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.”
What is your definition of "species"? What is the transitive closure of your definition. Is "species" a property of an individual entity?
Well, the whole concept of a subspecies is somewhat subjective. Here's one formal definition: "any natural subdivision of a species that exhibits small, but persistent, morphological variations from other subdivisions of the same species living in different geographical regions or times."
By a rigidly literal interpretation of that standard, all the various races and even ethnicities can be deemed human subspecies. Then again, depending on what criteria one applies, the superficial races largely identified via coloration are genetically all but nonexistent. Alternatively, since the human species is pretty much intermixed and continuous one might just as easily say (and usually would say) that there can be no such thing as a human subspecies.
I guess what I meant to reference is the idea that incipient speciation may have been taking place due to the lengthy isolation of Australian aborigines. Obviously, one would assume that given a more permanent isolation that a separation of species would indeed have taken place eventually. So, what I'm pondering is the degree to which that may have taken place, and more importantly (in the grand scheme of things) what timespan is necessary for that (in hominid populations) and how precisely does one demarcate the threshold at which a population of one species has segued into a population of a different species.
I probably shouldn't have said "purely biological standpoint" because subspecies are not a clear-cut, objective classification. Whatever the case, you appear to contemplate subspecies in much the same way I do, so we're pretty much on the same wavelength.
That's the question that the authors need to answer. As it stands now they say balcks and whites are different species. See my point? sloppy work or sloppy writing. Something needs to be fixed
Again, (giving you one more chance to be evasive), what is your definition of "species"?
Species...the usual, can interbreed with one another to create fertile offspring.
Different species...cannot interbreed and create fertile offspring.
I'm sure there are some biologists that can tighten this up a bit, but it is fairly clear.
Since the Goatsbeard observation, this one won't be a first observation of actual speciation. But it does appear to depend on the easier "prefer not to reproduce with" standard which creationists love to laugh at.
DK
The intriguing factor here, though, is that not only do the two populations "prefer not to reproduce with" one another, but when they are coerced into doing so a significent subset of the offspring are infertile, but not all of them. This appears more like an ongoing 'real-time' speciation event than anything else that's been observed (to my knowledge), and if so it's certainly a sympatric speciation at that.
There are laws against speeding, too, but I saw it happen once.
;-(
I don't plan to transit Uranus this evening. Some other time, maybe. (I have to be in the mood.)
Wow, you must find that article again! I've been wondering if that kind of phenomenon exists anywhere. And yes, that would be quite a controversial claim if a researcher were to publish it.
If it is true, I'd expect that aborigines would have less problems mating with Malaysians, BTW. I wonder about an aborigine + a !Kung? or aborigine + a Fuegian? I suppose we could start a "cross-cultural dating service" to try to discreetly generate some raw data... :-)
Different species...cannot interbreed and create fertile offspring.
There's nothing wrong with this definition as far as it goes. It does have the consequence that there are 3 groups of entities: X, Y, and Z such that X and Y can produce fertile offspring; Y and Z can produce fertile offspring; but X and Z cannot. (I first heard of this with plants about 50 years ago. There are some animal examples.)
One gets the interesting observation that x&y belong to the "same" species; y&z belong to the same species; but x&z do not. Specieshood is not an equivalence relation.
Quantitive questions do remain. Horses and donkeys generally produce infertile offspring. Do the few fertile mules and hinnies make horses and donkeys the same species.
Human speciation would take quite a bit longer than, say, fruit fly speciation. Our generations are 20 or 30 years each, and in that time a fly can have (I'm guessing) several hundred generations. The migrations out of Africa, assuming that was our origin, were recent enough that speciation just hasn't had time to happen. Presumably, if we hadn't experienced our relatively recent re-discovery (and reunion) of the human world, we'd have a few sub-species starting to emerge. But we developed navigation and linked up with our origins and our far-flung cousins quickly enough -- and our explorers were randy enough -- that I doubt we'll ever develop human sub-species. At least not until we get widly-disbursed interstellar settlements. Then it's inevitable.
BTW, I think I meant "Polynesians".
As a pot might demonstrate the rich black hue of the kettle?
Interracial marriage was not the issue. Slavers used to rape their female slaves in order to get them pregnant with mulatto children, which fetched a higher price.
It's a mistake to confuse what the article calls preference - i.e. behavioral barriers to mating - with human decision making. Behavioral isolation is hard wired into animals, whereas humans can choose with whom to mate.
I think we need to create a UN agency to get on this project right away. We have to save humanity from speciation.
Which is getting to my point. The article was sloppily worded. or the definition of "prefers" was incorrect.
"The clearest evidence for evolution are the creationists, for they have not evolved"
No, but we get to evolve in the afterlife.
Dear SirAllen:
I am happy for you and truly admire your choice in timing. Better late than never...
How about Neptunian budgetary concerns?
...or the alien invasion of Uranus?
;-)
Any truth to the rumors that RadioAstronomer has been gazing at Uranus?
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