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.

[Alter Kaker]

Ask any lawyer about "evidence".

Ask any jury about evidence. We are well past the point of any informed, reasonable person having reasonable doubt about the evidence for evolution.

You surely aren't saying scientists always agree with the evidence, are you?

I don't know what that means. Scientists don't "agree" or "disagree" with evidence -- evidence is evidence, it is not an argument, it simply exists.

[Alter Kaker]

Can't help myself.

I know you can't. Some people are just born stupid. You should really take it up with your father, wherever he is.

And I enjoy imaging the veins popping out of your neck every time you reply to me.

Really, you flatter yourself.

[mlc9852]

Your answer regarding whether or not scientists always agree with the evidence is disingenuous at best or maybe just a lie. Do you believe all scientists think humans descended from ape-like creatures? Not one scientist doesn't believe that? Or are those scientists just wrong.

[Antonello]

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 disease is not one of Lamark's ideas, iirc, any more than congenital syphilis.

My point, for clarification, is that is more likely that exposure to a pathogen would be an environmental pressure that selected genetic mutations that favored survivability rather than a direct instigator of the genetic change itself.

Kinda just borrowing your Occam's Razor to split hairs, so to speak.

You are a little off regarding inherited diseases, however. Something such as congenital syphilis is not genetically inherited. Rather, it is a result of the child being directly infected in utero. Remove the pathogen and future progeny won't be infected. Other heritable maladies, such as sickle cell anemia, actually are a genetic predisposition that is passed on to future generations. These, however, are genetic defects, not pathogenic infections.

[mlc9852]

You flatter yourself if you think you are so brilliant. You aren't. And my dad died when I was five.

[Alter Kaker]

That's one of the funniest things I have ever read.

[Alter Kaker]

You flatter yourself if you think you are so brilliant.

Compared to what? Next to you, a mound of manure would shine, and I'd like to think I'm at least a few IQ points ahead of manure.

And my dad died when I was five.

OK.

[Alter Kaker]

Do you believe all scientists think humans descended from ape-like creatures?

All scientists in the field, yes, because that's what 100% of the available evidence indicates. No evidence -- geological, historical, genetic, morphological, fossil, dendochronological (and I could go on with adjectives) suggests otherwise. Until you find some, you just look silly.

[RogueIsland]

Looks like Abe Vigoda.

[Thatcherite]

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

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.

[Thatcherite]

That's one of the funniest things I have ever read.

Seconded, what made it so brilliant is that the first few questions in it were totally believable and probably exist on a creationist website somewhere. That is the problem with trying to parody creationism; half the time it is a parody of itself. No anti-evolution view is so ignorant and bigoted that it cannot be put forward in all seriousness.

[Thatcherite]

You are a little off regarding inherited diseases, however. Something such as congenital syphilis is not genetically inherited. Rather, it is a result of the child being directly infected in utero. Remove the pathogen and future progeny won't be infected. Other heritable maladies, such as sickle cell anemia, actually are a genetic predisposition that is passed on to future generations. These, however, are genetic defects, not pathogenic infections.

Out of interest, how many diseases must the passengers on the ark have been carrying? Given that we are supposedly all descended from them, and that those who believe in the ark tend to deny large-scale evolution... ;)

[Antonello]

Out of interest, how many diseases must the passengers on the ark have been carrying? Given that we are supposedly all descended from them, and that those who believe in the ark tend to deny large-scale evolution... ;)

I generally tend to steer clear of theological or philosophical musing on science threads, but as long as you brought it up to me....

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?

[furball4paws]

Mark Twain had some fun comments on this, but bugs are often quite resistant and long lived, so it might not be necessary for all of Noah's family to be a walking petri dish.

[P-Marlowe]

Evolution at work!

Just 8000 years ago these animals were... frogs.

And now, after a mere 8000 years they have evolved into... frogs.

Amazing.

I guess this puts the final nail into the old creationist coffin, eh?

[Quark2005]

I guess this puts the final nail into the old creationist coffin, eh?

No, that happened decades ago.

This is just another molehill on the mountain of evidence that already supports the evolution of life on earth.

[staterightsfirst]

I won't add much to what Antonello said, other than you seem to be ignoring my statements that since the problem occurs with in vitro frogs, it's not an ecological factor, but rather a heritable one.

The developmental problems in the offspring between the northern and southern strains are based on genomic incompatibilities, not ecological factors. As Antonello said, a pathogen could easily be the driving force (in just a few generations, by your own words) to create genomic divergence.

Do you have reason to doubt this? Have you read the research article in enough depth to criticize their methodology and see evidence for your competing hypothesis?

[b_sharp]

"sounds like what can be demostrated with extant species (tiny steps) and your description of the fossil record are inconsistent.

I believe you are mistaking me for a creationist. The inconsistency is not mine, but creationists who have different standards for the fossil record and extant species.

My description of the fossil record meshes quite well with the tiny steps we see in extant species. Each of the fossils in a sequence are hundreds of thousands of years apart if not more. It is similar to viewing single frames of a movie and limiting the frames viewed to be thousands of frames apart.

[P-Marlowe]

This is just another molehill on the mountain of evidence that already supports the evolution of life on earth.

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.

The evidence that you point to is not evidence of the "evolution of life", but evidence of the evolutionary adaptability that was designed into the creation.

In this case you have a frog evolving over a period of 8000 years into a frog. Well, you multiply that change by 8000 and you've chewed up 64 million years and you still have a frog.

All you have shown is that evolutionary characteristics of survival have been designed into the creation. 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.

[PatrickHenry]

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