Charles Darwin and the Theory of Natural Selection

The history of life, as documented by fossils and other evidence, is a saga of a changing Earth billions of years old, inhabited by an evolving cast of living forms (Figure 1.17). This evolutionary view of life came into sharp focus in November 1859, when Charles Robert Darwin published one of the most
important and influential books ever written. Entitled On the Origin of Species by Means of Natural Selection, Darwin's book was an immediate bestseller and soon made "Darwinism" almost synonymous with the concept of evolution (Figure 1.18)

Figure 1.17 Digging into the past. Paleontologist Paul Sereno, of the University of Chicago. gingerly excavates the leg bones of a dinosaur fossil in Niger.

The Origin of Species articulated two main points. First, Darwin presented evidence to support his view that contemporary species arose from a succession ofancestors. (We will discuss the evidence for evolution in detail in next Chapter.)
Darwin called this evolutionary history of species "descent with modification" It was an insightful phrase, as it captured the duality of life's unity and diversity-unity in the kinship among species that descended from common ancestors; diversity in the modifications that evolved as species branched from their common ancestors (Figure 1.19).
Darwin's second main point was to proposea mechanism for descent with modification. He called this evolutionary mechanism natural selection.
Darwin synthesized his theory of natural selection from observations that by themselves were neither new nor profound. Others had the pieces of the puzzle, but Darwin saw how they fit together. He started with the following observations from nature: Individuals in a population vary in their traits, many of which seem to be heritable (passed on from parents to offspring), Also, a population can produce far more off spring than can survive to produce offspring of their own. With more individuals than the environment can support, competition is inevitable. Lastly, species generally suit their environments.

Figure 1.19 Unity and diversity in the orchid family. These three rain forest orchids are variations on a common floral theme. For example, each of these flowers has a liplike petal that helps attrad pollinating insedS and provides a landing platform for the pollinators.

Figure 1.20 Natural selection. This imaginary beetle population has colonized alocale where the soil has been blackened by a recent brush fire. Initially, the population varies extensively in the inherited coloration of the individuals, from very light gray to charcoal. For hungry birds that prey on the beetles, it is easiest to spot the beetles that are lightest in color

For instance, birds living where tough seeds are a good food source may have especially strong beaks. Darwin made inferences from these observations to arrive at his theory of evolution. He reasoned that individuals with inherited traits that are best suited to the local environment are more likely to survive and reproduce than less fit individuals. Over many generations, a higher and higher proportion of individuals in a population will have the advantageous traits. Evolution occurs as the unequal reproductive success of individuals adapts the population to its environment.
Darwin called this mechanism of evolutionary adaptation "natural selection" because the natural environment "selects" for the propagation of certain traits. The example in Figure 1.20 illustrates the ability of natural selection to "edit" a population's heritable variations in color. We see the products of natural selection in the exquisite adaptations of various organisms to the special circumstances of their way of Hfe and their environment