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.

[mlc9852]

Why do you believe that? Are you saying if a geologist doesn't believe we descended from apes, he can't find oil?

[GadareneDemoniac]

Give 'em a break, will ya? In another 8,000,000 years they'll be doing calculus and driving cars.

[RunningWolf]

Is there any science on these threads? ha ha ha ;)

[RunningWolf]

You know, he just might be

[RunningWolf]

You are an ape and a human, as am I.

Sounds just like what she (an anthropologist) told me at a University honky tonk 25 years ago, I held it against her ;)

[savedbygrace]

And you seem to be making assumptions about what I'm thinking. You so busy creating straw men out of whole cloth that you apparently don't have time to read what I actually wrote.

Either you're so dumb you're willing to accept what's written in an article ABOUT some research without reading the research itself, or you're just a koolaide drinking evolutionist who never learned to think for himself.

Got a link, or not? Yes or no, or don't bother to reply.

[Junior]

Wrong on all accounts, bucko. I know enough about research to know that if these guys were pulling stuff out of their asses (as you seem to think researchers do) they'd be put paid to by their peers.

To you, however, it's all assumptions isn't it?

[RunningWolf]

Do you have any purpose on these threads other than to yell out cheap shots from the peanut gallery?

I thought that was your job.. whats up with that?

Wolf

[RunningWolf]

Tell me how I have been rude.

You haven't don't worry

Wolf

[PatrickHenry]

Placemarker and plug for The List-O-Links.

[Dimensio]

But I love how she keeps responding to you saying that she's not responding to you!

Apparently this policy "started" when I claimed that all creationists are liars. Except that I didn't actually claim that, mlc9852 was lying when she said that I did. some creationists are liars and provided three examples. Rather than admit that yes, some creationists are dishonest or at least attempt address the examples that I gave to try and show that they weren't documented examples of dishonesty, she first changed the subject, and then lied and claimed that I had accused all creationists of being dishonest. Which gave me a fourth example of documented creationist dishonesty.

[furball4paws]

She let you get that close?

[Liberal Classic]

You have a point to make about the quality of my posts? Make it now.

[RunningWolf]

Hey mlc9852,

here one for all /the ape is man/ contingeint.

And if two trains leave Chicago traveling in opposite directions at 60 mph, and one contains a carload of monkeys, and one contains a carload of evolutionists, how far is it before both trains have occupants with the same IQ?

Well, calculating in for the variable=(evo cultist).., about a mile? /sarc>

Screaming chimps doing head flips..., eeh ooh ooh, eeeh ooh oh!
..,train sounds CHOO choo choo, CHOO choo choo choo choo, CHOO choo choo choo choo, WooWooohh! WooWooohh! WooWooohh!

Wolf

[staterightsfirst]

The abstract is available and full text (by subscription) is available at:

http://www.nature.com/nature/journal/v437/n7063/abs/nature04004.html

Copyright laws prohibit the reproduction of large sections of the journal article here, however, I do have subscription access to it, so if you have any specific questions, I would be happy to answer them.

Cheers!

[RunningWolf]

mlc9852; I am a human being created to be a human being. Can't speak for you.

Liberal Classic's interpretation
I am a human being... Can't speak for you.

Do you have any purpose on these threads other than to yell out cheap shots from the peanut gallery?

Liberal Classic to Wolf ; You have a point to make about the quality of my posts? Make it now.

No No, I have not read all of your posts, I'm sure some of them are very good. It seemed you were not all that serious here.

But if you were, take a look at when you took out 30% of mlc9852's words to make your 'cheap shot peanut gallery' comment, and then you make the call.
nite

Wolf

[dread78645]

Profound rebuttal placemark

[Smokin' Joe]

Because if they made it this far in just 8,000 years, just think! In a million years, they could be---frogs!

[Smokin' Joe]

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.

Do the southern frogs (or the northern ones for that matter) have some disease or parasite which could cause this effect? Something like what caused all the three-legged frogs a few years back?

[staterightsfirst]

No disease and no parasite required - over time, the genomes just diverge enough that combining the two doesn't work anymore.

They think this has to do with mitochondrial DNA, which is passed on exclusively by the mother (hence why northern female + southern male = OK but slow and southern female + northern male = no development).

Mitochondrial DNA is complimentary to the regular DNA - so say the original ancestor needs genes A and B to survive, mito. DNA produces A, and regular DNA supplies B.

Then, the southern frog mito DNA mutates slowly such that A becomes gene C - but that's okay, because B in regular DNA changes along with it to D such that they work even better than before - so frogs with DC were more successful than AB. Over time, DC becomes more and more common, because they work better.

However, northern still use AB - so when a southern female and a nothern male mate, the southern female contributes mitochondrial gene C, and the northern male contributes gene B - but B and C don't work together at all anymore and no offspring even develops!

Hope this helps...