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Evolution’s Mirror in a Fish’s Spines [Micro vs. Macro Evolution]
Howard Hughes Medical Institute ^ | 15 April 2004 | Staff

Posted on 04/15/2004 4:31:58 AM PDT by PatrickHenry

Howard Hughes Medical Institute researchers at Stanford University are closer to understanding one of evolution's biggest questions: How do genetic changes contribute to the generation of new traits in naturally occurring species?

By studying related populations of small fish, called sticklebacks, the scientists have learned how a variety of animals might have lost their hindlimbs during evolution. The researchers discovered that relatively small changes in the regulation of specific genes can lead to a phenomenon called hindlimb reduction. The work demonstrates that rapid skeletal changes can occur in one body structure without disrupting the essential role of the same genes elsewhere in the body.

The research team, led by Howard Hughes Medical Institute investigator David M. Kingsley, published its findings in the April 15, 2004, issue of the journal Nature. Kingsley and his colleagues at the Stanford University School of Medicine collaborated on the studies with researchers from the Fred Hutchinson Cancer Research Center, the Institute of Freshwater Fisheries in Iceland and the University of British Columbia.

“One of the central mysteries of evolutionary biology has been the relationship between microevolution and macroevolution,” wrote Neil H. Shubin and Randall D. Dahn of the University of Chicago in an accompanying perspective article in Nature. “[The researchers] might have discovered a smoking gun — a real example of a type of macroevolutionary change that is produced by genetic differences between populations.”

According to Kingsley, hindlimb reduction is a trait that has evolved repeatedly in different animal groups, including mammals, such as dolphins and whales that have returned to the sea, snakes, reptiles, amphibians and many fish species. “It's a major morphological change in the vertebrate skeleton,” Kingsley said. “And despite the fact that it has occurred repeatedly, the mechanisms responsible are not understood.”

Over the years, various laboratories studying gene mutations in animals have identified genes that govern limb development. But these mutations were almost invariably lethal, said Kingsley, leaving open the question of whether changes in those genes could possibly underlie skeletal evolution. Some scientists argued that since animals must always remain genetically fit, only subtle changes in genes that are relatively low in the hierarchy of developmental control could cause skeletal evolution.

“We felt that to understand what was happening in nature, we had to find a genetic model system where we could study naturally occurring populations instead of lab mutants,” said Kingsley. “We wanted examples that had evolved in nature and had been subject to whatever constraints of viability and fitness that any organism that has evolved in the wild would have.”

Kingsley and his colleagues found such an animal in the threespine stickleback fish. These small fish typically live in the ocean but breed in coastal streams. After the last ice age ended some 11,000 years ago, populations of sticklebacks rapidly colonized newly formed freshwater streams and lakes - through a process known as adaptive radiation.

The many stickleback populations underwent disparate and parallel evolutionary changes, among them partial or complete loss of their pelvic spines. These spines are thought to protect the fish from being devoured by predators. As Kingsley points out, however, pelvic spines may be a disadvantage if the fish live in environments that have very low calcium levels available for building the skeletal structures, or in locations with many large predatory insects that hunt sticklebacks by grabbing hold of the spines. “Although vastly different morphologies have evolved in different stickleback populations, they have evolved recently enough that you can still take those different populations, cross them and actually let the genetics of the trait guide you to the underlying events that have controlled the process,” said Kingsley.

The Stanford group collaborated with senior co-author Dolph Schluter at the University of British Columbia, and Bjarni Jo'nsson at Holar Agricultural College in Iceland, to set up multiple crosses between marine and freshwater stickleback populations that exhibited significantly different body structures, such as the presence or absence of the pelvic spine. By analyzing the genetic differences among the progeny with regard to spine development, Kingsley and his colleagues sought to understand the genetic basis of reshaping the hindfin in different populations.

That analysis revealed that a single region on the fish's chromosomes was responsible for most of the changes in spine and pelvic morphology. The researchers then began a search of that chromosome region for candidate genes known to be involved in hindlimb development in other animals. They ended their search when they identified the stickleback homolog of a gene known as Pitx1.

The researchers were able to determine the DNA sequence of the Pitx1 gene in marine fish with a normal hindfin and in freshwater fish with hindfin reduction. Although no changes were seen in the portions of the gene that code for the Pitx1 protein, comparative expression studies showed that the gene was no longer expressed properly at some locations in the freshwater fish, including the place where hindfins would normally develop.

Pitx1 is actually involved in a number of different processes, including pituitary development and craniofacial development,” said Kingsley. “So, any changes in the protein itself would affect all of those structures. We think that this finding shows how evolution has been able to make use of exactly the same major developmental regulator that is lethal if eliminated — and yet avoids lethality by tweaking the regulatory region of the gene. Therefore, such changes only affect the gene's expression in a very specific developmental site. It allows this very important gene to produce a morphological transformation, without producing effects that would reduce viability.”

The team's findings in natural populations of fish may add important insights into the evolutionary process, said Kingsley. “There are many theoretical predictions about whether evolution is controlled by many genes of little effect or few genes of major effect,” he said. “There are relatively few examples of natural populations in which genetic studies can actually let the organism tell you how complicated the genetics of their traits really are. I consider sticklebacks a great system for that kind of study.”

Further research will focus on identifying the specific DNA alterations responsible for causing changes in the regulation of Pitx1. This kind of analysis will be particularly challenging, Kingsley said, because the nature and function of these genetic regulatory regions is not well understood in either fish or other animals. However, knowing that such regions may be the basis for major evolutionary change adds new impetus to characterizing them more fully, and studying how they have changed in animals that have adapted to a range of different environments.


Stickleback fish (bottom) in some freshwater lakes have lost the large pelvic hindfin seen in their ocean ancestors (top).


TOPICS: Culture/Society; Miscellaneous; Philosophy
KEYWORDS: creationism; crevolist; darwin; evolution; macroevolution
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Bold font and underlining added by your humble freeper.

Everybody be nice.

1 posted on 04/15/2004 4:31:59 AM PDT by PatrickHenry
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To: VadeRetro; jennyp; Junior; longshadow; RadioAstronomer; Physicist; LogicWings; Doctor Stochastic; ..
PING. [This list is for the evolution side of evolution threads, and some other science topics like cosmology. Long-time list members get all pings, but can request evo-only status. New additions will be evo-only, but can request all pings. FReepmail me to be added or dropped. Specify all pings or you'll get evo-pings only.]
2 posted on 04/15/2004 4:32:43 AM PDT by PatrickHenry (Preserve the purity of your precious bodily fluids!)
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To: PatrickHenry
I wonder how Democrats evolved?
3 posted on 04/15/2004 4:35:09 AM PDT by The Raven
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To: The Raven
That's the related phenomenon of de-evolution at work. If you want to produce a herd of Democrats, first start with a colony of cherrystone clams, and then simply knock out all the genes related to intelligence or the nervous system.
4 posted on 04/15/2004 4:44:03 AM PDT by general_re (The doors to Heaven and Hell are adjacent and identical... - Nikos Kazantzakis)
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To: PatrickHenry
Interesting article, but you have to admit -- the Howard Hughes Medical Institute sounds like the beginning of a bad joke. What, do the staff going around opening doors with Kleenex and letting their toenails grow through their sneakers?
5 posted on 04/15/2004 4:48:50 AM PDT by prion
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To: PatrickHenry
Someone call Creationist Central and let them know they have to raise the bar again!

6 posted on 04/15/2004 4:57:28 AM PDT by <1/1,000,000th%
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To: <1/1,000,000th%
The more we learn, the more we learn how incredible the design is.
7 posted on 04/15/2004 5:58:56 AM PDT by DManA
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To: <1/1,000,000th%; PatrickHenry; VadeRetro; Ichneumon
Pool time. Which post number will the first creationist claim "this proves nothing?"
8 posted on 04/15/2004 6:03:21 AM PDT by Junior (Remember, you are unique, just like everyone else.)
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To: Junior
LOL. It wasn't quite that explicit, but the first seemingly ID oriented post beat yours.
9 posted on 04/15/2004 6:23:00 AM PDT by dmz
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To: PatrickHenry; <1/1,000,000th%
By studying related populations of small fish, called sticklebacks, the scientists have learned how a variety of animals might have lost their hindlimbs during evolution. The researchers discovered that relatively small changes in the regulation of specific genes can lead to a phenomenon called hindlimb reduction. The work demonstrates that rapid skeletal changes can occur in one body structure without disrupting the essential role of the same genes elsewhere in the body.

< -snip- >

“One of the central mysteries of evolutionary biology has been the relationship between microevolution and macroevolution,” wrote Neil H. Shubin and Randall D. Dahn of the University of Chicago in an accompanying perspective article in Nature. “[The researchers] might have discovered a smoking gun — a real example of a type of macroevolutionary change that is produced by genetic differences between populations.”

Too bad an otherwise interesting article is marred by this type of hype.

Is the bar for macroevolution now so low that a small genetic variation (with admittedly large consequences) within a species now qualifies as a "smoking gun?"

Those "related populations of small fish, called sticklebacks," are all geographically isolated populations of Gasteroseus aculeatus, the Three-spined Stickleback, a widespread and morphologically diverse species that is represented on both sides of the Atlantic, as well as along the Pacific Coast of North America (though it has been suggested that the latter is a separate species).

If the loss of pelvic spines in a few populations within a species is an example of macroevolution, then so is the taillessness of the Manx cat.

Shubin and Dahn have vastly overreached in grasping for their macroevolutionary "smoking gun." The evidence for evolutionary theory is compelling, but it won't be "proven" in this or the next 100 lifetimes.

More on the Three-Spined Stickleback here and here.


10 posted on 04/15/2004 6:35:47 AM PDT by Sabertooth (< /Kerry>)
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To: Sabertooth
!


11 posted on 04/15/2004 7:02:56 AM PDT by AndrewC (I am a Bertrand Russell agnostic, even an atheist.</sarcasm>)
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To: Sabertooth
Are you still looking for some kind of sudden jump? You accumulate a half-dozen micros or so and you've got a macro.

What are a lion and a tiger? They're reliably cross-fertile, although male tigers really don't turn on much to female lions. (An instance of how behavioral mate-selection factors intervene in questions of speciation.) The hybrid offspring (ligers and tigons) have low fertility, not enough for replacement in the wild but nevertheless higher than the fertility of mules, the classic just-beyond-speciation example.

Every degree of relationship between distinct populations is visible in nature now. The sticklebacks just present a particularly easy case to analyze.
12 posted on 04/15/2004 7:19:14 AM PDT by VadeRetro (Faster than a speeding building! Able to leap tall bullets in a single bound!)
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To: AndrewC
Is that before and after grooming?
13 posted on 04/15/2004 7:19:20 AM PDT by js1138 (In a minute there is time, for decisions and revisions which a minute will reverse. J Forbes Kerry)
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The *repeatable* evolution of isolated organisms. Interesting. The capabilities of natural selection seem to be endless! It's worked it's magic on certain frogs, lizards, monkeys and dolphins as well.

Gould is probably amazed as well, since he has said that the likelihood of evolution being *repeatable* in nature is highly unlikely.

Three words for the lurkers: "after their kind"

14 posted on 04/15/2004 7:29:12 AM PDT by Michael_Michaelangelo
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To: js1138
Examine the comb and pillow. Rogaine® indicated here.
15 posted on 04/15/2004 7:41:11 AM PDT by AndrewC (I am a Bertrand Russell agnostic, even an atheist.</sarcasm>)
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To: Sabertooth
Is the bar for macroevolution now so low that a small genetic variation (with admittedly large consequences) within a species now qualifies as a "smoking gun?"

What's the problem here? Creationists, and crypto-creationists (who hide behind the mask of ID) claim that there is no way these macro-type of changes can happen without outside intervention from a designer -- either a divine or some other (but never specified) type of designer. That's their entire argument -- that what they call "macro evolution" just flat-out isn't possible through natural evolutionary means.

This study demonstrates that at least one macro-type of change can indeed happen through natural means. What else is necessary to completely blow away the argument of the creationists and the ID advocates?

16 posted on 04/15/2004 7:51:27 AM PDT by PatrickHenry (Preserve the purity of your precious bodily fluids!)
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To: VadeRetro
Are you still looking for some kind of sudden jump? You accumulate a half-dozen micros or so and you've got a macro.

I'm apparently looking for a reliable definition of macroevolution.

Is it the evolution of a family, genus, or species into another?

Even by the formula you're now proposing, this article demonstrates macroevolution/6.

Is the Manx cat an example of macroevolution?

Many Creationists stipulate microevolution. Their dispute is precisely whether or not an aggregation of micros can constitute a macro.

Attempts to redefine macro such that "micro is macro anyway" are clumsy rhetorical gamesmanship.


17 posted on 04/15/2004 7:55:22 AM PDT by Sabertooth (< /Kerry>)
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To: PatrickHenry
This study demonstrates that at least one macro-type of change can indeed happen through natural means

Is "macro-type of change" synonymous with macroevolution?

This study demonstrates how different expressions of the same gene, Pitx1, within the same species, the three-spined stickleback, result in signicant morphological differences in subpopulations of that species.

Same gene. Same species.

That's a dandy example of microevolution, if it's evolution at all.

Can the sticklebacks' different expressions of Pitx1 be manipulated and/or reversed by subjecting the sticklebacks to different environmental conditions, such as higher calcium content in the water? If so, we would have demonstrated a very interesting neo-Lamarckian phenomenon whose implications for current evolutionary understanding might be profound.

We might even call it nanoevolution.


18 posted on 04/15/2004 8:09:34 AM PDT by Sabertooth (< /Kerry>)
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To: general_re
That was FUNNY !!!!
19 posted on 04/15/2004 8:12:06 AM PDT by The Raven
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To: Sabertooth
I'm apparently looking for a reliable definition of macroevolution.

Creo mode: Microevolution is whatever has been observed well enough that it can't be lawyered away. Macroevolution is an article of faith for which there is no evidence.

Serious mode: Speciation is a real point of no return, beyond which the divergence of two populations becomes irreversible. Everything after that (genus, family, order) is subjective judgment, a beauty contest.

I don't think the micro- macro- distinction is very useful, really. I used to think the creos made it up. However you phrase it, big divergences are the accumulated effect of small changes.

20 posted on 04/15/2004 8:38:11 AM PDT by VadeRetro (Faster than a speeding building! Able to leap tall bullets in a single bound!)
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