It Takes All Kinds
Beware of course documents that are misleading! In the first week of your course in rainforest biology, you looked at a number of plants and animals that were adapted for a specific niche in a specific biome. If you didn’t think critically as you read, you might have come away with the conclusion that individual organisms can adapt. It’s an intuitive idea, and a common use of language But is it possible?
Look at the picture at left. Those are penguins—quite small, but adult. They live almost as far north as the Equator. Their ancestors took a (one way) trip on the Humboldt Current (below, dark blue on the map) from Antarctica to Peru and the Galapagos Islands. There, the food was less plentiful and egg predators more so. Yet they survived. These penguins are only about 25 cm (10 inches) high. How did they change? How long did it take? What was the mechanism? For an answer, take a trip through time.
About 270 years ago, a Swede by the name of
Karl Linné (aka Linnaeus, born in 1707) invented the first modern system of
classification and documented the enormous variation in living things. He seems
to have been puzzled by the patterns and relationships he found in the
organisms
he classified, but there’s no record that he ever speculated on a model of
evolutionary change. (Linné was in enough trouble with religious folk since his
plant classification system relied on the sexual organs of flowers and couldn’t
be taught to proper young ladies!) But Europe was fascinated with fossils, and
before long, the elusive patterns that people were finding in rocks became hard
to deny. In Scotland, James Hutton (1726-1770) was writing detailed descriptions
of changes he saw in the rock strata in his farm. In France, George Cuvier
(1769-1832) refined Linné’s classification system and speculated that some of
the skeletons he had examined were from animals that no longer existed—that
change had occurred over time. None of these startling ideas was easily
accepted, but they persisted. Frenchman Jean Baptiste Lamarck (1744-1829)
developed the first model (detailed hypothesis) for how evolution might
have occurred. In Philosphie Zoologique, he speculated that organisms
might get traits that they needed, enhance traits that they used most often, and
pass on acquired traits to their offspring. He would have suggested that
equatorial penguins needed to be small. Of course, Lamarck had no idea
how a trait could be passed on. Gregor Mendel was still a child, and his work
wouldn’t be known for almost a century. But it was a start.
Proving Lamarck Wrong
Almost 50 years after Lamarck’s death, August Weissman proved him wrong. He cut the tails off of 22 generations of mice (over 1500 in all) only to find that the average length of tail in the 23rd generation was exactly the same as the average in the first generation. What happened to an organism during its lifetime could not change the traits it passed on.
But still, it was clear that populations could
change over time. By the time that Weissman did his research, the evidence from
the fossil record had accumulated. There were entire museums of fossils of
changing species like horses. Even though radioactive dating had not yet been
discovered, the relative ages of fossils could easily be determined by
looking at the position in which they occurred in massive cliffs. At almost the
same time that Weissman was destroying the tails of mice, Gregor Mendel was
diligently counting peas. He was defining the “unit character” that we call a
gene—a sequence of genetic material (almost always DNA) that determines a trait.
But he didn’t know how traits could change, either. It wasn’t until the 20th
Century that biologists were able to explain that.
James Watson and Francis Crick were the first to define how DNA could make accurate copies of itself in the 1950s—but only so accurate, and only most of the time. Every hundred thousand copies or so, an error occurs. Of course, since the organism works and is well adapted to its niche, almost every error is a disadvantage. But a very few, maybe one in a million, creates a better trait, or a potentially better variation. Can variation and evolution actually be based on such small chances? Remember, organisms have billions of cells and billions of years. So there are many mutations that might become the raw material for evolution.
But this explanation requires biologists to constantly check their logic. Organisms don’t adapt! They can’t get the mutations they need, or increase the chances of genes changing simply by using them alot. The environment, chance and even humans select from the variations that already exist in nature. Mutation comes first; evolution second.
So how about those penguins? In every generation a few were smaller and a few larger. The smaller ones were more likely to survive. It’s like the two (yes, two) Peppered Moths (Biston betularia) on the right. Some are darker, some are lighter. Which genes are more likely to be eaten? (You can’t reproduce in the stomach of a bird.)
In the rainforest, you'll find thousands of examples of adaptations that make survival just a little more likely.