Animation 13.1 Natural Selection

INTRODUCTION

In 1858, two men, Charles Darwin and Alfred Russel Wallace, independently proposed a mechanism for evolution. Darwin named this mechanism natural selection. Natural selection requires variation among individuals in a population. In a particular environment, some traits of individuals are more advantageous than others. The individuals with the advantageous traits are able to survive better, and therefore they are also able to produce more offspring. In subsequent generations, there are relatively more individuals with the inherited advantageous traits. In this way, the population changes, or evolves, from one generation to the next.

In the accompanying animation, we examine natural selection in Texas Longhorn cattle.

Textbook Reference: Key Concept 15.3 Changes in Gene Expression Often Shape Evolution, p.360

CONCLUSION

In the accompanying animation, we examined several examples of natural selection, each of which produces strikingly different results in the level of variation in the population.

· In stabilizing selection, natural selection preserves the characteristics of a population by favoring average individuals. Stabilizing selection reduces variation, but does not change the position of the population's mean.

· In directional selection, natural selection changes the characteristics of a population by favoring individuals that vary in one direction from the mean of the population. If directional selection operates over many generations, then an evolutionary trend results in the population.

· In disruptive selection, natural selection changes the characteristics of a population by favoring individuals that vary in both directions from the mean of the population.

The mechanism underlying each is the same: individuals with advantageous traits survive better and therefore are able reproduce more, increasing the frequency of individuals with the advantageous traits in the next generation.

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