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.

[Smokin' Joe]

I understand the basic genetic theory, but am shaving with occam's razor. Did anyone check?

[MHalblaub]

Don't cut yourself.


Occam refers to two competitive scientific theories. I can't see another one. If you got ID in mind. Please keep it there. ID has no explanatory power.

[staterightsfirst]

I read the research this article is based on, if that's what you mean...

I find it highly unlikely a parasite or other external force is involved when they do controlled matings in the lab.

[Smokin' Joe]

Call off yer dogs, son, I was just asking a question. Remember all the mutated frogs here in the states? Turned out they had some parasite and it wasn't pollution or global warming as some had postulated.

Even the researchers said that they found a mere 8000 years to be remarkable fast for the development of a new species.

Might the incompatibility be due to another organism, parasitic or otherwise, that the other frog population had not been exposed to affecting interpopulational fertility?

Moving them to a lab would not necessarily get rid of the parasites, virii, etc..

Did anyone check for that or were they in such a rush to have their ego step on someone's ID?

There is your other theory.

I asked, you flamed. Lovely science.

[Smokin' Joe]

Nonetheless, it is the application of evolutionary theory that makes the continued exploitation of mineral resources economical.

And here, all this time we thought it was the seismic data. Hmmmm. Have to have a talk with the guys in geophysics....

[savedbygrace]

Thank you. I'll order it as soon as I can afford it, hopefully within the next two or three weeks.

The abstract doesn't allude to any relationship between speciation and evolution in this particular case. Does the full text do that, and if so, what does it say?

In a quote in the article, Hoskins admits to speculation, but the abstract doesn't mention that, so I'm curious about the extent of this speculation. But I'm not sure what questions to ask. I think I'd need to read their reasoning, and see what assumptions were used.

The article says there are significant differences between the males in the two populations, but only mentions two, neither of which seems major on the face. What does the full text say about this?

Also, does the full text provide any data about genetic differences between the two populations? Are there any genetic differences?

[flevit]

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

[staterightsfirst]

Moving them to a lab would not necessarily get rid of the parasites, virii, etc..

First of all, generally, yes, it would, especially over several controlled matings, the ability to observe and assay embryos, in vitro fertilizations, etc.

Might the incompatibility be due to another organism, parasitic or otherwise, that the other frog population had not been exposed to affecting interpopulational fertility?

No. It's not fertility that's affected. It's a developmental problem in the offspring. Southern females and northern males have lots of babies - the babies don't (can't) develop into adults because of genetic deficiencies.

Consider also that their research is based on comparative molecular studies between the mitochondrial DNA of the northern frogs and the southern frogs. You mentioned Occam's razor. Is it more likely that an external force is causing such differences between them (especially developmental problems) or that the observable differential molecular sequence is causing a differential phenotype? :)

[Junior]

So basically you're not here to offere substantive evidence for your position(s); you're here simply hop up and down and throw insults from the sidelines.

I can count on one hand the number of anti-evolutionists on these threads who even attempted to have all their ducks in a row before posting. You ain't one of them.

[Smokin' Joe]

Wrong question. Is it more likely that these frogs have taken a unusually fast track to evolve into separate species or is an external factor causing embryonic and 'early childhood' developmental problems for the populations? I believe it was a nematode or some such which was causing frogs to develop extra limbs and other malformations here in the states. Industrial pollution (pesticides?) got the rap first, but turned out to not be the case. For instance, in this article fungus is cited as a problem.

Here, extra legs and here Scientific American, page 2 mentions parasites other causes are noted.

It takes no great leap to find potential for attribution here, well outside of aberrantly rapid evolution. Isolated populations will be exposed to different environmental factors, including vegetation, chemicals (natural and manmade, the latter in precipitation or runoff) and quite possibly different pathogens. One population exposed to a chemical (natural or manmade) or a pathogen, be it viral, bacterial, fungal, or parasitic, will naturally successfully breed only those who can adapt to or are resistant to that pathogen or chemical and still produce viable young.

That change could handily occur in just a few generations, not even 8000 years.

The other population, not exposed, might not fare as well if the pathogen or traces of the chemical are transferred in the act of breeding and that population has little or no natural resistance to the pathogen.

[Quark2005]

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

You have to remember that the fossil record doesn't preserve every step in the evolutionary chain. Most dead animals & plants just rot away; only a tiny fraction of 1% are preserved as fossil imprints. It is a spotty, haphazard means of preservation; it can easily give the appearance of "discrete" steps. Many of the intermediary steps are either lost to the winds of time or remain under the ground, waiting to be uncovered.

[highball]

Just ignore my posts. You really have no reason to respond to me, do you?

You're spreading ignorance. Ignorance of science, ignorance of the facts in this case. For the sake of the lurkers, we need to combat it.

If we don't, then the willful ignorance may win - it's far easier and more seductive.

[highball]

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.

She also refuses to condemn the lies of the school board in this case. She expressed a limp "lying is wrong" but won't admit that the school board lied, which is a new level of dishonesty.

I'll give her this, though: she's recently admitted that she has no interest in facts. That's some bizarre honesty about dishonesty - I don't know quite where to fit that statement in my catergories of creationist lies.

[MHalblaub]

"Call off yer dogs, son, I was just asking a question. Remember all the mutated frogs here in the states?"

I'll call off my frogs, dad, I just tried to heat your question to distill it.

The question in this article was not about why the southern females got problems. This really could due to "organism, prasitic or otherwise" cause. The evolutionary result was fascinating - a new specie. The entrapped southern colony is today unable to breed with the other frogs.

But that also may only have minor causes like "organism, prasitic or otherwise". Organism or parasitic is unlikely because neither male nor female frogs can breed and an organism is easy to detect under microscope. A viral infection is harder to detect. But why causes that only the entrapped specie to produce no offspring with the others? A special genetic resistance of the others?

So you see your hypothesis about 'other' causes has much more unknowns than the new specie hypothesis.

You know how Occam cuts?

[flevit]

all you have done is provided an explanation of why the fossil record doesn't match with extant demonstrations. if it is haphazard, it still shouldn't be selective, it should still provide evidence of the long chain of minute changes. These minute changes are the only demonstrable "evolution".

the fossil record, records dead things, the so-called "evolutionary chain" is one explanation (granted the most conventionally excepted explination) of the fossil record.

the bible told of mass extinction long before man (tried to divide it up into 6-7 mass extinctions)found the fossil record.

[Quark2005]

all you have done is provided an explanation of why the fossil record doesn't match with extant demonstrations.

That's all I've attempted to do. To say it doesn't "match" extant demonstrations is really not a totally accurate statement; a more accurate statement would be that the fossil record doesn't (usually) provide enough information in and of itself to draw gross conclusions about the nature of minute evolutionary changes one way or the other.

These minute changes are the only demonstrable "evolution".

This is the only direct evolution we observe, true; but the consequences of evolutionary theory are observable on many more lines of evidence than just microevolutionary adaptation/change and the fossil record provide. Morphological similarities and variations amongst related species and biogeographical distribution provide a wealth of information. With the advent of genome sequencing, direct tests can also be made using statistical mutation rates to see how long distinct lineages have been separate.

the fossil record, records dead things, the so-called "evolutionary chain" is one explanation (granted the most conventionally excepted explination) of the fossil record.

I know of no other scientific explanation that provides such a consistent model of life.

the bible told of mass extinction long before man (tried to divide it up into 6-7 mass extinctions)found the fossil record.

There are definitely multiple mass extinctions in the past, the Permian extinction being the most catastrophic. This knowledge is provided by extant geological evidence pointing to the strata where the relevant fossils are found. (Again, other lines of inquiry seem to point to the same conclusions...)

[flevit]

thats because any explanation of evidence that does not assume evolutionary/uniformitarian time line is conveniently labeled as "unscientific". So yes your statement "you know of no other "scientific" model..." is quite true.

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).

[Liberal Classic]

As oil becomes more difficult to find, better exploration techniques help keep oil production economical. Seismic data gives us the reflectivity of different layers of strata, but it doesn't tell us the whole story. When looking for oil, micropaleontology helps us understand the environment at the time sediments were laid down, and this helps us determine which formations are likely to contain oil deposits. Micropalenontology is also helpful when looking for coal, for the same reason.

[Quark2005]

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).

Sorry, geological evidence just doesn't accomodate one mass extinction. The fossils are deposited in multiple strata separated by hundreds of millions of years. Radiometry and sedimentation analysis of the surrounding rocks prove this. One catastrophe could not deposit huge numbers of layers of dirt, instantly condense them to rock, then deposit more layers of dirt that instantly turn into rock, etc. Not to mention that each strata in the column give radiometric dating rates that correspond to gradual accumulation. And the fact that particular fossils are only found in the appropriate strata (no method of deposition can explain that).

[Amish with an attitude]

Things have really begun to slip for you evolutionists when junk science starts to look like "a welcome break".