exchanging genes and eventually go their own evolutionary ways, thus forming separate species.
While the evolutionary record is brimming with examples of speciation events, biologists haven’t been able to put their fingers on just what initiates the reproductive isolation, Reed said. Several researchers have identified mutant forms of certain genes associated with the inability of fruit flies to hybridize with closely related species. However, in all cases those genes were discovered long after the two species diverged. Therefore, those genetic changes could either have caused the speciation, resulted from it or even be incidental changes that happened long after the two species diverged.
Reed said the difficulty is researchers need to catch the genetic schism while it’s still brewing. Now she and her advisor have managed to do just that.
In the wild, the two fruit flies Drosophila mojavensis and Drosophila arizonae rarely, if ever, interbreed, even though their ranges overlap in a broad swath along the northern Mexican coastline. In the lab, researchers can coax successful conjugal visits between members of the two groups. But even under laboratory conditions such matings aren’t always fruitful. Drosophila mojavensis mothers typically produce healthy offspring after mating with Drosophila arizonae males, but when Drosophila arizonae females mate with Drosphila mojavensis males, the resulting sons are sterile. Reed said such limited capacity for interbreeding suggests the two groups of flies are on the verge of becoming completely separate species.
Another finding adds support to that notion. Previous research had shown that for a strain of Drosophila mojavensis from southern California's Catalina Island, mothers always produce sterile sons when mated with Drosophila arizonae males.
Because the hybrid male's sterility depends on the mother’s genetic heritage, Reed and Markow conclude the genetic change, or polymorphism, responsible for creating sterile sons must not be firmly established in Drosophila mojavensis populations -- a telltale sign that the change is recent.
Reed wanted to know just how deeply the polymorphism causing male sterility had suffused the Catalina Island Drosophila mojavensis populations. In other words, do just some of the Catalina Island mothers produce sterile sons when mated to Drosophila arizonae males? When Reed did the experiment, she found that only about half the crosses resulted in sterile sons. That result implies that only half the females in the Catalina Island population had the gene (or genes) that confer sterility in the hybrid male offspring.
Surprisingly, when she tested Drosophila mojavensis females from other geographic regions, Reed found that a small fraction of those populations also exhibited the hybrid male sterility polymorphism. “That polymorphism exists in every population I looked at,” Reed said. “It just happens to be that whatever factors are causing sterility are at higher frequencies in the Catalina Island population.”
Further experiments demonstrated that the sterility trait is caused by more than one genetic change. “I think there are many genes -- 4 or 5 probably, maybe many more,” Reed said.
Now that the researchers are hot on the trail of a set of “speciation genes,” the next task will be to identify them. For help in the hunt, the team will take advantage of the newly begun Drosophila mojavensis genome sequencing project, which will provide a complete roadmap of every gene in the species.
“Evolutionary biologists are excited to figure out what causes what we see out there—the relative forces of selection and drift -- whether things are adapting to their environment or variation is random," Reed said.
"Another important component to that is how that variation is partitioned into separate species. Once you’re a separate species, you have an independent evolutionary trajectory to some other species -- an independent set of tools, or genetic potential, relative to other species. So this partitioning of genomes is an important cause of the variation we see in nature.”
Paraphrasing the immortal words of that D-Day veteran who responded to smart assed TV news guy, as played continually on Imus Monday AM: "Up your Seventh Planet!"
;-)
I checked, and "uranus.com" isn't available for registration. So it looks like someone's got uranus.
Different species...cannot interbreed and create fertile offspring.
Question:
Are all dogs of the same species?
If not, where do they draw the lines between different "species" of dogs? Most dogs can interbreed and produce fertile offspring, but I just can't imagine a Saint Bernard and a chihuahua... They are both dogs, but are they of the same species?
I just can't imagine a Saint Bernard and a chihuahuaI think it's a sign of good mental health not being able to imagine that :p ... However, they can still interbreed and indeed create fertile offspring, even if the 'act' itself has become somewhat cumbersome due to the enormous difference in size.
The various dog breeds we have are very recent, from a evolutionary perspective, so they have yet to speciate. Given enough time I don't think many biologists would disagree that the Saint Berhard and Chihuahua are likely subjects for speciation though :)
That is amazing. I learn something new every day!
Thanks for the information.
I've read that it's only possible if the female is the St. Bernard. She's large enough to produce live offspring. But a female chihuahua, assuming she somehow survives an encounter with a male St. Bernard, isn't be large enough to drop the resulting litter. Otherwise, I think that at the genetic level the mating is no problem at all.
Very interesting.
I wonder what the pups resulting from such a union would look like.
Nawww, this is a done deal. It joins the "new" species, or "gonna be a new" species pool. This pool contains Nereid worms, primroses, Drosophila paulistorum et al., Lake Nagubago cichlids, mice somewhere in New England etc. etc. And then again there is "nothing new" under the sun.
A fundamental goal in speciation research has been to determine the number of genes that contribute to barriers to gene exchange (or reproductive isolating mechanisms) between closely related species. Recent studies of two particular barriers to gene exchange, hybrid male sterility and female species preferences, have found that these characters are often highly polygenic, with many regions of the genome being associated with at least some effect (for reviews see refs. 1-4). Hybrid male sterility appears particularly highly polygenic when segments of one species are experimentally introgressed into the genetic background of another and made homozygous (e.g., refs. 5 and 6). Perhaps the most extensively studied groups for these characters have been the two races of Drosophila melanogaster and the three species of the Drosophila simulans clade; D. simulans, D. mauritiana, and D. sechellia, although many other species (including non-Drosophila) have also been investigated. However, these species may be atypical of Drosophila species in particular or species in general, because all the hybridizations studied involved homosequential taxa: taxa not differing in their gene arrangements
St. Barnard - Chihuahua: A ring species in the making.
Ring species in the making.
Wait - Gulls are no ring species:
The Herring Gull complex, consisting of some 20 species of so-called large white-headed gulls, has long been considered as the classic example of a ring-species whereby the ring was finally closed by the Herring Gull Larus argentatus invading NW.Europe from America. In a study based on mitochondrial DNA, Dr.'s Dorit Liebers, Peter de Knijff and Andreas Helbig show that this is not the case. In fact the ring was never closed and they predict that should it ever happen then the most likely scenario is that the Lesser Black-backed Gull Larus fuscus will settle in the New World where it's numbers have increased dramatically in recent times. Instead of one they have found two ancient refugia, an Atlantic and an Aralo-Caspian one from where gulls spread eastwards into the Mediterranean and into the Americas respectively.
source: Dorit Liebers, Peter de Knijff and Andreas Helbig. 2004. The herring gull complex is not a ring species. Proc.Royal Soc.Lond. 10.1098/rspb-2004.2679 (2004).
Here's a bit more.
Recent genetic study sheds light on the evolution of the Herring Gull complex
As reported in the May 2004 issue of Birding World, a recent genetic study (Liebers, de Knijff, and Helbig 2004) has shed some fascinating light on the evolution of the Herring Gull complex, and incidentally therefore on the relationships of some of the larger larids of South Korea.
The authors studied the mitochondrial DNA variation of 21 large gull taxa in an attempt to reconstruct their evolutionary history. Contrary to the expected "ring species" model, members of this complex apparently differentiated largely in geographical separation, not through isolation-by-distance. The divergence started from two glacial refugia, one in the North Atlantic and one in the Aral-Caspian region. Great Black-backed Gull Larus marinus, Yellow-legged Gull L. michahellis, Armenian Gull L. armenicus, and Herring Gull L. argentatus all originated from the North Atlantic refugium. However, the Lesser Black-backed Gull group (including - according to the authors - heuglini and barabensis), Caspian Gull L. cachinanns and a Pacific-North American clade (consisting of vegae, mongolicus, schistisagus, glaucoides, thayeri, and smithsonianus) derive from the Aral-Caspian refugium: it has only been this latter group that has attained circumpolar distribution.
Pesky DNA.
You evolution religionists are incredibly defensive. Where did I ever say that I disputed the findings? Or was even interested in doing so? I know that I cannot change your faith and I know that you cannot change mine. I know that God will use whatever methods (including natural selection) He chooses to use to fill any niche that needs to be filled. (I guess you could put me firmly in the creationist/intelligent design group)
However, none of that affects my original statement that the article was improperly worded or uses a definition of "prefers" that is not accurate. Whether or not the science is done correctly the report still should be written to make sense and eliminate confusion.
Missed this earlier and I think it deserves a response. I would object because blacks and whites can interbreed with no problems. There's not enough genetic difference for them to be considered different species. Other than external characteristics (color mostly) I don't know of anything else that is different. Same blood types etc. There is only the human species, not the white species or black species.
Educate me a little. Ring species? Hadn't seen the term before.
The various Ensatina salamanders of the Pacific coast all descended from a common ancestral population. As the species spread southward from Oregon and Washington, subpopulations adapted to their local environments on either side of the San Joaquin Valley. From one population to the next, in a circular pattern, these salamanders are still able to interbreed successfully. However, where the circle closes -- in the black zone on the map in Southern California -- the salamanders no longer interbreed successfully. The variation within a single species has produced differences as large as those between two separate species
pretty cool. Thanks.
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