Evolution Of Life

Individual organisms change during their lifetime, but their basic genetic characteristics do not change. However, populations of living organisms do change through time, or evolve. Evolution, or descent with modification, is the process in which the inherited characteristics within populations change over generations, such that genetically distinct populations and new species can develop.

Evolution as a theme in biology helps us understand how the various branches of the "tree of life" came into existence and have changed over time. It also explains how organisms alive today are related to those that lived in the past. Finally, it helps us understand the mechanisms that underlie the way organisms look and behave.

Natural Selection

The ability of populations of organisms to change over time is important for survival in a changing world. According to the theory of evolution by natural selection, organisms that have certain favorable traits are better able to survive and reproduce successfully than organisms that lack these traits.

One product of natural selection is the adaptation of organisms to their environment. Adaptations are traits that improve an individual's ability to survive and reproduce. For example, rabbits with white fur and short ears in a snowy place, such as the one in Figure 1-7a, may avoid predators and frostbitten ears more often than those with dark fur and long ears. Thus, the next generation of rabbits will have a greater percentage of animals carrying the genes for white fur and short ears. In contrast, the brown, long-eared rabbit, as shown in Figure 1-7b, would survive and reproduce more successfully in a hot desert environment.

The survival and reproductive success of organisms with favorable traits cause a change in populations of organisms over generations. This descent with modification is an important factor in explaining the diversity of organisms we see on Earth today.

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