Posted on 02/24/2006 4:12:32 AM PST by PatrickHenry
Charles Darwin would undoubtedly be both pleased and chagrined.
The famous scientist would be pleased because a study published online this week provides the first clear evidence that natural selection, his favored mechanism of evolution, drives the process of species formation in a wide variety of plants and animals. But he would be chagrined because it has taken nearly 150 years to do so.
What Darwin did in his revolutionary treatise, “On the Origin of Species,” was to explain how much of the extraordinary variety of biological traits possessed by plants and animals arises from a single process, natural selection. Since then a large number of studies and observations have supported and extended his original work. However, linking natural selection to the origin of the 30 to 100 million different species estimated to inhabit the earth, has proven considerably more elusive.
In the last 20 years, studies of a number of specific species have demonstrated that natural selection can cause sub-populations to adapt to new environments in ways that reduce their ability to interbreed, an essential first step in the formation of a new species. However, biologists have not known whether these cases represent special exceptions or illustrate a general rule.
The new study – published online in the Proceedings of the National Academy of Sciences – provides empirical support for the proposition that natural selection is a general force behind the formation of new species by analyzing the relationship between natural selection and the ability to interbreed in hundreds of different organisms – ranging from plants through insects, fish, frogs and birds – and finding that the overall link between them is positive.
“This helps fill a big gap that has existed in evolutionary studies,” says Daniel Funk, assistant professor of biological sciences at Vanderbilt University. He authored the study with Patrik Nosil from Simon Fraser University in British Columbia and William J. Etges from the University of Arkansas. “We have known for some time that when species invade a new environment or ecological niche, a common result is the formation of a great diversity of new species. However, we haven’t really understood how or whether the process of adaptation generally drives this pattern of species diversification.”
The specific question that Funk and his colleagues set out to answer is whether there is a positive link between the degree of adaptation to different environments by closely related groups and the extent to which they can interbreed, what biologists call reproductive isolation.
Funk and his colleagues saw a way to address this question by extending a method pioneered by two scientists in a now classic study of species formation in fruit flies published in 1989. The original method measured the way in which reproductive isolation varies with time. It proved to be very powerful and a number of other researchers applied it to additional species. Funk and his colleagues realized that if they used the results of these studies and added an ecological dimension then they would have an approach capable of measuring the link between natural selection and reproductive isolation.
“We thought that the idea itself was important, that this is a really powerful approach to a very major question,” says Funk, “but we thought that there was no way in the world that we were actually going to get statistically significant results.”
The reason for his doubt was the incompleteness and lack of uniformity of ecological data. “There are all these species out there and so few of them are known in intimate detail, so any kind of ecological characterization, through no fault of ecologists, will be limited in accuracy and precision,” Funk says.
Nevertheless, the researchers decided to do the best they could with the information available. So they collected information from the published literature on three basic ecological variables: habitat, diet and size. Then they used this information to calculate the differences in ecological adaptation between the hundreds of pairs of related species in the original studies.
When they compared these differences in adaptation with the degree of reproductive isolation for each pair and then added them up, the researchers found that the overall association was positive with a surprisingly high level of confidence: The odds that the association is simply due to chance are only one in 250, substantially higher than the standard confidence level of one chance in 20 that scientists demand.
“The fact that the association is statistically significant despite the crudeness of our estimates suggests that the true biological association is very strong,” Funk says. “Darwin’s famous book was called ‘On the Origin of Species,’ but it was really about natural selection on traits rather than species formation. Since our study suggests that natural selection is a general cause of species formation, it seems that Darwin chose an appropriate title after all.”
[Omitted contact info which is at the end of the article.]
Not in a way that is more scientifically meaningful than saying one would "predict" a find of cheeseburgers at a McDonald's Restaurant.
So you think trees and cows and dinosaurs dropped out of the sky? That's your explanation of the origin of species?
"Whatever they are called, even one instance of fertile offspring proves that horses and donkeys are NOT different species."
Sure they are. The fertility rate of the offspring is almost zero (60 documented cases in 500 years of a fertile mules), and far far below replacement levels.
"natural selection" ... is merely an arbitrary way of explaining what has already taken placeand
"predict[ing]" a find of cheeseburgers at a McDonald's Restaurant?
Certainly the notion of "species" is a slippery one (and, to a large degree, arbitrary) but I think it is generally agreed that horses and donkeys are different species because they have different numbers of chromosomes, mating isn't natural and hybrid offspring are nearly always sterile.
Imagine the following situation. One billion individuals in species A. One billion individuals in species B. Both species are genetically diverse, but there is a high probability that any two individuals of A (of the opposite sex) could successfully breed; likewise B. It just happens that there is one male in species A which would produce fertile offspring with 1 female in species B if they happened to meet and "get the urge". This is an extreme example of ring speciation. In such a situation would you seriously claim that A and B are still the same species? If one of the potential co-breeders died would you still claim that they were the same species, or would you admit to a speciation event at that point? This could lead to a bizarre situation where the two groups "flickered" in and out of being the same species as the rare potential co-breeders were born and died.
The whole concept of species is a difficult one to pin down, as examples like that demonstrate.
(unless, of course, you believe in the sort of speciation that gives you nearly 7 million different species of salmon, to say nothing of tuna).
The substance of your point about salmon is unclear to me. Are you asserting that there are seven million distinct groups of salmon that have difficulty interbreeding?
Let me give you an example of what I mean (which will explain your own limitation of thought on the matter).
We both know that mammals with some history of tree`dwelling generally have full color vision. Not only do they have different sorts of photo receptors (red/green/blue, contrast), they have neural structures in their brains that are necessary to use the information stream coming from their optical receptors.
Ground dwellers, do not have this sort of color vision system.
Interestingly enough birds have full color vision. They have essentially the same brain circuitry as mammals so they can make use of the information stream.
It's almost as if birds and tree dwelling mammals had the same genes, and the Darwinians crawl through their butts to give you stories about convergent, parallel, and "new purposes for old genes" ways of accounting for this.
So, let's say there's another way genes get taken from one genome and tucked into another one. We know there's a bacteria that can do this to the sex organs of insects (creating new species because the critical parts no longer fit), but what could transport bird genes to monkeys?
What could be a simple straightforward explanation of how all the tree dwellers end up with color vision. COuld it be a bacteria? Sex crazed parakeets? Or, could both birds and mammals who live in trees be drawing from the same source of disease to obtain free floating genes that have the same effect in both forms?
And, where would those genes be found?
Since there are undoubtedly billions of different kinds of genes just floating around in the oceans in the form of viruses, maybe there's a pathway from that pool of genes to the tree dwellers so they can quickly acquire, and use, genes that give them color vision and the neural pathways to use color receptors.
Next question might be just why there might be virus genes that go into making up the genetic structure to give us color vision.
dYou do know, of course, that you have vast stretches of DNA in your genome that appear to do nothing, but they are otherwise identical to DNA elements readily found in viruses we know about, not just the billions out in the ocean.
In fact, we (meaning everybody but the bacteria) have chromosomes shaped just like those found in viruses.
Ayway, good article to read is in this month's Discover magazine. It's about the mimivirus. Provocative stuff.
I disagree with the methodology. One instance is enough.
I think that is a quite satisfactory standard to use as a test for speciation that NO INSTANCE of crossbreeding is ever seen.
"Ever" is a very long time you know!
Ok. I was viewing "successful" as resulting in offspring. But yes, the offspring are sterile.
What you do is count up the number of streams and streamlets in the PAC NW where a salmon might ever have bred. Gives you a huge count. I use 7 million since that's the biggest number I've ever seen rationally argued for.
I dispute the method. Those salmon can be physically isolated a million years and I guarantee they'll still taste like salmon!
Indeed, by that standard the number of species on earth is probably 1.
How do you know this?
It's a slipper slope for sure!
Look, if they don't taste the same a million years from now, you get back to me ya' hear!
What's your concept of Zenos Paradox, muawiyah?
As far as evolution is concerned offspring that don't produce offspring are not successful reproduction.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.