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.

[Dimensio]

Of course not. I don't stand for liars, so you run away from me like a coward.

[Junior]

The timeline summarized in the article seems to be based on assumptions...

Which means you have some idea of the assumptions being made, or you wouldn't characterize the research as being "based on assumptions." That thought would never cross the mind of anyone who understood science, and is typically a creationist talking point.

So, either you have an idea of the assumptions you claim have been made, or you are just assuming that's how science works (in which case you would be wrong).

[highball]

And evidence is all in the interpretation, isn't it?

Um - no, it isn't. If that's what you really think, no wonder that you're a creationist.

[Dimensio]

The first time mlc9852 said that he (or she) was not replying to me, it was stated as though he (or she) had initially said it sometime in the past. Except that I checked and he (or she) hadn't. So he (or she) was lying then and now.

[mlc9852]

You wish

[Rocky]

It doesn't occur to anyone that there are assumptions built into this analyis?

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.

We don't know that they were all originally the same before the communities were divided. We don't know what the calls of the male frog sounded like before they were divided.

there may have been selection pressure to evolve a mating strategy...; This appears to have occurred in the contact region...

Speculation. Why is this kind of stuff given such high credibility by evolutionists? Can it be because it aligns with what they already believe?

[highball]

I've been told that she's a she, if it makes any difference.

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

[mlc9852]

I have refused to reply to your name-calling and insults for some time now. Honesty still isn't your strong suit, is it? Go back and review some prior posts. I always say the same thing. I don't respond to those who use name-calling and insults to try to make their point. And funny I don't "run away" from anyone else, isn't it? And many are much smarter than you.

[highball]

Honesty still isn't your strong suit, is it?

Come now - anyone who supports this school board ought to be very careful before they call another person a liar.

[b_sharp]

"I would wonder why his/her parents weren't watching him/her near the water.

That doesn't answer the question. Is it a sign of high intelligence for a human child to, on her own, figure out that a stick will help her avoid the deep spots in the stream? Yes or no?

"And writing goes back much farther than a few hundred years.

In our culture yes. However many cultures did not develop writing as a method of storage until much later. An example is the northern plains aboriginals who passed on stories verbally and used other methods of communication than writing. Does that make them less than human?

"Look, if you want to be an ape, that's fine. I prefer to be a human.

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

[TaxRelief]

No. Seriously. In what way were they "unable to mate with either of the original frog populations"?

[Liberal Classic]

Evolution has absolutely no impact on my life one way or another.

You see, this is how you're incorrect. The study of evolutionary theory has practical application not only in the medical sciences and biology, but fields beyond biology as well. For example, an important aspect of modern oil exploration is micropaleontology, the study of the microscopic remains of bacteria, diatoms, pollens, and protists. Micropaleontology is by far the largest field of paleontology, and has broad application in envrionmental engineering and mineral exploration. So, if you drive a car, or use electricity from coal-fired power plants, then evolution has had an impact on your life.

[Rock N Jones]

When I’m calling you-ou-ou-ou-ou-ou-ou
Will you answer true-ou-ou-ou-ou-ou-ou

[Alter Kaker]

So you don't think evolution means descended from one common ancestor? Great! I can agree with that!

Of course I think evolution posits common descent. Are you deliberately being obtuse or can you not help yourself?

[Alter Kaker]

I have repeatedly stated I have no way of knowing how old the earth is but believe it is more than 6,000 years.

If you have no way of knowing how old the earth is, how do you know it's older than 6000 years?

[VadeRetro]

... in less than 8,000 years

So you were there?

(But I see the real creos beat me to it, and with a straight face at that. How easy it is to do "the REAL science!")

[Alter Kaker]

Evolution has absolutely no impact on my life one way or another.

I love your head-in-the-sand mentality. Evolution affects you whether you like it or not. And you may be singing another tune if a strain of avian influenza evolves and wipes out your family.

And evidence is all in the interpretation, isn't it?

No. Evidence is evidence. Facts are facts. And you have neither on your side.

[Palisades]

A frog is a frog is a frog. What is your point? Did the frog sprout wings and fly? No? Hmmmm.....

Don't you think you're overly simplifying things here? There are a variety of frog species. Do you really consider all of them to be the same?

[Palisades]

Look, if you want to be an ape, that's fine. I prefer to be a human.

False dichotomy.

You are a human, and an ape.

A relatively hairless, clever, tool-using ape. But an ape, nonetheless.

[BluH2o]

... began selecting mates with a the southern call type.

So those Yankee accents drove those little trollops wild ... in Australia north is north & south is south ... but north is actually closer to the equator ... so those southern gentlemen frogs are Yankees (by our standards) ... get my drift? Oh, never mind ...