Natural selection is the main mechanism by which populations adapt and evolve.

Before they started disappearing, the birds of Guam were a diverse and resplendent bunch. The island, about one-fifth the size of Rhode Island, was home to 18 native species of birds, each specially suited to life on the island.

Populations usually contain individuals that are genetically different from one another. According to the evolutionary theory first put forth by Charles Darwin and Alfred Russel Wallace, and subsequently supported by a tremendous amount of evidence from a wide variety of scientific disciplines, selective pressure on a population—a nonrandom influence that affects who survives or reproduces—favors individuals with certain inherited traits over others (such as better camouflage, tolerance for drought, or enhanced sense of smell). These individuals have differential reproductive success compared to other individuals: They are best suited for their environment and leave more offspring than those who are less suited for their environment.

selective pressure

A nonrandom influence that affects who survives or reproduces.

WHERE IS GUAM?

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The traits that an environment favors are called adaptations, and the process by which organisms best adapted to the environment survive to pass on their traits is known as natural selection. Evolutionary biology helps us understand the diversity of life on Earth and how populations change over time. It is one of the pillars of biological science, and the vast amount of evidence in support of both the occurrence of evolution and the mechanisms by which it happens has elevated this explanation to the level of scientific theory (see Chapter 2).

adaptation

A trait that helps an individual survive or reproduce.

natural selection

The process by which organisms best adapted to the environment (the fittest) survive to reproduce, leaving more offspring than less well-adapted individuals.

For most populations, more offspring are born than can survive, since resources are limited and many species produce large numbers of young. Since only some individuals will survive, over time, the population will contain more and more of these better-adapted individuals and their offspring. Ultimately, this changes how common certain variants of genes (these variants are called alleles) are in the population: The frequency (percentage in the population) of some genes increases and that of others decreases. When this occurs, the population has experienced evolution, or changes in the gene frequencies within a population from one generation to the next. Natural selection may be stabilizing, directional, or disruptive, depending on which genetic traits are favored or selected against. INFOGRAPHIC 11.1

genes

Stretches of DNA, the hereditary material of cells, that each direct the production of a particular protein and influence an individual’s traits.

evolution

Differences in the gene frequencies within a population from one generation to the next.

gene frequencies

The assortment and abundance of particular variants of genes relative to each other within a population.

NATURAL SELECTION AT WORK

When the environment presents a selective force (e.g., a new predator, changing temperatures, change in food supply), natural selection is the primary force by which populations adapt. The survivors are those who were lucky enough to have genetic traits that allowed them to survive in their changing environment. (Others who did not possess the trait were not as likely to survive to reproduce.) Because survivors pass on those adaptations to their offspring, the gene frequencies of the population change in the next generation, which means some traits are more common and others are less common than they used to be. When this happens, the population is said to have evolved.

KEY CONCEPT 11.1

Natural selection is a mechanism by which a population can adapt to changes in its environment. Individuals that are best suited for their environment due to inherited traits leave more offspring (which also possess these traits) than other, less-suited individuals, producing subsequent generations that contain more individuals with the useful traits.

Identify the gene frequencies of the “original population” for each color morph (dark gray, dark brown, light brown, dark tan, and light tan) by counting the number of each and expressing it as a percentage of the whole. Now do the same for the “later generation.” Has evolution occurred? Explain.

In the Original population the gene frequencies are: dark gray = 2/12 (16.7%); dark brown = 3/12 (25%); light brown = 3/12 (25%); dark tan = 2/12 (16.7%); light tan = 2/12 (16.7%). In the "Later generation" the gene frequencies are: dark gray 0/13 (0%); dark brown = 0/13 (0%); light brown = 7/13 (53.8%); dark tan = 4/13 (30.8%); light tan = 2/13 (15.4%). Evolution of this population has occurred since the gene frequencies have changed - there are more of the light brown and dark tan beetles and less of (or in some cases none of) the others.

It is important to note that individuals are selected for, but populations evolve; individuals do not change their own genetic makeup to produce new necessary adaptations, such as bigger size or pesticide resistance. If they get the opportunity to reproduce, they pass on their traits to the next generation. If they cannot tolerate environmental changes, as was the case with the first bird species to disappear from Guam (the bridled white-eye), they die or fail to reproduce and do not pass on their genes. Individuals may be able to adjust their behavior to accommodate environmental changes, but if a trait is not genetically controlled, and therefore is not heritable, it will not influence the composition of the next generation.