Picky female frogs drive evolution of new species in less than 8,000 years

UC Berkeley News Center ^ | 27 October 2005 | Robert Sanders

[PatrickHenry]

Picky female frogs in a tiny rainforest outpost of Australia have driven the evolution of a new species in 8,000 years or less, according to scientists from the University of Queensland, the University of California, Berkeley, and the Queensland Parks and Wildlife Service.

"That's lightning-fast," said co-author Craig Moritz, professor of integrative biology at UC Berkeley and director of the Museum of Vertebrate Zoology. "To find a recently evolved species like this is exceptional, at least in my experience."

The yet-to-be-named species arose after two isolated populations of the green-eyed tree frog reestablished contact less than 8,000 years ago and found that their hybrid offspring were less viable. To avoid hybridizing with the wrong frogs and ensure healthy offspring, one group of females preferentially chose mates from their own lineage. Over several thousand years, this behavior created a reproductively isolated population - essentially a new species - that is unable to mate with either of the original frog populations.

This example suggests that rapid speciation is often driven by recontact between long-isolated populations, Moritz said. Random drift between isolated populations can produce small variations over millions of years, whereas recontact can amplify the difference over several thousands of years to generate a distinct species.

"The overarching question is: Why are there so many species in the tropics?" Moritz said. "This work has led me to think that the reason is complex topography with lots of valleys and steep slopes, where you have species meeting in lots of little pockets, so that you get all these independent evolutionary experiments going on. Perhaps that helps explain why places like the Andes are so extraordinarily diverse."

When isolated populations of the green-eyed tree frog (gray and brown) met again 8,000 years ago, they found that each had changed in subtle ways. The calls of the male frogs were different, and more importantly, hybrid offspring were less viable. One population that was cut off from its southern kin (pink) found a way to ensure healthy young. Females, who choose mates based only on their call, began selecting mates with a the southern call type. Over thousands of years, this behavior exaggerated the pre-existing differences in call, lead to smaller body size in males of the "isolated southern population" and resulted in rapid speciation between the two populations of the southern lineage (pink and brown). (Nicolle Rager Fuller/National Science Foundation)

Moritz; lead author Conrad Hoskin, a graduate student at the University of Queensland in St. Lucia, Australia; and colleagues Megan Higgie of the University of Queensland and Keith McDonald of the Queensland Parks and Wildlife Service, reported their findings in the Oct. 27 issue of Nature.

The green-eyed tree frog, Litoria genimaculata, lives in the Wet Tropics area of northeast Queensland, a rugged tropical region of Australia along the Pacific Ocean's Great Barrier Reef. The frog, which is green with reddish-brown splotches, is common around streams and grows to about 2 1/2 inches in length.

Because of geographic isolation that began between 1 and 2 million years ago with the retreat of rainforest to higher elevations, two separate frog lineages developed in the northern and southern parts of the species' coastal range - only to be reconnected less than 8,000 years ago as the climate got wetter and warmer and the rainforest expanded.

Hoskin and his colleagues found that the northern and southern calls of the male frog, which are what females pay attention to in the mating game, had become different from each other. Yet despite this difference, reflected in the call's duration, note rate and dominant frequency, the two lineages could still breed with one another.

The southern females, however, were more picky about their mates than the northern females. And in one area of contact that had become isolated from the southern range, the southern females were extremely picky, to the extent that they almost never mated with northern males.

In laboratory breeding experiments, the biologists discovered the reason for this choosiness: While northern and southern lineages could breed successfully, they apparently had diverged enough during their million-year separation that offspring of southern females and northern males failed to develop beyond the tadpole stage. Though crosses involving northern females and southern males successfully produced frogs, the offspring developed more slowly than the offspring of pairs of northern frogs.

Field studies confirmed the laboratory results. Researchers could find no hybrid frogs in the contact zones that were the offspring of southern mothers, judging by the absence of any southern mitochondrial DNA, which is contributed only by the mother.

Hoskin and colleagues argue that because southern females have the most to lose in such cross-breeding, there may have been selection pressure to evolve a mating strategy to minimize dead-end mating with northern males. This appears to have occurred in the contact region where a population of the southern lineage had become isolated from the rest of its lineage and had developed a preference for certain male calls. The male frog call in this population has diverged significantly from both the northern and southern lineage calls.

"If females have a reason not to get the mating wrong, and they have some way of telling the males apart - the call - the theory is that this should create evolutionary pressure for the female choice to evolve so that they pick the right males," Moritz said.

This so-called reinforcement has been controversial since the time of Charles Darwin, with some biologists claiming that it requires too many steps for evolution to get it right.

"Some have argued that it's just too complicated and that it is not really necessary, and there have been few convincing demonstrations. In their view, differences between populations arise because of natural selection or genetic drift or mutation or some combination of those three, and reproductive isolation is just some glorious accident that arises from that," Moritz said. "We do have very compelling evidence. We have addressed various lines of evidence and conclude that there has been reinforcement and that has given rise to a new species based on very strong female choice."

As a comparison, they looked at a second contact zone on the border between north and south, where frogs were not isolated from either lineage.

"Reinforcement does not appear to occur at the more 'classic' contact between northern and southern lineages, and we speculate that this may be due to gene flow from the extensive range of the southern lineage into the contact zone," Hoskin said. "This problem does not exist at the other contact because the southern lineage population is very small and occurs primarily within the contact zone."

Because the frogs in the isolated contact area had a distinctively different call, and because they were effectively isolated from surrounding populations by mating preference, Hoskin and colleagues concluded that female choice led to this new species.

Interestingly, evolutionary theory would predict that the southern and northern frog populations would drift apart into two distinct species. In the case of the green-eyed tree frog, Moritz said, a subpopulation of the southern species drifted away not only from the northern species, but also from the southern. That was unexpected, he said.

Moritz noted that geographic isolation in this "dinky bit of rainforest in Australia" has split many species, and that reinforcement at zones of recontact may be generating other new species.

"In this tropical system, we have had long periods of isolation between populations, and each one, when they come back together, have got a separate evolutionary experiment going on. And some of those pan out and some don't. But if they head off in different directions, the products themselves can be new species. And I think that's kinda cool. It gives us a mechanism for very rapid speciation."

The research was supported by the U.S. National Science Foundation, the University of Queensland and the Australian Cooperative Research Centre for Tropical Rainforest Ecology and Management.

[CarolinaGuitarman]

"Actually you have absolutely NO evidence that supports the "evolution of life" on earth. None. I've challenged you guys a thousand times on these threads and the stock answer is that the question is irrelevant."

That isn't true, and you know it. We have time and time again provided you with the evidence, and you just ignore it. Nobody has said that it is *irrelevant* because no evolutionist thinks the matter of evidence is irrelevant. The evidence for evolution is overwhelming except for those who don't want to know.

If you are talking about the origins of life, that doesn't have anything to do with the ToE. The evolution of life and the origins of life are two different things. Evolution is agnostic about the origins.

[dread78645]

XenuDidit placemark

[Rocky]

If the communities were at one time combined then the calls must have been the same. It is easy to determine if the two groups have shared genes in the past.

Just because they were living together in the past doesn't mean the calls were the same. It's speculation. The business about genes is a red herring. Are you suggesting you can tell what kind of mating call they had in the past, based on DNA?

[RogueIsland]

Actually you have absolutely NO evidence that supports the "evolution of life" on earth. None. I've challenged you guys a thousand times on these threads and the stock answer is that the question is irrelevant.

Ha ha ha ha, ho, ha, hee, ha, ha, ha ha ha ha, ho, ha, hee, ha, ha!!! Too funny.

Oh, wait. You were serious.

[b_sharp]

"I do not share your certainty of more than one mass extinction. hydrology, sedimentation, of a liquid catatrophy that large would be tough to predict the outcome, and could form layering that could easily be mistaken as seperate events, not to mention the effect that would do to elements and their decay (un-knowable).

It sounds like you are reaching. For there to have been only one flood, many physical laws would have to be suspended. There is no way for creationists to explain the dating differences of the layers combined with the types of fossils found in the layers. The separation of fossils is not along any lines that make sense if you assume a single catastrophic event.

If you want just so stories, consider the various speculations by the so-called creation scientists. They not on make up just-so stories, they suspend just about all we know and accept of a number of physical laws including the Second Law of Thermodynamics.

Many extreme pressure tests have been conducted on radioactive materials. No significant changes to their decay rates have been found, certainly nothing that would result in errors as large as creation supposes. Have you considered the enormous heat that would result from increased decay rates? If decay rates of any of the materials used in radiometric dating were fast enough during the flood, the energy released would have boiled the oceans and every living thing on the ark. At the end, there would be no water left (the heat would break the water molecules apart and most of the hydrogen would escape) and no life left.

[Quark2005]

You have not provided a single shred of evidence for the "evolution of life", you have only provided evidence for the inherent design of the adaptability of the creation.

Me personally, no, I'm not an evolutionary biologist.

However, there is a wealth of information out there regarding what course evolution has taken in the distant past. There is a wealth of information showing that genome is equipped to make evolutionary changes and evidence in the inherent structure of our DNA that it has done so in the past. All the data intersects at the same conclusion - that life on earth has evolved and it continues to do so.

You have a case of a mere 8000 years (a blink of an eye in the earth's history) producing a marked case of speciation - the first step of evolution. What mechanism do you believe exists that would prevent further change (call it "adaptation" or whatever you want) from happening? Do you think it's possible that frogs and toads have a common ancestor? Or did God create frogs and toads in their different "present forms"?

[b_sharp]

"I would dare say you have NO IDEA what a "one catastrophe" could or could not do, one castatrophy, could have many different compositions weather,deluges,respites,deluges, earthquakes,tremors,earthquakes,eruptions,steamvents,respites, repeat. not to mention how this would effect radiometric decay(unknowable pressures,temperatures,leaching,etc..).multiple strata doesn't demand long ages.

As I've mentioned before, extreme pressure and temperature tests have been done and no appreciable change in decay rates have been found. Leaching can be controlled as long as the properties of the layers the fossils are found in is known. Multiple strata does require long times to solidify. A single flood would not explain the evidence of tilt followed by erosion followed by sediment deposition followed by lava. Nor would it explain the lack of very distinctive pillow lavas that should occur if everything was water covered.

"fossil fish scream out rapid burial and preservation/hardening, the are preserved with no sign of decay.

Conditions for this to happen are observed today. It does not take a flood for this to happen.

[mlc9852]

Again, you didn't answer the question. But your insults are quite amusing. I think you fear right-wing religious Americans. You would prefer American become like England - it won't.

[BikerNYC]

Oh, wait. You were serious.

That's what makes it so sad.

[b_sharp]

"I was referring to an ongoing infection of a population, or resistance thereto. Similar to the resistance europeans had to smallpox, but the Indians did not, only affecting the ability of the species to interbreed. The separate populations may be separated by viral infection as well. "

Inherited resistance to virii is a good example of evolution in action.

[Alter Kaker]

Extending your implication that the flood did occur, and there was an ark, what makes you discount that the ancestors to today's diseases couldn't have been waterborne, or at least hitched rides on aquatic hosts?

Waterborne gonorrhea? Find me some of that and a lot of disreptuable men now have a credible excuse to give their wives.

[b_sharp]

"Just because they were living together in the past doesn't mean the calls were the same. It's speculation. The business about genes is a red herring. Are you suggesting you can tell what kind of mating call they had in the past, based on DNA?

If I understand what you are trying to say, you feel that all females in the previous single population responded to more than one call. If the females in one of the two populations only respond to one specific call, while females in the other group only respond to the other call, gene flow is disrupted and they are confirmed as two different species.

However, mating calls are very specific and generally tell a female which male belongs to her group and will not respond to other calls. Unless these particular frogs are an anomaly in this respect we conclude they had only one call. However, the number of initial calls is not relevant to speciation, the response or non-response to a call determines whether gene flow is disrupted.

DNA can not tell whether they had a single call or not, but that is not what I said. DNA will tell us if they in fact were a single intra-breeding population and mDNA will tell us when they separated. The thing about the genes was not a red herring, you simply didn't understand the significance of the mention of them.

[Alter Kaker]

Just 8000 years ago these animals were... frogs. And now, after a mere 8000 years they have evolved into... frogs.

Just a few hundred thousand years ago, our ancestors were primates, and we're still primates, so I guess that means you're a living example of Homo ergaster. Right?

[RunningWolf]

so I guess that means you're a living example of Homo ergaster. Right?

Hey now.., getting into the stratosphere of high level evo debate, were any on this train?

And if two trains leave Chicago traveling in opposite directions at 60 mph, and one contains a carload of apes, and one contains a carload of evolutionists ........

[Thatcherite]

Lets run through the sequence again, with some added emphasis to make it easier for the lurkers (and the very dim):

You: Do you see this as a political or scientific matter? I think it is more political for you.

Me: It gets quite comic at times. You are like one of the characters in a Smullyan logical puzzle who only entertains false beliefs. This matter is 100% scientific. Religious folks of all political persuasions attack evolution because they think (perhaps correctly though many Christians say not) that it attacks the basis of their faith. The characteristic the evolution nay-sayers tend to share is a total lack of interest in the evidence, and a pretence that they are interested in science.

You: Again, you didn't answer the question. But your insults are quite amusing. I think you fear right-wing religious Americans. You would prefer American become like England - it won't.

You aren't very good at this reading thing yet, are you. Obviously the work of a legal secretary doesn't require comprehension skills. Feel free to provide any example of me not answering one of your questions. It is easy to see how scientific evidence passes you by... You don't seem to be capable of reading and understanding your own posts, let alone mine.

And I don't fear right-wing religious Americans. Lots of them post on the evo side in this forum. Bible Idolators are a different matter however.

[Thatcherite]

Where do you see a conflict?

You agreed that you reject the earth sciences. Then you went back to saying that your only beef is with evolution, specifically from apes to people. You need to make your mind up which parts of science you accept and which parts you reject.

[mlc9852]

I don't totally reject earth science and think it is quite interesting.

[PatrickHenry]

Placemarker and plug for The List-O-Links.

[Thombo2]

"A welcome break from the trial."I'll second that.Good article.This is a very good example of evolution at work.

[Alter Kaker]

I don't totally reject earth science and think it is quite interesting.

But nevertheless you reject most of it. You seem to have a Chinese Buffet approach to science, where you pick and choose only small smidgens that seem of "interest" to you.

Nobody cares what you're interested in.