PART II - DARWIN'S FINCHES
by Nancy A. Schiller and Clyde Freeman Herreid
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The Galapagos
PART II - DARWIN'S FINCHES |
"So, Kate, what do you think of the islands?" asked Miguel.
"I think they're fantastic. The Spanish were right to call them 'The Encantadas'." Kate had just entered the library of the Charles Darwin Research Station to find its director sitting at a table.
"Yes, 'The Enchanted Isles' does seem to suit them. The volcanoes have always seemed bewitched to me. Sit down for a minute and tell me how your plans are shaping up." Miguel gestured toward a chair across from him.
"Well, I was on Albemarle two days ago, and I think I could get my research done up there. The chance to study character displacement on the five volcanoes is really exciting. Imagine, five different races of tortoises on the same island. A lot of the tortoises seem to have already been marked by previous researchers, and you seem to have a good handle on some of the history already."
"Yes. In addition, we have some good DNA data on the tortoises here, especially those in our breeding pens. You've seen them, I'm sure."
"Yes, I have. I think it's a terrific breakthrough that at last you can identify which islands some of these animals came from. In fact, it should be possible to identify the parents if enough data were collected. But, who's doing this work? Is it done here at the station?"
"No. We send tissue samples out to labs in the United States for DNA fingerprinting. We don't have the lab facilities here to do Southern Blot procedures or PCR. But tell me, are you committed to working on the tortoises? There are a lot of easier things to study here. The iguanas and the birds are more accessible."
"I've thought about that, especially since the Grants have been so successful working on the finches. How's their work coming?" Kate leaned forward eagerly.
Miguel removed his glasses, tilted his chair back, and began: "I'm sure you know that Peter and Rosemary started trapping and banding all of the finches on Daphne Major over 20 years ago. Sometimes they had only 200 birds on the island; sometimes there were over 2000. And they could recognize them all! The Grants have been able to watch evolution happen; something that Darwin could only imagine."
"He thought evolution was too slow to see," Kate said. "In fact, Creationists have argued that neither evolution nor Divine Creation could be tested. The Grants' work certainly puts a lie to that."
"Yup. Darwin certainly underestimated the speed of evolution. These islands appeared less than five million years ago, and life is evolving here as fast and furiously as the volcanoes because life forms are trapped on separate islands. The top of each volcano is a prison for most of the creatures."
"There never was a bridge to the mainland, was there?"
"No. Anything that has arrived here had to cross at least 600 miles of water from South America. That's how the first finches arrived here, presumably blown off the mainland during a storm."
"So, in the space of a few million years all of the different species had to have adapted to the different conditions on the different islands."
"That's true, but realize the differences are not all due to adaptations and natural selection. I figure a lot of it has to do with genetic drift."
"Of course. But whatever was going on, no one figured it would be as fast as the Grants have discovered. I think one of the neatest techniques that the Grants brought to their study was a way of measuring how the seeds of the different plant species vary in their hardness."
"I agree with you, Kate. Peter and an engineer at McGill University designed a nutcracker, a sort of pliers with a scale attached. They found small soft seeds of Portulaca needed only a force of 0.35 newtons to be cracked. Any of the finch species could do that. But the big hard seeds of Cordia lutea needed 14 newtons force and only a few large finches can muster that much force. Once they knew how to measure hardness, the Grant team could check the abundance of the seeds and see exactly what the birds selected and ate."
"But that varied with the season, didn't it?"
"Exactly. In the wet season, when small seeds were abundant, the average seed hardness was 0.5, but in the dry season it jumped to over 6 newtons. Lots of finches couldn't eat many of those seeds. If it hadn't been for the fact that the six species began to switch to other foods, severe starvation would have occurred. A great example of resource partitioning--one of your pet subjects, Kate.
"In fact, Rosemary Grant found that switching even occurred among individuals of a single species. In times of plenty, all cactus finches eat the same seeds, but when these seeds are scarce, things change. Those with long beaks open the fruits and probe the cactus flowers, while those with larger, deeper beaks crack the big tough cactus seeds, and those with still deeper beaks strip the bark from the trees to get grubs."
"Everything gets worse when a severe drought happens, doesn't it? Natural selection really gets intense."
"Absolutely. It virtually wipes out the birds with the smaller beaks. The great drought of 1977 taught the Grants that. The population plunged to 200 birds. Before the drought, the average beak size of the fortis species was 10.68 mm long and 9.42 mm deep. After the drought, it was 11.07 mm long and 9.96 mm deep. Variations too small to see with the naked eye made the difference between life and death. It altered the sex ratio, too, since males have larger, stronger beaks. After the drought, there were seven times more males than females! In the years following the drought the competition between males for mates was fierce. Amazingly, the females started choosing the males with the largest beaks for breeding. The smaller-beaked males didn't have a chance to mate at all."
Kate nodded. "That's a perfect example of sexual selection in action. Both natural election and sexual selection were working in the same direction: producing birds with large beaks. But were these genetic differences or were they due to the food supply? I mean, wouldn't you expect the well-fed birds to have larger bills, and then they would be the ones to breed and successfully raise young?"
"Of course. One of their graduate students wanted to test that, but it didn't work out, so they have had to resort to indirect evidence to demonstrate that the changes were genetic.
"But let me tell you about the big El Nino events of 1983," Miguel continued. "A huge amount of rain fell that year, a once-in-a-century event! The island went from a desert to a jungle in a few weeks. Food was everywhere. The birds started breeding like crazy and there were suddenly 2000 finches on Daphne Major. When normal, dry conditions returned, there suddenly wasn't enough food to go around. The birds had overshot the carrying capacity of the island.
"But here's the interesting part: now, birds with smaller beaks were favored. Big males and big females started dying because of insufficient food. That's because there were a lot more small seeds than large ones lying around. The floods of El Nino had washed away many of the large seed-producing cactus trees. Even though the big birds could eat small seeds, they were at a disadvantage because of their size. They had to eat more to survive."
"A lot of hybridization started occurring after El Nino, didn't it?" Kate said.
"Yes, to everyone's great surprise. Hybrids were rare during the lean years. The reproductive barriers kept the species distinct. The isolating mechanisms were primarily differences in song and beak size. But once the conditions dramatically improved, diversity was favored. A lot of finches began to breed with individuals of different species. They didn't seem to care as much what kind of song or beak their mate had. Some people even speculated that the mutation rate actually increased when environmental conditions shifted dramatically.
"Take a look at the finches outside the window," Miguel added. "Here at the research station it's hard to separate the different species because of the hybrids. They sort of fuse together here, where conditions are always good because people feed them. Kate, if you really want a great thesis problem, you couldn't do much better than study finch hybridization around the village. I'm sure that humans are having a terrific impact on both speciation and extinction."
Charles Darwin Research Station
http://www.darwinfoundation.org/
Image Credits::
Galapagos Finch, photograph by Clyde F. Herreid. Used with permission.
