SJU Reads: Beak of the Finch

SJU Reads: Beak of the Finch

Adam Capriola

Biology 1021

Dr. McCann

Question 1: How did natural selection change the morphology of the finch populations on DaphneIsland from 1977 to 1978?

In 1977, there was a drought on Daphne Major. Plants began to wilt and shrivel, leaving the finches without much food to eat. The fortis did not breed because of this lack of food. They needed to feed themselves first before worrying about breeding. During the previous year, there had been 10 grams of seed per square meter of lava, but during this year there were only 6 grams of seed per square meter and June and only 3 grams of seed per square meter in December. This dearth of seeds led to a decrease in the selection of seeds available. Most of the easy to eat seeds were already eaten, leaving only the harder to eat seeds. One main seed left to eat was the Tribulus.

In the past, the fortis would almost never attempt to crack open a Tribulus, but now they were forced to try and open them. The smallest fortis could not open Tribulus, and were thus forced to try and eat Chamaesyce seeds. The Chamaesyce plant gave off a milky, sticky latex when its leaves and stems were cut, which got onto the birds’ feathers and helped lead to their deaths. The scientists gathered numbers on the finches and noticed that the populations were dropping, especially the smallest sized finches. The smallest finches were having the most trouble surviving the drought. It was noted that the fortis with the biggest beaks and larger bodies had the highest survival rates.

The next year when the scientists went back to the island, they noticed that the female fortis were very specific with the males they mated with. They only mated with the bigger males. This caused the population of fortis to be bigger than they were in the past. This morphology was cause due to the fact that the bigger fortis had the best chance of survival during the drought. Naturally, the female mated with the males that had the best characteristics to survive during the drought. Eventually, bigger fortis became inferior and the population switched back to smaller fortis.

Question 2: Describe Endler's "natural" selection experiments with guppies. What did he show?

Endler studied guppies in the wild. He noticed that near the headwaters of the streams where there is only one enemy of the guppy, the guppies for the most part were colorful with large, bright spots of blue. The guppies downstream, however, that had to deal with multiple enemies were for the most part smaller and less bright with spots of black and red. Endler thus concluded that guppy females are attracted to colorful males, but that same trait also attracts predators. Therefore guppies that live in areas with few enemies are more colorful and extravagant because they stand less chance of being eaten and have a better chance of finding a mate. Guppies that live in areas with lots of enemies need to be able to blend in with gravel on the bottom of the stream in order to survive.

In order to test this theory, Endler created artificial streams. He took samples of guppies from every stream and bred them together in an environment free of enemies, creating a heterogeneous mixture of guppies. All the guppies had random attributes of spot size, location, and color. Endler then separated the guppies into different streams. Each stream had different types of gravel at the bottom. Endler introduced predators into some of the streams. In the streams where he introduced enemies, the guppies began to change. They became smaller, less colorful, and their spots changed to match the gravel. In the streams without enemies, the guppies grew bigger, their spots did not change, and they became more colorful. Endler had reproduced what had occurred in the wild. He had proved that the variation in guppies was a direct result of their environment. This showed that natural selection was true.

Question 3: How have biologists reconciled the apparent paradox of rapid evolution in the short term with much slower evolutionary rates measured in the fossil record?

Biologist had once thought that evolution took place over very long periods of time, when in fact, evolution happens quickly. In order to explain this, the analogy of a volcano is used. When close up to a volcano, one can see the violence and intense movement of lava inside and the massive amounts of smoke billowing out. There is a lot of movement and action. When looking from a distance, however, the volcano seems to be passive. Smoke coming out looks to be at a standstill, and one has to watch for a long time to notice much change.

As shown by the finches, evolution happens very fast. In just one year, the size of a finch’s beak can increase by 5 percent. This is a fairly big change. Just as quickly however, the size of the beak can decrease by 5 percent. The finch goes through a back and forth struggle of change to survive. Over the long run, the change in beak size may only be a tenth or hundredth of a percent. This is why the fossil record does not show much change. It takes a very long time for significant changes to become permanent.