2009-05-22 / Columnists

Drawing On Science For Kids

Evolution: Mechanisms
Commentary By Stephen S. Yaeger

The first part of this series gave an overview of natural selection. Natural selection explains in part how evolution occurs and may be included as one mechanism for evolution. But Darwin could not explain how or why variations in species occur. At the time that Darwin and Wallace were formulating their theory of evolution a young monk in the Augustinian monastery in Austria was pursuing his studies in genetics. Had Darwin known of Gregor Mendel's results (they were published, but were not widely distributed or recognized) he could have explained a number of things that remained beyond his knowledge, including why there are varieties within certain species and how do species change over time.

Today we know that genetics, in particular, plays a major role in evolution. Since 1970 scientists have been comparing DNA of species revealing relationships among living things. Darwin's theory has now been modified to include the principles of genetics. If a population's gene pool is static or in equilibrium, no evolutionary change is possible since there is no change in the genes. But if the equilibrium is altered evolution can occur. One such change is due to

mutations (changes in DNA or chromosome structure). Mutations occur randomly and add new genetic material to the population. Generally mutations are harmful and are not passed on. But if the geno-

type or genetic makeup of an individual is changed and the change is beneficial and passed on to new generations, then the genetic equilibrium may be disrupted. In such a case evolution may occur since natural selection will favor the change.

An error in the DNA or chromosome may result in an error in the phenotype or outward appearance of the individual. Sometimes the mutation is beneficial and may not harm the individual, but most of the times mutations are harmful to the individual. Suppose letters in a sentence represent genes. You have no problem understanding the sentence, "This house is larger than the one next to it." Changing a letter ("gene") makes it read, "This mouse is larger than the one next to it." And it still makes sense. But now read, "The fastest horse won the race." and "The fastest house won the race."

Now, suppose the sentence ("chromosome"), "This mouse is larger than the one next to it." was altered by changing the position of two words and it read, "This one is larger than the mouse next to it." It would still make sense. But if the sentence were altered as "This is larger mouse than the one next to it." it would make no sense.

Imagine a population of large, small, and average size spiders. The smaller spiders may not be seen by birds and escape being eaten. But these spiders may have a hard time getting prey themselves. As a result their phenotypes pass on very little of their genes to future generations of spiders. The larger spiders can easily capture and eat their prey, but they are easily seen by birds and suffer the consequences. Their genes, too, are not readily passed on to future generations. The average size spider has a selective advantage over their brethren as they are capable of getting prey; remain, more or less, invisible to birds; and are able, then, to pass on their genes with no problem.

The larger a hummingbird's tongue, the deeper it can insert it into a flower to get at the nectar. Long-tongued hummingbirds have a selective advantage over the shorter-tongued varieties. Adult limpets live on rocks. Light colored limpets are safe on light-colored rocks, while dark-colored limpets are safe on dark-colored rocks. But tan-colored limpets are at a major disadvantage being easily spotted by predators. In this case natural selection favors the two extreme cases.

Speciation or the evolution of new species occurs when interbreeding or the production of fertile offspring is somehow prevented. If some physical barrier such as a river, desert, or mountain range somehow breaks up a population into two or more groups, each group may be prevented from interbreeding. In time they may develop their own specific gene pools and become new species. Five million years ago a species of squirrel lived on what became the Grand Canyon plateau. When the Colorado River eroded the land and formed the canyon, the squirrel population gradually separated into two groups unable to interbreed. Today the north rim hosts the Kaibab squirrel with black bellies and white tails. On the South rim the Abert squirrel with white bellies and dark tails makes its home. The two species, once members of a single population, developed as a result of geographic isolation.

The Hawaiian honeycreepers, Darwin's finches of the Galapagos, and the hundreds of cichlid fish varieties in Africa's Lake Victoria are examples of

adaptive radiation where a species once similar to an ancestral type slowly became more and more distinct as new species.

Darwin said that small, adaptive changes accumulate within populations over a long period of time. He said that speciation is a slow, gradual process. An example of this process, called gradualism is the classic evolution of the horse from a small, dog-size animal to the modern horse as evidenced in the fossil record. In the 1990s Stephen J. Gould proposed his theory of Punctuated Equilibium. Gould said that evolution does not occur gradually over long periods of time, but rather in short, abrupt bursts over a short period of time (keep in mind that geologic time includes millions of years) in response to environmental changes. His theory, too, has fossil evidence support. Today scientists recognize both gradualism and punctuated equilibrium as prime movers of evolution. Next month: Human evolution. Questions/comments? E-mail Steve: Drawingon science@aol.com.

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