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.

[highball]

You'd think that one example of something that evolved would be the final nail in the coffin, but of course 'they' are driven by something other than science. It never was about science.

The creationist rallying cry used to be "show me an example! Show me an example!"

Now it will be "Um... show me another example!"

I don't mind plain ignorance - everybody has to start learning sometime. Willful ignorance, however, is just sad.

[js1138]

If ignorance were only painful...

[PatrickHenry]

If ignorance were only painful...

It often is. Read my tagline.

[BikerNYC]

Well, according tho what I read...it's still a frog. What did the frog evolve into? Not a very strong case for evolution.

Not all frogs are alike: 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.

[narby]

Wow! I didn't know scientists lived that long!

You must have missed that point entirely about how the rain forest had been separated by climate shifts and then rejoined by their expansion 8000 years ago. That separated the frog populations and this article examined the repercussions of the forest coming back together.

No doubt there's plenty of evidence of forest boundaries over the years available.

Of course if you think the earth is only 6000 years old, then I see your problem.

[mlc9852]

"that is unable to mate with either of the original frog populations."

Maybe they aren't really trying.

[js1138]

So how com there aren't any new kingdoms evolving, huh?

[Conservativehomeschoolmama]

It's still a frog. It did not turn into a salamander or a snake. I would put this article under *adaptation* not evolution.

[skinkinthegrass]

What? No Helen Thomas pic? Yall are slippin....

Nahhh, She's in the Ancient/Classic Greek Mythology Section under, Gorgons Sisters, and their Ilk.

[balch3]

Darwinist barf alert.

[mlc9852]

Why is everyone so excited about a frog becoming a frog?

[mlc9852]

Why would I think the earth is 6,000 years old? Think I just fell off the turnip truck?

[b_sharp]

"Well, according tho what I read...it's still a frog. What did the frog evolve into? Not a very strong case for evolution."

Actually it's a very good example of evolution. What it is not a good example of is the creationist strawman version of evolution. Tell me, what would you accept as a good example of speciation?

[RightWhale]

It's obviously not about science. It's also not about God or religion, IMHO. The question is, what is it really about. I suspect at heart it is anti-individualism.

[BikerNYC]

So whether or not evolution occurs depends upon whether or not we describe an animal that can no longer mate with others with a new name? It can't mate with the other species. I thought that was part of what a new species was all about.

Would you think evolution had more viability if we called the new animal a nerfworker and not a frog? Or does evolution no occur until the new species is given a new name, but before the new name is bestowed upon the new creature, there is no evidence that evolution occurs?

[highball]

So how com there aren't any new kingdoms evolving, huh?

Once creationists as a group admit that new species are indeed evolving, we'll talk about that....

[Theophilus]

"...found that their hybrid offspring were less viable"

Hmmmmmm, this guy may not believe in intelligent design, but he definitely believes in intelligent frogs practicing eugenics.

[Antonello]

I don't understand. The southern group was isolated from the other group for a long period of time, but didn't develop this "speciation" -- they were supposedly all still compatable.

But then, having found each other again, the same behavior (breeding only among southern frogs) led to a speciation in only 8000 years?

What was different between the physical separation and the "choice" separation?

Isn't it much more likely that the southern population already have females which were incompatable with the other group? And when they started inbreeding again, those who couldn't get pregnant from the northern group would have babies which couldn't get pregnant from the northern group?

Meanwhile those who still COULD would randomly get inpregnated from their own group OR the northern group, and since the cross-breed children were inferior they tended to die out?

So over time simple natural selection tended to select toward those southern frogs who could only get pregnant by inbreeding?

How do they know what the ones from 8000 years ago did?

And it is rational that frogs would somehow notice their children were dying if they bred with the "wrong" frogs, and some of them would be smart enough to change their behavior and only breed with the "right" frogs?

That seems like a stretch even for true believers.

I understand it to go something like this:

A group of frogs got physically isolated. During that time, each subgroup developed subtle genetic differences independently of each other. However, because the two groups physically could not interbreed, there was no environmental pressure for the females to differentiate between the two groups. This means that while differences did develop, there was no reason for natural selection to encourage a trait that caused the groups to discriminate in regards to interbreeding.

Once the groups reunited, however, this changed. Since the hybrid spawn lacked viability, the females that displayed the trait of being more particular about mating only within their group stood a better chance of passing along their genes, including this 'picky gene'. This altered the environmental conditions for the males as well, by causing those that were readily discernible by the females to be preferred. This resulted in their genes, with the differentiating identifier, to be passed along more readily than those that weren't.

This all culminated in an acceleration of the natural selection process resulting in the sped up genetic development of a new species.

Had these two groups not come back into contact, this new environmental pressure would not have been a factor, as both the 'easy' and 'picky' females would still be producing viable offspring (since they would not have had the choice of a male that would result in nonviable offspring).

[highball]

I would put this article under *adaptation* not evolution.

That's because you obviously don't understand what evolution is.

Please re-read the article. The frogs evolved from one species to another. The very essence of Darwin's work (which was entitled "The Origin of Species", after all). And, incidentally, creationists mistakenly claim this type of evolution doesn't happen.

There's a good list o'links where you can learn about it - I'll see if I can dig it up.

[Trimegistus]

In the News/Activism forum, on a thread titled Picky female frogs drive evolution of new species in less than 8,000 years, Conservativehomeschoolmama wrote:

"It's still a frog. It did not turn into a salamander or a snake. I would put this article under *adaptation* not evolution."

"Frog" is not a species. It's a whole group of species, just like salamanders and snakes. Here we have direct evidence of one species of frog splitting into two. And, yes, it's an adaptation to environmental change. THAT'S WHAT EVOLUTION IS ALL ABOUT!

Trying to dodge the issue via semantic games is dishonest. God will boil you in molten sulfur for a billion years for being dishonest.